Make repack less likely to corrupt repository

[alt-git.git] / builtin-pack-objects.c

#include "builtin.h"
#include "cache.h"
#include "attr.h"
#include "object.h"
#include "blob.h"
#include "commit.h"
#include "tag.h"
#include "tree.h"
#include "delta.h"
#include "pack.h"
#include "pack-revindex.h"
#include "csum-file.h"
#include "tree-walk.h"
#include "diff.h"
#include "revision.h"
#include "list-objects.h"
#include "progress.h"
#include "refs.h"

#ifdef THREADED_DELTA_SEARCH
#include "thread-utils.h"
#include <pthread.h>
#endif

static const char pack_usage[] = "\
git pack-objects [{ -q | --progress | --all-progress }] \n\
        [--max-pack-size=N] [--local] [--incremental] \n\
        [--window=N] [--window-memory=N] [--depth=N] \n\
        [--no-reuse-delta] [--no-reuse-object] [--delta-base-offset] \n\
        [--threads=N] [--non-empty] [--revs [--unpacked | --all]*] [--reflog] \n\
        [--stdout | base-name] [--include-tag] \n\
        [--keep-unreachable | --unpack-unreachable] \n\
        [<ref-list | <object-list]";

struct object_entry {
        struct pack_idx_entry idx;
        unsigned long size;     /* uncompressed size */
        struct packed_git *in_pack;     /* already in pack */
        off_t in_pack_offset;
        struct object_entry *delta;     /* delta base object */
        struct object_entry *delta_child; /* deltified objects who bases me */
        struct object_entry *delta_sibling; /* other deltified objects who
                                             * uses the same base as me
                                             */
        void *delta_data;       /* cached delta (uncompressed) */
        unsigned long delta_size;       /* delta data size (uncompressed) */
        unsigned long z_delta_size;     /* delta data size (compressed) */
        unsigned int hash;      /* name hint hash */
        enum object_type type;
        enum object_type in_pack_type;  /* could be delta */
        unsigned char in_pack_header_size;
        unsigned char preferred_base; /* we do not pack this, but is available
                                       * to be used as the base object to delta
                                       * objects against.
                                       */
        unsigned char no_try_delta;
};

/*
 * Objects we are going to pack are collected in objects array (dynamically
 * expanded).  nr_objects & nr_alloc controls this array.  They are stored
 * in the order we see -- typically rev-list --objects order that gives us
 * nice "minimum seek" order.
 */
static struct object_entry *objects;
static struct pack_idx_entry **written_list;
static uint32_t nr_objects, nr_alloc, nr_result, nr_written;

static int non_empty;
static int reuse_delta = 1, reuse_object = 1;
static int keep_unreachable, unpack_unreachable, include_tag;
static int local;
static int incremental;
static int ignore_packed_keep;
static int allow_ofs_delta;
static const char *base_name;
static int progress = 1;
static int window = 10;
static uint32_t pack_size_limit, pack_size_limit_cfg;
static int depth = 50;
static int delta_search_threads = 1;
static int pack_to_stdout;
static int num_preferred_base;
static struct progress *progress_state;
static int pack_compression_level = Z_DEFAULT_COMPRESSION;
static int pack_compression_seen;

static unsigned long delta_cache_size = 0;
static unsigned long max_delta_cache_size = 0;
static unsigned long cache_max_small_delta_size = 1000;

static unsigned long window_memory_limit = 0;

/*
 * The object names in objects array are hashed with this hashtable,
 * to help looking up the entry by object name.
 * This hashtable is built after all the objects are seen.
 */
static int *object_ix;
static int object_ix_hashsz;

/*
 * stats
 */
static uint32_t written, written_delta;
static uint32_t reused, reused_delta;


static void *get_delta(struct object_entry *entry)
{
        unsigned long size, base_size, delta_size;
        void *buf, *base_buf, *delta_buf;
        enum object_type type;

        buf = read_sha1_file(entry->idx.sha1, &type, &size);
        if (!buf)
                die("unable to read %s", sha1_to_hex(entry->idx.sha1));
        base_buf = read_sha1_file(entry->delta->idx.sha1, &type, &base_size);
        if (!base_buf)
                die("unable to read %s", sha1_to_hex(entry->delta->idx.sha1));
        delta_buf = diff_delta(base_buf, base_size,
                               buf, size, &delta_size, 0);
        if (!delta_buf || delta_size != entry->delta_size)
                die("delta size changed");
        free(buf);
        free(base_buf);
        return delta_buf;
}

static unsigned long do_compress(void **pptr, unsigned long size)
{
        z_stream stream;
        void *in, *out;
        unsigned long maxsize;

        memset(&stream, 0, sizeof(stream));
        deflateInit(&stream, pack_compression_level);
        maxsize = deflateBound(&stream, size);

        in = *pptr;
        out = xmalloc(maxsize);
        *pptr = out;

        stream.next_in = in;
        stream.avail_in = size;
        stream.next_out = out;
        stream.avail_out = maxsize;
        while (deflate(&stream, Z_FINISH) == Z_OK)
                ; /* nothing */
        deflateEnd(&stream);

        free(in);
        return stream.total_out;
}

/*
 * The per-object header is a pretty dense thing, which is
 *  - first byte: low four bits are "size", then three bits of "type",
 *    and the high bit is "size continues".
 *  - each byte afterwards: low seven bits are size continuation,
 *    with the high bit being "size continues"
 */
static int encode_header(enum object_type type, unsigned long size, unsigned char *hdr)
{
        int n = 1;
        unsigned char c;

        if (type < OBJ_COMMIT || type > OBJ_REF_DELTA)
                die("bad type %d", type);

        c = (type << 4) | (size & 15);
        size >>= 4;
        while (size) {
                *hdr++ = c | 0x80;
                c = size & 0x7f;
                size >>= 7;
                n++;
        }
        *hdr = c;
        return n;
}

/*
 * we are going to reuse the existing object data as is.  make
 * sure it is not corrupt.
 */
static int check_pack_inflate(struct packed_git *p,
                struct pack_window **w_curs,
                off_t offset,
                off_t len,
                unsigned long expect)
{
        z_stream stream;
        unsigned char fakebuf[4096], *in;
        int st;

        memset(&stream, 0, sizeof(stream));
        inflateInit(&stream);
        do {
                in = use_pack(p, w_curs, offset, &stream.avail_in);
                stream.next_in = in;
                stream.next_out = fakebuf;
                stream.avail_out = sizeof(fakebuf);
                st = inflate(&stream, Z_FINISH);
                offset += stream.next_in - in;
        } while (st == Z_OK || st == Z_BUF_ERROR);
        inflateEnd(&stream);
        return (st == Z_STREAM_END &&
                stream.total_out == expect &&
                stream.total_in == len) ? 0 : -1;
}

static void copy_pack_data(struct sha1file *f,
                struct packed_git *p,
                struct pack_window **w_curs,
                off_t offset,
                off_t len)
{
        unsigned char *in;
        unsigned int avail;

        while (len) {
                in = use_pack(p, w_curs, offset, &avail);
                if (avail > len)
                        avail = (unsigned int)len;
                sha1write(f, in, avail);
                offset += avail;
                len -= avail;
        }
}

static unsigned long write_object(struct sha1file *f,
                                  struct object_entry *entry,
                                  off_t write_offset)
{
        unsigned long size, limit, datalen;
        void *buf;
        unsigned char header[10], dheader[10];
        unsigned hdrlen;
        enum object_type type;
        int usable_delta, to_reuse;

        if (!pack_to_stdout)
                crc32_begin(f);

        type = entry->type;

