[PATCH] Simplify "reverse-diff" logic in the diff core.

[git/debian.git] / diff.c

/*
 * Copyright (C) 2005 Junio C Hamano
 */
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
#include <limits.h>
#include "cache.h"
#include "diff.h"
#include "delta.h"

static const char *diff_opts = "-pu";
static unsigned char null_sha1[20] = { 0, };
#define MAX_SCORE 10000
#define DEFAULT_MINIMUM_SCORE 5000

static const char *external_diff(void)
{
        static const char *external_diff_cmd = NULL;
        static int done_preparing = 0;

        if (done_preparing)
                return external_diff_cmd;

        /*
         * Default values above are meant to match the
         * Linux kernel development style.  Examples of
         * alternative styles you can specify via environment
         * variables are:
         *
         * GIT_DIFF_OPTS="-c";
         */
        if (gitenv("GIT_EXTERNAL_DIFF"))
                external_diff_cmd = gitenv("GIT_EXTERNAL_DIFF");

        /* In case external diff fails... */
        diff_opts = gitenv("GIT_DIFF_OPTS") ? : diff_opts;

        done_preparing = 1;
        return external_diff_cmd;
}

/* Help to copy the thing properly quoted for the shell safety.
 * any single quote is replaced with '\'', and the caller is
 * expected to enclose the result within a single quote pair.
 *
 * E.g.
 *  original     sq_expand     result
 *  name     ==> name      ==> 'name'
 *  a b      ==> a b       ==> 'a b'
 *  a'b      ==> a'\''b    ==> 'a'\''b'
 */
static char *sq_expand(const char *src)
{
        static char *buf = NULL;
        int cnt, c;
        const char *cp;
        char *bp;

        /* count bytes needed to store the quoted string. */
        for (cnt = 1, cp = src; *cp; cnt++, cp++)
                if (*cp == '\'')
                        cnt += 3;

        buf = xmalloc(cnt);
        bp = buf;
        while ((c = *src++)) {
                if (c != '\'')
                        *bp++ = c;
                else {
                        bp = strcpy(bp, "'\\''");
                        bp += 4;
                }
        }
        *bp = 0;
        return buf;
}

static struct diff_tempfile {
        const char *name;
        char hex[41];
        char mode[10];
        char tmp_path[50];
} diff_temp[2];

struct diff_spec {
        unsigned char blob_sha1[20];
        unsigned short mode;     /* file mode */
        unsigned sha1_valid : 1; /* if true, use blob_sha1 and trust mode;
                                  * if false, use the name and read from
                                  * the filesystem.
                                  */
        unsigned file_valid : 1; /* if false the file does not exist */
};

static void builtin_diff(const char *name_a,
                         const char *name_b,
                         struct diff_tempfile *temp,
                         int rename_score)
{
        int i, next_at, cmd_size;
        const char *diff_cmd = "diff -L'%s%s' -L'%s%s'";
        const char *diff_arg  = "'%s' '%s'||:"; /* "||:" is to return 0 */
        const char *input_name_sq[2];
        const char *path0[2];
        const char *path1[2];
        const char *name_sq[2];
        char *cmd;

        name_sq[0] = sq_expand(name_a);
        name_sq[1] = sq_expand(name_b);

        /* diff_cmd and diff_arg have 6 %s in total which makes
         * the sum of these strings 12 bytes larger than required.
         * we use 2 spaces around diff-opts, and we need to count
         * terminating NUL, so we subtract 9 here.
         */
        cmd_size = (strlen(diff_cmd) + strlen(diff_opts) +
                        strlen(diff_arg) - 9);
        for (i = 0; i < 2; i++) {
                input_name_sq[i] = sq_expand(temp[i].name);
                if (!strcmp(temp[i].name, "/dev/null")) {
                        path0[i] = "/dev/null";
                        path1[i] = "";
                } else {
                        path0[i] = i ? "b/" : "a/";
                        path1[i] = name_sq[i];
                }
                cmd_size += (strlen(path0[i]) + strlen(path1[i]) +
                             strlen(input_name_sq[i]));
        }

        cmd = xmalloc(cmd_size);

        next_at = 0;
        next_at += snprintf(cmd+next_at, cmd_size-next_at,
                            diff_cmd,
                            path0[0], path1[0], path0[1], path1[1]);
        next_at += snprintf(cmd+next_at, cmd_size-next_at,
                            " %s ", diff_opts);
        next_at += snprintf(cmd+next_at, cmd_size-next_at,
                            diff_arg, input_name_sq[0], input_name_sq[1]);

        printf("diff --git a/%s b/%s\n", name_a, name_b);
        if (!path1[0][0])
                printf("new file mode %s\n", temp[1].mode);
        else if (!path1[1][0])
                printf("deleted file mode %s\n", temp[0].mode);
        else {
                if (strcmp(temp[0].mode, temp[1].mode)) {
                        printf("old mode %s\n", temp[0].mode);
                        printf("new mode %s\n", temp[1].mode);
                }
                if (strcmp(name_a, name_b)) {
                        if (0 < rename_score)
                                printf("rename similarity index %d%%\n",
                                       (int)(0.5+
                                             rename_score*100.0/MAX_SCORE));
                        printf("rename old %s\n", name_a);
                        printf("rename new %s\n", name_b);
                }
                if (strncmp(temp[0].mode, temp[1].mode, 3))
                        /* we do not run diff between different kind
                         * of objects.
                         */
                        exit(0);
        }
        fflush(NULL);
        execlp("/bin/sh","sh", "-c", cmd, NULL);
}

/*
 * Given a name and sha1 pair, if the dircache tells us the file in
 * the work tree has that object contents, return true, so that
 * prepare_temp_file() does not have to inflate and extract.
 */
static int work_tree_matches(const char *name, const unsigned char *sha1)
{
        struct cache_entry *ce;
        struct stat st;
        int pos, len;

        /* We do not read the cache ourselves here, because the
         * benchmark with my previous version that always reads cache
         * shows that it makes things worse for diff-tree comparing
         * two linux-2.6 kernel trees in an already checked out work
         * tree.  This is because most diff-tree comparisons deal with
         * only a small number of files, while reading the cache is
         * expensive for a large project, and its cost outweighs the
         * savings we get by not inflating the object to a temporary
         * file.  Practically, this code only helps when we are used
         * by diff-cache --cached, which does read the cache before
         * calling us.
         */
        if (!active_cache)
                return 0;

        len = strlen(name);
        pos = cache_name_pos(name, len);
        if (pos < 0)
                return 0;
        ce = active_cache[pos];
        if ((lstat(name, &st) < 0) ||
            !S_ISREG(st.st_mode) ||
            ce_match_stat(ce, &st) ||
            memcmp(sha1, ce->sha1, 20))
                return 0;
        return 1;
}

static void prep_temp_blob(struct diff_tempfile *temp,
                           void *blob,
                           unsigned long size,
                           unsigned char *sha1,
                           int mode)
{
        int fd;

        strcpy(temp->tmp_path, ".diff_XXXXXX");
        fd = mkstemp(temp->tmp_path);
        if (fd < 0)
                die("unable to create temp-file");
        if (write(fd, blob, size) != size)
                die("unable to write temp-file");
        close(fd);
        temp->name = temp->tmp_path;
        strcpy(temp->hex, sha1_to_hex(sha1));
        temp->hex[40] = 0;
        sprintf(temp->mode, "%06o", mode);
}

static void prepare_temp_file(const char *name,
                              struct diff_tempfile *temp,
                              struct diff_spec *one)
{
        if (!one->file_valid) {
        not_a_valid_file:
                /* A '-' entry produces this for file-2, and
                 * a '+' entry produces this for file-1.
                 */
                temp->name = "/dev/null";
                strcpy(temp->hex, ".");
                strcpy(temp->mode, ".");
                return;
        }

        if (!one->sha1_valid ||
            work_tree_matches(name, one->blob_sha1)) {
                struct stat st;
                temp->name = name;
                if (lstat(temp->name, &st) < 0) {
                        if (errno == ENOENT)
                                goto not_a_valid_file;
                        die("stat(%s): %s", temp->name, strerror(errno));
                }
                if (S_ISLNK(st.st_mode)) {
                        int ret;
                        char *buf, buf_[1024];
                        buf = ((sizeof(buf_) < st.st_size) ?
                               xmalloc(st.st_size) : buf_);
                        ret = readlink(name, buf, st.st_size);
                        if (ret < 0)
                                die("readlink(%s)", name);
                        prep_temp_blob(temp, buf, st.st_size,
                                       (one->sha1_valid ?
                                        one->blob_sha1 : null_sha1),
                                       (one->sha1_valid ?
                                        one->mode : S_IFLNK));
                }
                else {
                        if (!one->sha1_valid)
                                strcpy(temp->hex, sha1_to_hex(null_sha1));
                        else
                                strcpy(temp->hex, sha1_to_hex(one->blob_sha1));
                        sprintf(temp->mode, "%06o",
                                S_IFREG |ce_permissions(st.st_mode));
                }
                return;
        }
        else {
                void *blob;
                char type[20];
                unsigned long size;