        /* write limit if limited packsize and not first object */
        if (!pack_size_limit || !nr_written)
                limit = 0;
        else if (pack_size_limit <= write_offset)
                /*
                 * the earlier object did not fit the limit; avoid
                 * mistaking this with unlimited (i.e. limit = 0).
                 */
                limit = 1;
        else
                limit = pack_size_limit - write_offset;

        if (!entry->delta)
                usable_delta = 0;       /* no delta */
        else if (!pack_size_limit)
               usable_delta = 1;        /* unlimited packfile */
        else if (entry->delta->idx.offset == (off_t)-1)
                usable_delta = 0;       /* base was written to another pack */
        else if (entry->delta->idx.offset)
                usable_delta = 1;       /* base already exists in this pack */
        else
                usable_delta = 0;       /* base could end up in another pack */

        if (!reuse_object)
                to_reuse = 0;   /* explicit */
        else if (!entry->in_pack)
                to_reuse = 0;   /* can't reuse what we don't have */
        else if (type == OBJ_REF_DELTA || type == OBJ_OFS_DELTA)
                                /* check_object() decided it for us ... */
                to_reuse = usable_delta;
                                /* ... but pack split may override that */
        else if (type != entry->in_pack_type)
                to_reuse = 0;   /* pack has delta which is unusable */
        else if (entry->delta)
                to_reuse = 0;   /* we want to pack afresh */
        else
                to_reuse = 1;   /* we have it in-pack undeltified,
                                 * and we do not need to deltify it.
                                 */

        if (!to_reuse) {
                if (!usable_delta) {
                        buf = read_sha1_file(entry->idx.sha1, &type, &size);
                        if (!buf)
                                die("unable to read %s", sha1_to_hex(entry->idx.sha1));
                        /*
                         * make sure no cached delta data remains from a
                         * previous attempt before a pack split occured.
                         */
                        free(entry->delta_data);
                        entry->delta_data = NULL;
                        entry->z_delta_size = 0;
                } else if (entry->delta_data) {
                        size = entry->delta_size;
                        buf = entry->delta_data;
                        entry->delta_data = NULL;
                        type = (allow_ofs_delta && entry->delta->idx.offset) ?
                                OBJ_OFS_DELTA : OBJ_REF_DELTA;
                } else {
                        buf = get_delta(entry);
                        size = entry->delta_size;
                        type = (allow_ofs_delta && entry->delta->idx.offset) ?
                                OBJ_OFS_DELTA : OBJ_REF_DELTA;
                }

                if (entry->z_delta_size)
                        datalen = entry->z_delta_size;
                else
                        datalen = do_compress(&buf, size);

                /*
                 * The object header is a byte of 'type' followed by zero or
                 * more bytes of length.
                 */
                hdrlen = encode_header(type, size, header);

                if (type == OBJ_OFS_DELTA) {
                        /*
                         * Deltas with relative base contain an additional
                         * encoding of the relative offset for the delta
                         * base from this object's position in the pack.
                         */
                        off_t ofs = entry->idx.offset - entry->delta->idx.offset;
                        unsigned pos = sizeof(dheader) - 1;
                        dheader[pos] = ofs & 127;
                        while (ofs >>= 7)
                                dheader[--pos] = 128 | (--ofs & 127);
                        if (limit && hdrlen + sizeof(dheader) - pos + datalen + 20 >= limit) {
                                free(buf);
                                return 0;
                        }
                        sha1write(f, header, hdrlen);
                        sha1write(f, dheader + pos, sizeof(dheader) - pos);
                        hdrlen += sizeof(dheader) - pos;
                } else if (type == OBJ_REF_DELTA) {
                        /*
                         * Deltas with a base reference contain
                         * an additional 20 bytes for the base sha1.
                         */
                        if (limit && hdrlen + 20 + datalen + 20 >= limit) {
                                free(buf);
                                return 0;
                        }
                        sha1write(f, header, hdrlen);
                        sha1write(f, entry->delta->idx.sha1, 20);
                        hdrlen += 20;
                } else {
                        if (limit && hdrlen + datalen + 20 >= limit) {
                                free(buf);
                                return 0;
                        }
                        sha1write(f, header, hdrlen);
                }
                sha1write(f, buf, datalen);
                free(buf);
        }
        else {
                struct packed_git *p = entry->in_pack;
                struct pack_window *w_curs = NULL;
                struct revindex_entry *revidx;
                off_t offset;

                if (entry->delta) {
                        type = (allow_ofs_delta && entry->delta->idx.offset) ?
                                OBJ_OFS_DELTA : OBJ_REF_DELTA;
                        reused_delta++;
                }
                hdrlen = encode_header(type, entry->size, header);
                offset = entry->in_pack_offset;
                revidx = find_pack_revindex(p, offset);
                datalen = revidx[1].offset - offset;
                if (!pack_to_stdout && p->index_version > 1 &&
                    check_pack_crc(p, &w_curs, offset, datalen, revidx->nr))
                        die("bad packed object CRC for %s", sha1_to_hex(entry->idx.sha1));
                offset += entry->in_pack_header_size;
                datalen -= entry->in_pack_header_size;
                if (type == OBJ_OFS_DELTA) {
                        off_t ofs = entry->idx.offset - entry->delta->idx.offset;
                        unsigned pos = sizeof(dheader) - 1;
                        dheader[pos] = ofs & 127;
                        while (ofs >>= 7)
                                dheader[--pos] = 128 | (--ofs & 127);
                        if (limit && hdrlen + sizeof(dheader) - pos + datalen + 20 >= limit)
                                return 0;
                        sha1write(f, header, hdrlen);
                        sha1write(f, dheader + pos, sizeof(dheader) - pos);
                        hdrlen += sizeof(dheader) - pos;
                } else if (type == OBJ_REF_DELTA) {
                        if (limit && hdrlen + 20 + datalen + 20 >= limit)
                                return 0;
                        sha1write(f, header, hdrlen);
                        sha1write(f, entry->delta->idx.sha1, 20);
                        hdrlen += 20;
                } else {
                        if (limit && hdrlen + datalen + 20 >= limit)
                                return 0;
                        sha1write(f, header, hdrlen);
                }

                if (!pack_to_stdout && p->index_version == 1 &&
                    check_pack_inflate(p, &w_curs, offset, datalen, entry->size))
                        die("corrupt packed object for %s", sha1_to_hex(entry->idx.sha1));
                copy_pack_data(f, p, &w_curs, offset, datalen);
                unuse_pack(&w_curs);
                reused++;
        }
        if (usable_delta)
                written_delta++;
        written++;
        if (!pack_to_stdout)
                entry->idx.crc32 = crc32_end(f);
        return hdrlen + datalen;
}

static int write_one(struct sha1file *f,
                               struct object_entry *e,
                               off_t *offset)
{
        unsigned long size;

        /* offset is non zero if object is written already. */
        if (e->idx.offset || e->preferred_base)
                return 1;

        /* if we are deltified, write out base object first. */
        if (e->delta && !write_one(f, e->delta, offset))
                return 0;

        e->idx.offset = *offset;
        size = write_object(f, e, *offset);
        if (!size) {
                e->idx.offset = 0;
                return 0;
        }
        written_list[nr_written++] = &e->idx;

        /* make sure off_t is sufficiently large not to wrap */
        if (*offset > *offset + size)
                die("pack too large for current definition of off_t");
        *offset += size;
        return 1;
}

/* forward declaration for write_pack_file */
static int adjust_perm(const char *path, mode_t mode);

static void write_pack_file(void)
{
        uint32_t i = 0, j;
        struct sha1file *f;
        off_t offset;
        struct pack_header hdr;
        uint32_t nr_remaining = nr_result;
        time_t last_mtime = 0;

        if (progress > pack_to_stdout)
                progress_state = start_progress("Writing objects", nr_result);
        written_list = xmalloc(nr_objects * sizeof(*written_list));

        do {
                unsigned char sha1[20];
                char *pack_tmp_name = NULL;

                if (pack_to_stdout) {
                        f = sha1fd_throughput(1, "<stdout>", progress_state);
                } else {
                        char tmpname[PATH_MAX];
                        int fd;
                        snprintf(tmpname, sizeof(tmpname),
                                 "%s/pack/tmp_pack_XXXXXX", get_object_directory());
                        fd = xmkstemp(tmpname);
                        pack_tmp_name = xstrdup(tmpname);
                        f = sha1fd(fd, pack_tmp_name);
                }

                hdr.hdr_signature = htonl(PACK_SIGNATURE);
                hdr.hdr_version = htonl(PACK_VERSION);
                hdr.hdr_entries = htonl(nr_remaining);
                sha1write(f, &hdr, sizeof(hdr));
                offset = sizeof(hdr);
                nr_written = 0;
                for (; i < nr_objects; i++) {
                        if (!write_one(f, objects + i, &offset))
                                break;
                        display_progress(progress_state, written);
                }

                /*
                 * Did we write the wrong # entries in the header?
                 * If so, rewrite it like in fast-import
                 */
                if (pack_to_stdout) {
                        sha1close(f, sha1, CSUM_CLOSE);
                } else if (nr_written == nr_remaining) {
                        sha1close(f, sha1, CSUM_FSYNC);
                } else {
                        int fd = sha1close(f, sha1, 0);
                        fixup_pack_header_footer(fd, sha1, pack_tmp_name,
                                                 nr_written, sha1, offset);
                        close(fd);
                }

                if (!pack_to_stdout) {
                        mode_t mode = umask(0);
                        struct stat st;
                        char *idx_tmp_name, tmpname[PATH_MAX];

                        umask(mode);
                        mode = 0444 & ~mode;

                        idx_tmp_name = write_idx_file(NULL, written_list,
                                                      nr_written, sha1);