                blob = read_sha1_file(one->blob_sha1, type, &size);
                if (!blob || strcmp(type, "blob"))
                        die("unable to read blob object for %s (%s)",
                            name, sha1_to_hex(one->blob_sha1));
                prep_temp_blob(temp, blob, size, one->blob_sha1, one->mode);
                free(blob);
        }
}

static void remove_tempfile(void)
{
        int i;

        for (i = 0; i < 2; i++)
                if (diff_temp[i].name == diff_temp[i].tmp_path) {
                        unlink(diff_temp[i].name);
                        diff_temp[i].name = NULL;
                }
}

static void remove_tempfile_on_signal(int signo)
{
        remove_tempfile();
}

static int detect_rename;
static int reverse_diff;
static int diff_raw_output = -1;
static const char **pathspec;
static int speccnt;
static int minimum_score;

static int matches_pathspec(const char *name)
{
        int i;
        int namelen;

        if (speccnt == 0)
                return 1;

        namelen = strlen(name);
        for (i = 0; i < speccnt; i++) {
                int speclen = strlen(pathspec[i]);
                if (! strncmp(pathspec[i], name, speclen) &&
                    speclen <= namelen &&
                    (name[speclen] == 0 || name[speclen] == '/'))
                        return 1;
        }
        return 0;
}

/* An external diff command takes:
 *
 * diff-cmd name infile1 infile1-sha1 infile1-mode \
 *               infile2 infile2-sha1 infile2-mode [ rename-to ]
 *
 */
static void run_external_diff(const char *name,
                              const char *other,
                              struct diff_spec *one,
                              struct diff_spec *two,
                              int rename_score)
{
        struct diff_tempfile *temp = diff_temp;
        pid_t pid;
        int status;
        static int atexit_asked = 0;

        if (!matches_pathspec(name) && (!other || !matches_pathspec(other)))
                return;

        if (one && two) {
                prepare_temp_file(name, &temp[0], one);
                prepare_temp_file(other ? : name, &temp[1], two);
                if (! atexit_asked &&
                    (temp[0].name == temp[0].tmp_path ||
                     temp[1].name == temp[1].tmp_path)) {
                        atexit_asked = 1;
                        atexit(remove_tempfile);
                }
                signal(SIGINT, remove_tempfile_on_signal);
        }

        fflush(NULL);
        pid = fork();
        if (pid < 0)
                die("unable to fork");
        if (!pid) {
                const char *pgm = external_diff();
                if (pgm) {
                        if (one && two) {
                                const char *exec_arg[9];
                                const char **arg = &exec_arg[0];
                                *arg++ = pgm;
                                *arg++ = name;
                                *arg++ = temp[0].name;
                                *arg++ = temp[0].hex;
                                *arg++ = temp[0].mode;
                                *arg++ = temp[1].name;
                                *arg++ = temp[1].hex;
                                *arg++ = temp[1].mode;
                                if (other)
                                        *arg++ = other;
                                *arg = 0;
                                execvp(pgm, (char *const*) exec_arg);
                        }
                        else
                                execlp(pgm, pgm, name, NULL);
                }
                /*
                 * otherwise we use the built-in one.
                 */
                if (one && two)
                        builtin_diff(name, other ? : name, temp, rename_score);
                else
                        printf("* Unmerged path %s\n", name);
                exit(0);
        }
        if (waitpid(pid, &status, 0) < 0 ||
            !WIFEXITED(status) || WEXITSTATUS(status)) {
                /* Earlier we did not check the exit status because
                 * diff exits non-zero if files are different, and
                 * we are not interested in knowing that.  It was a
                 * mistake which made it harder to quit a diff-*
                 * session that uses the git-apply-patch-script as
                 * the GIT_EXTERNAL_DIFF.  A custom GIT_EXTERNAL_DIFF
                 * should also exit non-zero only when it wants to
                 * abort the entire diff-* session.
                 */
                remove_tempfile();
                fprintf(stderr, "external diff died, stopping at %s.\n", name);
                exit(1);
        }
        remove_tempfile();
}