                        snprintf(tmpname, sizeof(tmpname), "%s-%s.pack",
                                 base_name, sha1_to_hex(sha1));
                        free_pack_by_name(tmpname);
                        if (adjust_perm(pack_tmp_name, mode))
                                die("unable to make temporary pack file readable: %s",
                                    strerror(errno));
                        if (rename(pack_tmp_name, tmpname))
                                die("unable to rename temporary pack file: %s",
                                    strerror(errno));

                        /*
                         * Packs are runtime accessed in their mtime
                         * order since newer packs are more likely to contain
                         * younger objects.  So if we are creating multiple
                         * packs then we should modify the mtime of later ones
                         * to preserve this property.
                         */
                        if (stat(tmpname, &st) < 0) {
                                warning("failed to stat %s: %s",
                                        tmpname, strerror(errno));
                        } else if (!last_mtime) {
                                last_mtime = st.st_mtime;
                        } else {
                                struct utimbuf utb;
                                utb.actime = st.st_atime;
                                utb.modtime = --last_mtime;
                                if (utime(tmpname, &utb) < 0)
                                        warning("failed utime() on %s: %s",
                                                tmpname, strerror(errno));
                        }

                        snprintf(tmpname, sizeof(tmpname), "%s-%s.idx",
                                 base_name, sha1_to_hex(sha1));
                        if (adjust_perm(idx_tmp_name, mode))
                                die("unable to make temporary index file readable: %s",
                                    strerror(errno));
                        if (rename(idx_tmp_name, tmpname))
                                die("unable to rename temporary index file: %s",
                                    strerror(errno));

                        free(idx_tmp_name);
                        free(pack_tmp_name);
                        puts(sha1_to_hex(sha1));
                }

                /* mark written objects as written to previous pack */
                for (j = 0; j < nr_written; j++) {
                        written_list[j]->offset = (off_t)-1;
                }
                nr_remaining -= nr_written;
        } while (nr_remaining && i < nr_objects);

        free(written_list);
        stop_progress(&progress_state);
        if (written != nr_result)
                die("wrote %"PRIu32" objects while expecting %"PRIu32,
                        written, nr_result);
        /*
         * We have scanned through [0 ... i).  Since we have written
         * the correct number of objects,  the remaining [i ... nr_objects)
         * items must be either already written (due to out-of-order delta base)
         * or a preferred base.  Count those which are neither and complain if any.
         */
        for (j = 0; i < nr_objects; i++) {
                struct object_entry *e = objects + i;
                j += !e->idx.offset && !e->preferred_base;
        }
        if (j)
                die("wrote %"PRIu32" objects as expected but %"PRIu32
                        " unwritten", written, j);
}

static int locate_object_entry_hash(const unsigned char *sha1)
{
        int i;
        unsigned int ui;
        memcpy(&ui, sha1, sizeof(unsigned int));
        i = ui % object_ix_hashsz;
        while (0 < object_ix[i]) {
                if (!hashcmp(sha1, objects[object_ix[i] - 1].idx.sha1))
                        return i;
                if (++i == object_ix_hashsz)
                        i = 0;
        }
        return -1 - i;
}

static struct object_entry *locate_object_entry(const unsigned char *sha1)
{
        int i;

        if (!object_ix_hashsz)
                return NULL;

        i = locate_object_entry_hash(sha1);
        if (0 <= i)
                return &objects[object_ix[i]-1];
        return NULL;
}

static void rehash_objects(void)
{
        uint32_t i;
        struct object_entry *oe;

        object_ix_hashsz = nr_objects * 3;
        if (object_ix_hashsz < 1024)
                object_ix_hashsz = 1024;
        object_ix = xrealloc(object_ix, sizeof(int) * object_ix_hashsz);
        memset(object_ix, 0, sizeof(int) * object_ix_hashsz);
        for (i = 0, oe = objects; i < nr_objects; i++, oe++) {
                int ix = locate_object_entry_hash(oe->idx.sha1);
                if (0 <= ix)
                        continue;
                ix = -1 - ix;
                object_ix[ix] = i + 1;
        }
}

static unsigned name_hash(const char *name)
{
        unsigned char c;
        unsigned hash = 0;

        if (!name)
                return 0;

        /*
         * This effectively just creates a sortable number from the
         * last sixteen non-whitespace characters. Last characters
         * count "most", so things that end in ".c" sort together.
         */
        while ((c = *name++) != 0) {
                if (isspace(c))
                        continue;
                hash = (hash >> 2) + (c << 24);
        }
        return hash;
}

static void setup_delta_attr_check(struct git_attr_check *check)
{
        static struct git_attr *attr_delta;

        if (!attr_delta)
                attr_delta = git_attr("delta", 5);

        check[0].attr = attr_delta;
}

static int no_try_delta(const char *path)
{
        struct git_attr_check check[1];

        setup_delta_attr_check(check);
        if (git_checkattr(path, ARRAY_SIZE(check), check))
                return 0;
        if (ATTR_FALSE(check->value))
                return 1;
        return 0;
}

static int add_object_entry(const unsigned char *sha1, enum object_type type,
                            const char *name, int exclude)
{
        struct object_entry *entry;
        struct packed_git *p, *found_pack = NULL;
        off_t found_offset = 0;
        int ix;
        unsigned hash = name_hash(name);

        ix = nr_objects ? locate_object_entry_hash(sha1) : -1;
        if (ix >= 0) {
                if (exclude) {
                        entry = objects + object_ix[ix] - 1;
                        if (!entry->preferred_base)
                                nr_result--;
                        entry->preferred_base = 1;
                }
                return 0;
        }

        if (!exclude && local && has_loose_object_nonlocal(sha1))
                return 0;

        for (p = packed_git; p; p = p->next) {
                off_t offset = find_pack_entry_one(sha1, p);
                if (offset) {
                        if (!found_pack) {
                                found_offset = offset;
                                found_pack = p;
                        }
                        if (exclude)
                                break;
                        if (incremental)
                                return 0;
                        if (local && !p->pack_local)
                                return 0;
                        if (ignore_packed_keep && p->pack_local && p->pack_keep)
                                return 0;
                }
        }

        if (nr_objects >= nr_alloc) {
                nr_alloc = (nr_alloc  + 1024) * 3 / 2;
                objects = xrealloc(objects, nr_alloc * sizeof(*entry));
        }

        entry = objects + nr_objects++;
        memset(entry, 0, sizeof(*entry));
        hashcpy(entry->idx.sha1, sha1);
        entry->hash = hash;
        if (type)
                entry->type = type;
        if (exclude)
                entry->preferred_base = 1;
        else
                nr_result++;
        if (found_pack) {
                entry->in_pack = found_pack;
                entry->in_pack_offset = found_offset;
        }

        if (object_ix_hashsz * 3 <= nr_objects * 4)
                rehash_objects();
        else
                object_ix[-1 - ix] = nr_objects;

        display_progress(progress_state, nr_objects);

        if (name && no_try_delta(name))
                entry->no_try_delta = 1;

        return 1;
}

struct pbase_tree_cache {
        unsigned char sha1[20];
        int ref;
        int temporary;
        void *tree_data;
        unsigned long tree_size;
};

static struct pbase_tree_cache *(pbase_tree_cache[256]);
static int pbase_tree_cache_ix(const unsigned char *sha1)
{
        return sha1[0] % ARRAY_SIZE(pbase_tree_cache);
}
static int pbase_tree_cache_ix_incr(int ix)
{
        return (ix+1) % ARRAY_SIZE(pbase_tree_cache);
}

static struct pbase_tree {
        struct pbase_tree *next;
        /* This is a phony "cache" entry; we are not
         * going to evict it nor find it through _get()
         * mechanism -- this is for the toplevel node that
         * would almost always change with any commit.
         */
        struct pbase_tree_cache pcache;
} *pbase_tree;

static struct pbase_tree_cache *pbase_tree_get(const unsigned char *sha1)
{
        struct pbase_tree_cache *ent, *nent;
        void *data;
        unsigned long size;
        enum object_type type;
        int neigh;
        int my_ix = pbase_tree_cache_ix(sha1);
        int available_ix = -1;

        /* pbase-tree-cache acts as a limited hashtable.
         * your object will be found at your index or within a few
         * slots after that slot if it is cached.
         */
        for (neigh = 0; neigh < 8; neigh++) {
                ent = pbase_tree_cache[my_ix];
                if (ent && !hashcmp(ent->sha1, sha1)) {
                        ent->ref++;
                        return ent;
                }
                else if (((available_ix < 0) && (!ent || !ent->ref)) ||
                         ((0 <= available_ix) &&
                          (!ent && pbase_tree_cache[available_ix])))
                        available_ix = my_ix;
                if (!ent)
                        break;
                my_ix = pbase_tree_cache_ix_incr(my_ix);
        }

        /* Did not find one.  Either we got a bogus request or
         * we need to read and perhaps cache.
         */
        data = read_sha1_file(sha1, &type, &size);
        if (!data)
                return NULL;
        if (type != OBJ_TREE) {
                free(data);
                return NULL;
        }

        /* We need to either cache or return a throwaway copy */

        if (available_ix < 0)
                ent = NULL;
        else {
                ent = pbase_tree_cache[available_ix];
                my_ix = available_ix;
        }

        if (!ent) {
                nent = xmalloc(sizeof(*nent));
                nent->temporary = (available_ix < 0);
        }
        else {
                /* evict and reuse */
                free(ent->tree_data);
                nent = ent;
        }
        hashcpy(nent->sha1, sha1);
        nent->tree_data = data;
        nent->tree_size = size;
        nent->ref = 1;
        if (!nent->temporary)
                pbase_tree_cache[my_ix] = nent;
        return nent;
}

static void pbase_tree_put(struct pbase_tree_cache *cache)
{
        if (!cache->temporary) {
                cache->ref--;
                return;
        }
        free(cache->tree_data);
        free(cache);
}

static int name_cmp_len(const char *name)
{
        int i;
        for (i = 0; name[i] && name[i] != '\n' && name[i] != '/'; i++)
                ;
        return i;
}

static void add_pbase_object(struct tree_desc *tree,
                             const char *name,
                             int cmplen,
                             const char *fullname)
{
        struct name_entry entry;
        int cmp;

        while (tree_entry(tree,&entry)) {
                if (S_ISGITLINK(entry.mode))
                        continue;
                cmp = tree_entry_len(entry.path, entry.sha1) != cmplen ? 1 :
                      memcmp(name, entry.path, cmplen);
                if (cmp > 0)
                        continue;
                if (cmp < 0)
                        return;
                if (name[cmplen] != '/') {
                        add_object_entry(entry.sha1,
                                         object_type(entry.mode),
                                         fullname, 1);
                        return;
                }
                if (S_ISDIR(entry.mode)) {
                        struct tree_desc sub;
                        struct pbase_tree_cache *tree;
                        const char *down = name+cmplen+1;
                        int downlen = name_cmp_len(down);

                        tree = pbase_tree_get(entry.sha1);
                        if (!tree)
                                return;
                        init_tree_desc(&sub, tree->tree_data, tree->tree_size);

                        add_pbase_object(&sub, down, downlen, fullname);
                        pbase_tree_put(tree);
                }
        }
}

static unsigned *done_pbase_paths;
static int done_pbase_paths_num;
static int done_pbase_paths_alloc;
static int done_pbase_path_pos(unsigned hash)
{
        int lo = 0;
        int hi = done_pbase_paths_num;
        while (lo < hi) {
                int mi = (hi + lo) / 2;
                if (done_pbase_paths[mi] == hash)
                        return mi;
                if (done_pbase_paths[mi] < hash)
                        hi = mi;
                else
                        lo = mi + 1;
        }
        return -lo-1;
}

static int check_pbase_path(unsigned hash)
{
        int pos = (!done_pbase_paths) ? -1 : done_pbase_path_pos(hash);
        if (0 <= pos)
                return 1;
        pos = -pos - 1;
        if (done_pbase_paths_alloc <= done_pbase_paths_num) {
                done_pbase_paths_alloc = alloc_nr(done_pbase_paths_alloc);
                done_pbase_paths = xrealloc(done_pbase_paths,
                                            done_pbase_paths_alloc *
                                            sizeof(unsigned));
        }
        done_pbase_paths_num++;
        if (pos < done_pbase_paths_num)
                memmove(done_pbase_paths + pos + 1,
                        done_pbase_paths + pos,
                        (done_pbase_paths_num - pos - 1) * sizeof(unsigned));
        done_pbase_paths[pos] = hash;
        return 0;
}

static void add_preferred_base_object(const char *name)
{
        struct pbase_tree *it;
        int cmplen;
        unsigned hash = name_hash(name);

        if (!num_preferred_base || check_pbase_path(hash))
                return;

        cmplen = name_cmp_len(name);
        for (it = pbase_tree; it; it = it->next) {
                if (cmplen == 0) {
                        add_object_entry(it->pcache.sha1, OBJ_TREE, NULL, 1);
                }
                else {
                        struct tree_desc tree;
                        init_tree_desc(&tree, it->pcache.tree_data, it->pcache.tree_size);
                        add_pbase_object(&tree, name, cmplen, name);
                }
        }
}

static void add_preferred_base(unsigned char *sha1)
{
        struct pbase_tree *it;
        void *data;
        unsigned long size;
        unsigned char tree_sha1[20];

        if (window <= num_preferred_base++)
                return;

        data = read_object_with_reference(sha1, tree_type, &size, tree_sha1);
        if (!data)
                return;

        for (it = pbase_tree; it; it = it->next) {
                if (!hashcmp(it->pcache.sha1, tree_sha1)) {
                        free(data);
                        return;
                }
        }

        it = xcalloc(1, sizeof(*it));
        it->next = pbase_tree;
        pbase_tree = it;

        hashcpy(it->pcache.sha1, tree_sha1);
        it->pcache.tree_data = data;
        it->pcache.tree_size = size;
}

static void check_object(struct object_entry *entry)
{
        if (entry->in_pack) {
                struct packed_git *p = entry->in_pack;
                struct pack_window *w_curs = NULL;
                const unsigned char *base_ref = NULL;
                struct object_entry *base_entry;
                unsigned long used, used_0;
                unsigned int avail;
                off_t ofs;
                unsigned char *buf, c;

                buf = use_pack(p, &w_curs, entry->in_pack_offset, &avail);

                /*
                 * We want in_pack_type even if we do not reuse delta
                 * since non-delta representations could still be reused.
                 */
                used = unpack_object_header_gently(buf, avail,
                                                   &entry->in_pack_type,
                                                   &entry->size);

                /*
                 * Determine if this is a delta and if so whether we can
                 * reuse it or not.  Otherwise let's find out as cheaply as
                 * possible what the actual type and size for this object is.
                 */
                switch (entry->in_pack_type) {
                default:
                        /* Not a delta hence we've already got all we need. */
                        entry->type = entry->in_pack_type;
                        entry->in_pack_header_size = used;
                        unuse_pack(&w_curs);
                        return;
                case OBJ_REF_DELTA:
                        if (reuse_delta && !entry->preferred_base)
                                base_ref = use_pack(p, &w_curs,
                                                entry->in_pack_offset + used, NULL);
                        entry->in_pack_header_size = used + 20;
                        break;
                case OBJ_OFS_DELTA:
                        buf = use_pack(p, &w_curs,
                                       entry->in_pack_offset + used, NULL);
                        used_0 = 0;
                        c = buf[used_0++];
                        ofs = c & 127;
                        while (c & 128) {
                                ofs += 1;
                                if (!ofs || MSB(ofs, 7))
                                        die("delta base offset overflow in pack for %s",
                                            sha1_to_hex(entry->idx.sha1));
                                c = buf[used_0++];
                                ofs = (ofs << 7) + (c & 127);
                        }
                        if (ofs >= entry->in_pack_offset)
                                die("delta base offset out of bound for %s",
                                    sha1_to_hex(entry->idx.sha1));
                        ofs = entry->in_pack_offset - ofs;
                        if (reuse_delta && !entry->preferred_base) {
                                struct revindex_entry *revidx;
                                revidx = find_pack_revindex(p, ofs);
                                base_ref = nth_packed_object_sha1(p, revidx->nr);
                        }
                        entry->in_pack_header_size = used + used_0;
                        break;
                }

                if (base_ref && (base_entry = locate_object_entry(base_ref))) {
                        /*
                         * If base_ref was set above that means we wish to
                         * reuse delta data, and we even found that base
                         * in the list of objects we want to pack. Goodie!
                         *
                         * Depth value does not matter - find_deltas() will
                         * never consider reused delta as the base object to
                         * deltify other objects against, in order to avoid
                         * circular deltas.
                         */
                        entry->type = entry->in_pack_type;
                        entry->delta = base_entry;
                        entry->delta_sibling = base_entry->delta_child;
                        base_entry->delta_child = entry;
                        unuse_pack(&w_curs);
                        return;
                }

                if (entry->type) {
                        /*
                         * This must be a delta and we already know what the
                         * final object type is.  Let's extract the actual
                         * object size from the delta header.
                         */
                        entry->size = get_size_from_delta(p, &w_curs,
                                        entry->in_pack_offset + entry->in_pack_header_size);
                        unuse_pack(&w_curs);
                        return;
                }

                /*
                 * No choice but to fall back to the recursive delta walk
                 * with sha1_object_info() to find about the object type
                 * at this point...
                 */
                unuse_pack(&w_curs);
        }

        entry->type = sha1_object_info(entry->idx.sha1, &entry->size);
        /*
         * The error condition is checked in prepare_pack().  This is
         * to permit a missing preferred base object to be ignored
         * as a preferred base.  Doing so can result in a larger
         * pack file, but the transfer will still take place.
         */
}

static int pack_offset_sort(const void *_a, const void *_b)
{
        const struct object_entry *a = *(struct object_entry **)_a;
        const struct object_entry *b = *(struct object_entry **)_b;

        /* avoid filesystem trashing with loose objects */
        if (!a->in_pack && !b->in_pack)
                return hashcmp(a->idx.sha1, b->idx.sha1);

        if (a->in_pack < b->in_pack)
                return -1;
        if (a->in_pack > b->in_pack)
                return 1;
        return a->in_pack_offset < b->in_pack_offset ? -1 :
                        (a->in_pack_offset > b->in_pack_offset);
}

static void get_object_details(void)
{
        uint32_t i;
        struct object_entry **sorted_by_offset;

        sorted_by_offset = xcalloc(nr_objects, sizeof(struct object_entry *));
        for (i = 0; i < nr_objects; i++)
                sorted_by_offset[i] = objects + i;
        qsort(sorted_by_offset, nr_objects, sizeof(*sorted_by_offset), pack_offset_sort);

        for (i = 0; i < nr_objects; i++)
                check_object(sorted_by_offset[i]);

        free(sorted_by_offset);
}

/*
 * We search for deltas in a list sorted by type, by filename hash, and then
 * by size, so that we see progressively smaller and smaller files.
 * That's because we prefer deltas to be from the bigger file
 * to the smaller -- deletes are potentially cheaper, but perhaps
 * more importantly, the bigger file is likely the more recent
 * one.  The deepest deltas are therefore the oldest objects which are
 * less susceptible to be accessed often.
 */
static int type_size_sort(const void *_a, const void *_b)
{
        const struct object_entry *a = *(struct object_entry **)_a;
        const struct object_entry *b = *(struct object_entry **)_b;

        if (a->type > b->type)
                return -1;
        if (a->type < b->type)
                return 1;
        if (a->hash > b->hash)
                return -1;
        if (a->hash < b->hash)
                return 1;
        if (a->preferred_base > b->preferred_base)
                return -1;
        if (a->preferred_base < b->preferred_base)
                return 1;
        if (a->size > b->size)
                return -1;
        if (a->size < b->size)
                return 1;
        return a < b ? -1 : (a > b);  /* newest first */
}

struct unpacked {
        struct object_entry *entry;
        void *data;
        struct delta_index *index;
        unsigned depth;
};

static int delta_cacheable(unsigned long src_size, unsigned long trg_size,
                           unsigned long delta_size)
{
        if (max_delta_cache_size && delta_cache_size + delta_size > max_delta_cache_size)
                return 0;

        if (delta_size < cache_max_small_delta_size)
                return 1;

        /* cache delta, if objects are large enough compared to delta size */
        if ((src_size >> 20) + (trg_size >> 21) > (delta_size >> 10))
                return 1;

        return 0;
}

#ifdef THREADED_DELTA_SEARCH

static pthread_mutex_t read_mutex = PTHREAD_MUTEX_INITIALIZER;
#define read_lock()             pthread_mutex_lock(&read_mutex)
#define read_unlock()           pthread_mutex_unlock(&read_mutex)

static pthread_mutex_t cache_mutex = PTHREAD_MUTEX_INITIALIZER;
#define cache_lock()            pthread_mutex_lock(&cache_mutex)
#define cache_unlock()          pthread_mutex_unlock(&cache_mutex)

static pthread_mutex_t progress_mutex = PTHREAD_MUTEX_INITIALIZER;
#define progress_lock()         pthread_mutex_lock(&progress_mutex)
#define progress_unlock()       pthread_mutex_unlock(&progress_mutex)

#else

#define read_lock()             (void)0
#define read_unlock()           (void)0
#define cache_lock()            (void)0
#define cache_unlock()          (void)0
#define progress_lock()         (void)0
#define progress_unlock()       (void)0

#endif

static int try_delta(struct unpacked *trg, struct unpacked *src,
                     unsigned max_depth, unsigned long *mem_usage)
{
        struct object_entry *trg_entry = trg->entry;
        struct object_entry *src_entry = src->entry;
        unsigned long trg_size, src_size, delta_size, sizediff, max_size, sz;
        unsigned ref_depth;
        enum object_type type;
        void *delta_buf;

        /* Don't bother doing diffs between different types */
        if (trg_entry->type != src_entry->type)
                return -1;

        /*
         * We do not bother to try a delta that we discarded
         * on an earlier try, but only when reusing delta data.
         */
        if (reuse_delta && trg_entry->in_pack &&
            trg_entry->in_pack == src_entry->in_pack &&
            trg_entry->in_pack_type != OBJ_REF_DELTA &&
            trg_entry->in_pack_type != OBJ_OFS_DELTA)
                return 0;

        /* Let's not bust the allowed depth. */
        if (src->depth >= max_depth)
                return 0;

        /* Now some size filtering heuristics. */
        trg_size = trg_entry->size;
        if (!trg_entry->delta) {
                max_size = trg_size/2 - 20;
                ref_depth = 1;
        } else {
                max_size = trg_entry->delta_size;
                ref_depth = trg->depth;
        }
        max_size = max_size * (max_depth - src->depth) /
                                                (max_depth - ref_depth + 1);
        if (max_size == 0)
                return 0;
        src_size = src_entry->size;
        sizediff = src_size < trg_size ? trg_size - src_size : 0;
        if (sizediff >= max_size)
                return 0;
        if (trg_size < src_size / 32)
                return 0;

        /* Load data if not already done */
        if (!trg->data) {
                read_lock();
                trg->data = read_sha1_file(trg_entry->idx.sha1, &type, &sz);
                read_unlock();
                if (!trg->data)
                        die("object %s cannot be read",
                            sha1_to_hex(trg_entry->idx.sha1));
                if (sz != trg_size)
                        die("object %s inconsistent object length (%lu vs %lu)",
                            sha1_to_hex(trg_entry->idx.sha1), sz, trg_size);
                *mem_usage += sz;
        }
        if (!src->data) {
                read_lock();
                src->data = read_sha1_file(src_entry->idx.sha1, &type, &sz);
                read_unlock();
                if (!src->data)
                        die("object %s cannot be read",
                            sha1_to_hex(src_entry->idx.sha1));
                if (sz != src_size)
                        die("object %s inconsistent object length (%lu vs %lu)",
                            sha1_to_hex(src_entry->idx.sha1), sz, src_size);
                *mem_usage += sz;
        }
        if (!src->index) {
                src->index = create_delta_index(src->data, src_size);
                if (!src->index) {
                        static int warned = 0;
                        if (!warned++)
                                warning("suboptimal pack - out of memory");
                        return 0;
                }
                *mem_usage += sizeof_delta_index(src->index);
        }

        delta_buf = create_delta(src->index, trg->data, trg_size, &delta_size, max_size);
        if (!delta_buf)
                return 0;

        if (trg_entry->delta) {
                /* Prefer only shallower same-sized deltas. */
                if (delta_size == trg_entry->delta_size &&
                    src->depth + 1 >= trg->depth) {
                        free(delta_buf);
                        return 0;
                }
        }

        /*
         * Handle memory allocation outside of the cache
         * accounting lock.  Compiler will optimize the strangeness
         * away when THREADED_DELTA_SEARCH is not defined.
         */
        free(trg_entry->delta_data);
        cache_lock();
        if (trg_entry->delta_data) {
                delta_cache_size -= trg_entry->delta_size;
                trg_entry->delta_data = NULL;
        }
        if (delta_cacheable(src_size, trg_size, delta_size)) {
                delta_cache_size += delta_size;
                cache_unlock();
                trg_entry->delta_data = xrealloc(delta_buf, delta_size);
        } else {
                cache_unlock();
                free(delta_buf);
        }

        trg_entry->delta = src_entry;
        trg_entry->delta_size = delta_size;
        trg->depth = src->depth + 1;

        return 1;
}

static unsigned int check_delta_limit(struct object_entry *me, unsigned int n)
{
        struct object_entry *child = me->delta_child;
        unsigned int m = n;
        while (child) {
                unsigned int c = check_delta_limit(child, n + 1);
                if (m < c)
                        m = c;
                child = child->delta_sibling;
        }
        return m;
}

static unsigned long free_unpacked(struct unpacked *n)
{
        unsigned long freed_mem = sizeof_delta_index(n->index);
        free_delta_index(n->index);
        n->index = NULL;
        if (n->data) {
                freed_mem += n->entry->size;
                free(n->data);
                n->data = NULL;
        }
        n->entry = NULL;
        n->depth = 0;
        return freed_mem;
}

static void find_deltas(struct object_entry **list, unsigned *list_size,
                        int window, int depth, unsigned *processed)
{
        uint32_t i, idx = 0, count = 0;
        unsigned int array_size = window * sizeof(struct unpacked);
        struct unpacked *array;
        unsigned long mem_usage = 0;

        array = xmalloc(array_size);
        memset(array, 0, array_size);

        for (;;) {
                struct object_entry *entry;
                struct unpacked *n = array + idx;
                int j, max_depth, best_base = -1;

                progress_lock();
                if (!*list_size) {
                        progress_unlock();
                        break;
                }
                entry = *list++;
                (*list_size)--;
                if (!entry->preferred_base) {
                        (*processed)++;
                        display_progress(progress_state, *processed);
                }
                progress_unlock();

                mem_usage -= free_unpacked(n);
                n->entry = entry;

                while (window_memory_limit &&
                       mem_usage > window_memory_limit &&
                       count > 1) {
                        uint32_t tail = (idx + window - count) % window;
                        mem_usage -= free_unpacked(array + tail);
                        count--;
                }

                /* We do not compute delta to *create* objects we are not
                 * going to pack.
                 */
                if (entry->preferred_base)
                        goto next;

                /*
                 * If the current object is at pack edge, take the depth the
                 * objects that depend on the current object into account
                 * otherwise they would become too deep.
                 */
                max_depth = depth;
                if (entry->delta_child) {
                        max_depth -= check_delta_limit(entry, 0);
                        if (max_depth <= 0)
                                goto next;
                }

                j = window;
                while (--j > 0) {
                        int ret;
                        uint32_t other_idx = idx + j;
                        struct unpacked *m;
                        if (other_idx >= window)
                                other_idx -= window;
                        m = array + other_idx;
                        if (!m->entry)
                                break;
                        ret = try_delta(n, m, max_depth, &mem_usage);
                        if (ret < 0)
                                break;
                        else if (ret > 0)
                                best_base = other_idx;
                }

                /*
                 * If we decided to cache the delta data, then it is best
                 * to compress it right away.  First because we have to do
                 * it anyway, and doing it here while we're threaded will
                 * save a lot of time in the non threaded write phase,
                 * as well as allow for caching more deltas within
                 * the same cache size limit.
                 * ...
                 * But only if not writing to stdout, since in that case
                 * the network is most likely throttling writes anyway,
                 * and therefore it is best to go to the write phase ASAP
                 * instead, as we can afford spending more time compressing
                 * between writes at that moment.
                 */
                if (entry->delta_data && !pack_to_stdout) {
                        entry->z_delta_size = do_compress(&entry->delta_data,
                                                          entry->delta_size);
                        cache_lock();
                        delta_cache_size -= entry->delta_size;
                        delta_cache_size += entry->z_delta_size;
                        cache_unlock();
                }

                /* if we made n a delta, and if n is already at max
                 * depth, leaving it in the window is pointless.  we
                 * should evict it first.
                 */
                if (entry->delta && max_depth <= n->depth)
                        continue;

                /*
                 * Move the best delta base up in the window, after the
                 * currently deltified object, to keep it longer.  It will
                 * be the first base object to be attempted next.
                 */
                if (entry->delta) {
                        struct unpacked swap = array[best_base];
                        int dist = (window + idx - best_base) % window;
                        int dst = best_base;
                        while (dist--) {
                                int src = (dst + 1) % window;
                                array[dst] = array[src];
                                dst = src;
                        }
                        array[dst] = swap;
                }

                next:
                idx++;
                if (count + 1 < window)
                        count++;
                if (idx >= window)
                        idx = 0;
        }

        for (i = 0; i < window; ++i) {
                free_delta_index(array[i].index);
                free(array[i].data);
        }
        free(array);
}

#ifdef THREADED_DELTA_SEARCH

/*
 * The main thread waits on the condition that (at least) one of the workers
 * has stopped working (which is indicated in the .working member of
 * struct thread_params).
 * When a work thread has completed its work, it sets .working to 0 and
 * signals the main thread and waits on the condition that .data_ready
 * becomes 1.
 */

struct thread_params {
        pthread_t thread;
        struct object_entry **list;
        unsigned list_size;
        unsigned remaining;
        int window;
        int depth;
        int working;
        int data_ready;
        pthread_mutex_t mutex;
        pthread_cond_t cond;
        unsigned *processed;
};

static pthread_cond_t progress_cond = PTHREAD_COND_INITIALIZER;

static void *threaded_find_deltas(void *arg)
{
        struct thread_params *me = arg;

        while (me->remaining) {
                find_deltas(me->list, &me->remaining,
                            me->window, me->depth, me->processed);

                progress_lock();
                me->working = 0;
                pthread_cond_signal(&progress_cond);
                progress_unlock();

                /*
                 * We must not set ->data_ready before we wait on the
                 * condition because the main thread may have set it to 1
                 * before we get here. In order to be sure that new
                 * work is available if we see 1 in ->data_ready, it
                 * was initialized to 0 before this thread was spawned
                 * and we reset it to 0 right away.
                 */
                pthread_mutex_lock(&me->mutex);
                while (!me->data_ready)
                        pthread_cond_wait(&me->cond, &me->mutex);
                me->data_ready = 0;
                pthread_mutex_unlock(&me->mutex);
        }
        /* leave ->working 1 so that this doesn't get more work assigned */
        return NULL;
}

static void ll_find_deltas(struct object_entry **list, unsigned list_size,
                           int window, int depth, unsigned *processed)
{
        struct thread_params p[delta_search_threads];
        int i, ret, active_threads = 0;

        if (delta_search_threads <= 1) {
                find_deltas(list, &list_size, window, depth, processed);
                return;
        }

        /* Partition the work amongst work threads. */
        for (i = 0; i < delta_search_threads; i++) {
                unsigned sub_size = list_size / (delta_search_threads - i);

                p[i].window = window;
                p[i].depth = depth;
                p[i].processed = processed;
                p[i].working = 1;
                p[i].data_ready = 0;

                /* try to split chunks on "path" boundaries */
                while (sub_size && sub_size < list_size &&
                       list[sub_size]->hash &&
                       list[sub_size]->hash == list[sub_size-1]->hash)
                        sub_size++;

                p[i].list = list;
                p[i].list_size = sub_size;
                p[i].remaining = sub_size;

                list += sub_size;
                list_size -= sub_size;
        }

        /* Start work threads. */
        for (i = 0; i < delta_search_threads; i++) {
                if (!p[i].list_size)
                        continue;
                pthread_mutex_init(&p[i].mutex, NULL);
                pthread_cond_init(&p[i].cond, NULL);
                ret = pthread_create(&p[i].thread, NULL,
                                     threaded_find_deltas, &p[i]);
                if (ret)
                        die("unable to create thread: %s", strerror(ret));
                active_threads++;
        }

        /*
         * Now let's wait for work completion.  Each time a thread is done
         * with its work, we steal half of the remaining work from the
         * thread with the largest number of unprocessed objects and give
         * it to that newly idle thread.  This ensure good load balancing
         * until the remaining object list segments are simply too short
         * to be worth splitting anymore.
         */
        while (active_threads) {
                struct thread_params *target = NULL;
                struct thread_params *victim = NULL;
                unsigned sub_size = 0;

                progress_lock();
                for (;;) {
                        for (i = 0; !target && i < delta_search_threads; i++)
                                if (!p[i].working)
                                        target = &p[i];
                        if (target)
                                break;
                        pthread_cond_wait(&progress_cond, &progress_mutex);
                }

                for (i = 0; i < delta_search_threads; i++)
                        if (p[i].remaining > 2*window &&
                            (!victim || victim->remaining < p[i].remaining))
                                victim = &p[i];
                if (victim) {
                        sub_size = victim->remaining / 2;
                        list = victim->list + victim->list_size - sub_size;
                        while (sub_size && list[0]->hash &&
                               list[0]->hash == list[-1]->hash) {
                                list++;
                                sub_size--;
                        }
                        if (!sub_size) {
                                /*
                                 * It is possible for some "paths" to have
                                 * so many objects that no hash boundary
                                 * might be found.  Let's just steal the
                                 * exact half in that case.
                                 */
                                sub_size = victim->remaining / 2;
                                list -= sub_size;
                        }
                        target->list = list;
                        victim->list_size -= sub_size;
                        victim->remaining -= sub_size;
                }
                target->list_size = sub_size;
                target->remaining = sub_size;
                target->working = 1;
                progress_unlock();

                pthread_mutex_lock(&target->mutex);
                target->data_ready = 1;
                pthread_cond_signal(&target->cond);
                pthread_mutex_unlock(&target->mutex);

                if (!sub_size) {
                        pthread_join(target->thread, NULL);
                        pthread_cond_destroy(&target->cond);
                        pthread_mutex_destroy(&target->mutex);
                        active_threads--;
                }
        }
}

#else
#define ll_find_deltas(l, s, w, d, p)   find_deltas(l, &s, w, d, p)
#endif

static int add_ref_tag(const char *path, const unsigned char *sha1, int flag, void *cb_data)
{
        unsigned char peeled[20];

        if (!prefixcmp(path, "refs/tags/") && /* is a tag? */
            !peel_ref(path, peeled)        && /* peelable? */
            !is_null_sha1(peeled)          && /* annotated tag? */
            locate_object_entry(peeled))      /* object packed? */
                add_object_entry(sha1, OBJ_TAG, NULL, 0);
        return 0;
}

static void prepare_pack(int window, int depth)
{
        struct object_entry **delta_list;
        uint32_t i, nr_deltas;
        unsigned n;

        get_object_details();

        if (!nr_objects || !window || !depth)
                return;

        delta_list = xmalloc(nr_objects * sizeof(*delta_list));
        nr_deltas = n = 0;

        for (i = 0; i < nr_objects; i++) {
                struct object_entry *entry = objects + i;

                if (entry->delta)
                        /* This happens if we decided to reuse existing
                         * delta from a pack.  "reuse_delta &&" is implied.
                         */
                        continue;

                if (entry->size < 50)
                        continue;

                if (entry->no_try_delta)
                        continue;

                if (!entry->preferred_base) {
                        nr_deltas++;
                        if (entry->type < 0)
                                die("unable to get type of object %s",
                                    sha1_to_hex(entry->idx.sha1));
                }

                delta_list[n++] = entry;
        }

        if (nr_deltas && n > 1) {
                unsigned nr_done = 0;
                if (progress)
                        progress_state = start_progress("Compressing objects",
                                                        nr_deltas);
                qsort(delta_list, n, sizeof(*delta_list), type_size_sort);
                ll_find_deltas(delta_list, n, window+1, depth, &nr_done);
                stop_progress(&progress_state);
                if (nr_done != nr_deltas)
                        die("inconsistency with delta count");
        }
        free(delta_list);
}

static int git_pack_config(const char *k, const char *v, void *cb)
{
        if(!strcmp(k, "pack.window")) {
                window = git_config_int(k, v);
                return 0;
        }
        if (!strcmp(k, "pack.windowmemory")) {
                window_memory_limit = git_config_ulong(k, v);
                return 0;
        }
        if (!strcmp(k, "pack.depth")) {
                depth = git_config_int(k, v);
                return 0;
        }
        if (!strcmp(k, "pack.compression")) {
                int level = git_config_int(k, v);
                if (level == -1)
                        level = Z_DEFAULT_COMPRESSION;
                else if (level < 0 || level > Z_BEST_COMPRESSION)
                        die("bad pack compression level %d", level);
                pack_compression_level = level;
                pack_compression_seen = 1;
                return 0;
        }
        if (!strcmp(k, "pack.deltacachesize")) {
                max_delta_cache_size = git_config_int(k, v);
                return 0;
        }
        if (!strcmp(k, "pack.deltacachelimit")) {
                cache_max_small_delta_size = git_config_int(k, v);
                return 0;
        }
        if (!strcmp(k, "pack.threads")) {
                delta_search_threads = git_config_int(k, v);
                if (delta_search_threads < 0)
                        die("invalid number of threads specified (%d)",
                            delta_search_threads);
#ifndef THREADED_DELTA_SEARCH
                if (delta_search_threads != 1)
                        warning("no threads support, ignoring %s", k);
#endif
                return 0;
        }
        if (!strcmp(k, "pack.indexversion")) {
                pack_idx_default_version = git_config_int(k, v);
                if (pack_idx_default_version > 2)
                        die("bad pack.indexversion=%"PRIu32,
                                pack_idx_default_version);
                return 0;
        }
        if (!strcmp(k, "pack.packsizelimit")) {
                pack_size_limit_cfg = git_config_ulong(k, v);
                return 0;
        }
        return git_default_config(k, v, cb);
}

static void read_object_list_from_stdin(void)
{
        char line[40 + 1 + PATH_MAX + 2];
        unsigned char sha1[20];

        for (;;) {
                if (!fgets(line, sizeof(line), stdin)) {
                        if (feof(stdin))
                                break;
                        if (!ferror(stdin))
                                die("fgets returned NULL, not EOF, not error!");
                        if (errno != EINTR)
                                die("fgets: %s", strerror(errno));
                        clearerr(stdin);
                        continue;
                }
                if (line[0] == '-') {
                        if (get_sha1_hex(line+1, sha1))
                                die("expected edge sha1, got garbage:\n %s",
                                    line);
                        add_preferred_base(sha1);
                        continue;
                }
                if (get_sha1_hex(line, sha1))
                        die("expected sha1, got garbage:\n %s", line);

                add_preferred_base_object(line+41);
                add_object_entry(sha1, 0, line+41, 0);
        }
}

#define OBJECT_ADDED (1u<<20)

static void show_commit(struct commit *commit)
{
        add_object_entry(commit->object.sha1, OBJ_COMMIT, NULL, 0);
        commit->object.flags |= OBJECT_ADDED;
}

static void show_object(struct object_array_entry *p)
{
        add_preferred_base_object(p->name);
        add_object_entry(p->item->sha1, p->item->type, p->name, 0);
        p->item->flags |= OBJECT_ADDED;
}

static void show_edge(struct commit *commit)
{
        add_preferred_base(commit->object.sha1);
}

struct in_pack_object {
        off_t offset;
        struct object *object;
};

struct in_pack {
        int alloc;
        int nr;
        struct in_pack_object *array;
};

static void mark_in_pack_object(struct object *object, struct packed_git *p, struct in_pack *in_pack)
{
        in_pack->array[in_pack->nr].offset = find_pack_entry_one(object->sha1, p);
        in_pack->array[in_pack->nr].object = object;
        in_pack->nr++;
}

/*
 * Compare the objects in the offset order, in order to emulate the
 * "git rev-list --objects" output that produced the pack originally.
 */
static int ofscmp(const void *a_, const void *b_)
{
        struct in_pack_object *a = (struct in_pack_object *)a_;
        struct in_pack_object *b = (struct in_pack_object *)b_;

        if (a->offset < b->offset)
                return -1;
        else if (a->offset > b->offset)
                return 1;
        else
                return hashcmp(a->object->sha1, b->object->sha1);
}

static void add_objects_in_unpacked_packs(struct rev_info *revs)
{
        struct packed_git *p;
        struct in_pack in_pack;
        uint32_t i;

        memset(&in_pack, 0, sizeof(in_pack));

        for (p = packed_git; p; p = p->next) {
                const unsigned char *sha1;
                struct object *o;

                for (i = 0; i < revs->num_ignore_packed; i++) {
                        if (matches_pack_name(p, revs->ignore_packed[i]))
                                break;
                }
                if (revs->num_ignore_packed <= i)
                        continue;
                if (open_pack_index(p))
                        die("cannot open pack index");

                ALLOC_GROW(in_pack.array,
                           in_pack.nr + p->num_objects,
                           in_pack.alloc);

                for (i = 0; i < p->num_objects; i++) {
                        sha1 = nth_packed_object_sha1(p, i);
                        o = lookup_unknown_object(sha1);
                        if (!(o->flags & OBJECT_ADDED))
                                mark_in_pack_object(o, p, &in_pack);
                        o->flags |= OBJECT_ADDED;
                }
        }

        if (in_pack.nr) {
                qsort(in_pack.array, in_pack.nr, sizeof(in_pack.array[0]),
                      ofscmp);
                for (i = 0; i < in_pack.nr; i++) {
                        struct object *o = in_pack.array[i].object;
                        add_object_entry(o->sha1, o->type, "", 0);
                }
        }
        free(in_pack.array);
}

static void loosen_unused_packed_objects(struct rev_info *revs)
{
        struct packed_git *p;
        uint32_t i;
        const unsigned char *sha1;

        for (p = packed_git; p; p = p->next) {
                for (i = 0; i < revs->num_ignore_packed; i++) {
                        if (matches_pack_name(p, revs->ignore_packed[i]))
                                break;
                }
                if (revs->num_ignore_packed <= i)
                        continue;

                if (open_pack_index(p))
                        die("cannot open pack index");

                for (i = 0; i < p->num_objects; i++) {
                        sha1 = nth_packed_object_sha1(p, i);
                        if (!locate_object_entry(sha1))
                                if (force_object_loose(sha1, p->mtime))
                                        die("unable to force loose object");
                }
        }
}

static void get_object_list(int ac, const char **av)
{
        struct rev_info revs;
        char line[1000];
        int flags = 0;

        init_revisions(&revs, NULL);
        save_commit_buffer = 0;
        setup_revisions(ac, av, &revs, NULL);

        while (fgets(line, sizeof(line), stdin) != NULL) {
                int len = strlen(line);
                if (len && line[len - 1] == '\n')
                        line[--len] = 0;
                if (!len)
                        break;
                if (*line == '-') {
                        if (!strcmp(line, "--not")) {
                                flags ^= UNINTERESTING;
                                continue;
                        }
                        die("not a rev '%s'", line);
                }
                if (handle_revision_arg(line, &revs, flags, 1))
                        die("bad revision '%s'", line);
        }

        if (prepare_revision_walk(&revs))
                die("revision walk setup failed");
        mark_edges_uninteresting(revs.commits, &revs, show_edge);
        traverse_commit_list(&revs, show_commit, show_object);

        if (keep_unreachable)
                add_objects_in_unpacked_packs(&revs);
        if (unpack_unreachable)
                loosen_unused_packed_objects(&revs);
}

static int adjust_perm(const char *path, mode_t mode)
{
        if (chmod(path, mode))
                return -1;
        return adjust_shared_perm(path);
}

int cmd_pack_objects(int argc, const char **argv, const char *prefix)
{
        int use_internal_rev_list = 0;
        int thin = 0;
        uint32_t i;
        const char **rp_av;
        int rp_ac_alloc = 64;
        int rp_ac;

        rp_av = xcalloc(rp_ac_alloc, sizeof(*rp_av));

        rp_av[0] = "pack-objects";
        rp_av[1] = "--objects"; /* --thin will make it --objects-edge */
        rp_ac = 2;

        git_config(git_pack_config, NULL);
        if (!pack_compression_seen && core_compression_seen)
                pack_compression_level = core_compression_level;

        progress = isatty(2);
        for (i = 1; i < argc; i++) {
                const char *arg = argv[i];

                if (*arg != '-')
                        break;

                if (!strcmp("--non-empty", arg)) {
                        non_empty = 1;
                        continue;
                }
                if (!strcmp("--local", arg)) {
                        local = 1;
                        continue;
                }
                if (!strcmp("--incremental", arg)) {
                        incremental = 1;
                        continue;
                }
                if (!strcmp("--honor-pack-keep", arg)) {
                        ignore_packed_keep = 1;
                        continue;
                }
                if (!prefixcmp(arg, "--compression=")) {
                        char *end;
                        int level = strtoul(arg+14, &end, 0);
                        if (!arg[14] || *end)
                                usage(pack_usage);
                        if (level == -1)
                                level = Z_DEFAULT_COMPRESSION;
                        else if (level < 0 || level > Z_BEST_COMPRESSION)
                                die("bad pack compression level %d", level);
                        pack_compression_level = level;
                        continue;
                }
                if (!prefixcmp(arg, "--max-pack-size=")) {
                        char *end;
                        pack_size_limit_cfg = 0;
                        pack_size_limit = strtoul(arg+16, &end, 0) * 1024 * 1024;
                        if (!arg[16] || *end)
                                usage(pack_usage);
                        continue;
                }
                if (!prefixcmp(arg, "--window=")) {
                        char *end;
                        window = strtoul(arg+9, &end, 0);
                        if (!arg[9] || *end)
                                usage(pack_usage);
                        continue;
                }
                if (!prefixcmp(arg, "--window-memory=")) {
                        if (!git_parse_ulong(arg+16, &window_memory_limit))
                                usage(pack_usage);
                        continue;
                }
                if (!prefixcmp(arg, "--threads=")) {
                        char *end;
                        delta_search_threads = strtoul(arg+10, &end, 0);
                        if (!arg[10] || *end || delta_search_threads < 0)
                                usage(pack_usage);
#ifndef THREADED_DELTA_SEARCH
                        if (delta_search_threads != 1)
                                warning("no threads support, "
                                        "ignoring %s", arg);
#endif
                        continue;
                }
                if (!prefixcmp(arg, "--depth=")) {
                        char *end;
                        depth = strtoul(arg+8, &end, 0);
                        if (!arg[8] || *end)
                                usage(pack_usage);
                        continue;
                }
                if (!strcmp("--progress", arg)) {
                        progress = 1;
                        continue;
                }
                if (!strcmp("--all-progress", arg)) {
                        progress = 2;
                        continue;
                }
                if (!strcmp("-q", arg)) {
                        progress = 0;
                        continue;
                }
                if (!strcmp("--no-reuse-delta", arg)) {
                        reuse_delta = 0;
                        continue;
                }
                if (!strcmp("--no-reuse-object", arg)) {
                        reuse_object = reuse_delta = 0;
                        continue;
                }
                if (!strcmp("--delta-base-offset", arg)) {
                        allow_ofs_delta = 1;
                        continue;
                }
                if (!strcmp("--stdout", arg)) {
                        pack_to_stdout = 1;
                        continue;
                }
                if (!strcmp("--revs", arg)) {
                        use_internal_rev_list = 1;
                        continue;
                }
                if (!strcmp("--keep-unreachable", arg)) {
                        keep_unreachable = 1;
                        continue;
                }
                if (!strcmp("--unpack-unreachable", arg)) {
                        unpack_unreachable = 1;
                        continue;
                }
                if (!strcmp("--include-tag", arg)) {
                        include_tag = 1;
                        continue;
                }
                if (!strcmp("--unpacked", arg) ||
                    !prefixcmp(arg, "--unpacked=") ||
                    !strcmp("--reflog", arg) ||
                    !strcmp("--all", arg)) {
                        use_internal_rev_list = 1;
                        if (rp_ac >= rp_ac_alloc - 1) {
                                rp_ac_alloc = alloc_nr(rp_ac_alloc);
                                rp_av = xrealloc(rp_av,
                                                 rp_ac_alloc * sizeof(*rp_av));
                        }
                        rp_av[rp_ac++] = arg;
                        continue;
                }
                if (!strcmp("--thin", arg)) {
                        use_internal_rev_list = 1;
                        thin = 1;
                        rp_av[1] = "--objects-edge";
                        continue;
                }
                if (!prefixcmp(arg, "--index-version=")) {
                        char *c;
                        pack_idx_default_version = strtoul(arg + 16, &c, 10);
                        if (pack_idx_default_version > 2)
                                die("bad %s", arg);
                        if (*c == ',')
                                pack_idx_off32_limit = strtoul(c+1, &c, 0);
                        if (*c || pack_idx_off32_limit & 0x80000000)
                                die("bad %s", arg);
                        continue;
                }
                usage(pack_usage);
        }

        /* Traditionally "pack-objects [options] base extra" failed;
         * we would however want to take refs parameter that would
         * have been given to upstream rev-list ourselves, which means
         * we somehow want to say what the base name is.  So the
         * syntax would be:
         *
         * pack-objects [options] base <refs...>
         *
         * in other words, we would treat the first non-option as the
         * base_name and send everything else to the internal revision
         * walker.
         */

        if (!pack_to_stdout)
                base_name = argv[i++];

        if (pack_to_stdout != !base_name)
                usage(pack_usage);

        if (!pack_to_stdout && !pack_size_limit)
                pack_size_limit = pack_size_limit_cfg;

        if (pack_to_stdout && pack_size_limit)
                die("--max-pack-size cannot be used to build a pack for transfer.");

        if (!pack_to_stdout && thin)
                die("--thin cannot be used to build an indexable pack.");

        if (keep_unreachable && unpack_unreachable)
                die("--keep-unreachable and --unpack-unreachable are incompatible.");

#ifdef THREADED_DELTA_SEARCH
        if (!delta_search_threads)      /* --threads=0 means autodetect */
                delta_search_threads = online_cpus();
#endif

        prepare_packed_git();

        if (progress)
                progress_state = start_progress("Counting objects", 0);
        if (!use_internal_rev_list)
                read_object_list_from_stdin();
        else {
                rp_av[rp_ac] = NULL;
                get_object_list(rp_ac, rp_av);
        }
        if (include_tag && nr_result)
                for_each_ref(add_ref_tag, NULL);
        stop_progress(&progress_state);

        if (non_empty && !nr_result)
                return 0;
        if (nr_result)
                prepare_pack(window, depth);
        write_pack_file();
        if (progress)
                fprintf(stderr, "Total %"PRIu32" (delta %"PRIu32"),"
                        " reused %"PRIu32" (delta %"PRIu32")\n",
                        written, written_delta, reused, reused_delta);
        return 0;
}