/*
 * We do not detect circular renames.  Just hold created and deleted
 * entries and later attempt to match them up.  If they do not match,
 * then spit them out as deletes or creates as original.
 */

static struct diff_spec_hold {
        struct diff_spec_hold *next;
        struct diff_spec it;
        unsigned long size;
        int flags;
#define MATCHED 1
#define SHOULD_FREE 2
#define SHOULD_MUNMAP 4
        void *data;
        char path[1];
} *createdfile, *deletedfile;

static void hold_diff(const char *name,
                      struct diff_spec *one,
                      struct diff_spec *two)
{
        struct diff_spec_hold **list, *elem;

        if (one->file_valid && two->file_valid)
                die("internal error");

        if (!detect_rename) {
                run_external_diff(name, NULL, one, two, -1);
                return;
        }
        elem = xmalloc(sizeof(*elem) + strlen(name));
        strcpy(elem->path, name);
        elem->size = 0;
        elem->data = NULL;
        elem->flags = 0;
        if (one->file_valid) {
                list = &deletedfile;
                elem->it = *one;
        }
        else {
                list = &createdfile;
                elem->it = *two;
        }
        elem->next = *list;
        *list = elem;
}

static int populate_data(struct diff_spec_hold *s)
{
        char type[20];

        if (s->data)
                return 0;
        if (s->it.sha1_valid) {
                s->data = read_sha1_file(s->it.blob_sha1, type, &s->size);
                s->flags |= SHOULD_FREE;
        }
        else {
                struct stat st;
                int fd;
                fd = open(s->path, O_RDONLY);
                if (fd < 0)
                        return -1;
                if (fstat(fd, &st)) {
                        close(fd);
                        return -1;
                }
                s->size = st.st_size;
                s->data = mmap(NULL, s->size, PROT_READ, MAP_PRIVATE, fd, 0);
                close(fd);
                if (!s->size)
                        s->data = "";
                else
                        s->flags |= SHOULD_MUNMAP;
        }
        return 0;
}

static void free_data(struct diff_spec_hold *s)
{
        if (s->flags & SHOULD_FREE)
                free(s->data);
        else if (s->flags & SHOULD_MUNMAP)
                munmap(s->data, s->size);
        s->flags &= ~(SHOULD_FREE|SHOULD_MUNMAP);
        s->data = 0;
}

static void flush_remaining_diff(struct diff_spec_hold *elem,
                                 int on_created_list)
{
        static struct diff_spec null_file_spec;

        null_file_spec.file_valid = 0;
        for ( ; elem ; elem = elem->next) {
                free_data(elem);
                if (elem->flags & MATCHED)
                        continue;
                if (on_created_list)
                        run_external_diff(elem->path, NULL,
                                          &null_file_spec, &elem->it, -1);
                else
                        run_external_diff(elem->path, NULL,
                                          &elem->it, &null_file_spec, -1);
        }
}

static int is_exact_match(struct diff_spec_hold *src,
                          struct diff_spec_hold *dst)
{
        if (src->it.sha1_valid && dst->it.sha1_valid &&
            !memcmp(src->it.blob_sha1, dst->it.blob_sha1, 20))
                return 1;
        if (populate_data(src) || populate_data(dst))
                /* this is an error but will be caught downstream */
                return 0;
        if (src->size == dst->size &&
            !memcmp(src->data, dst->data, src->size))
                return 1;
        return 0;
}

int estimate_similarity(struct diff_spec_hold *src, struct diff_spec_hold *dst)
{
        /* src points at a deleted file and dst points at a created
         * file.  They may be quite similar, in which case we want to
         * say src is renamed to dst.
         *
         * Compare them and return how similar they are, representing
         * the score as an integer between 0 and 10000, except
         * where they match exactly it is considered better than anything
         * else.
         */
        void *delta;
        unsigned long delta_size;
        int score;

        delta_size = ((src->size < dst->size) ?
                      (dst->size - src->size) : (src->size - dst->size));

        /* We would not consider rename followed by more than
         * minimum_score/MAX_SCORE edits; that is, delta_size must be smaller
         * than (src->size + dst->size)/2 * minimum_score/MAX_SCORE,
         * which means...
         */

        if ((src->size+dst->size)*minimum_score < delta_size*MAX_SCORE*2)
                return 0;

        delta = diff_delta(src->data, src->size,
                           dst->data, dst->size,
                           &delta_size);
        free(delta);

        /* This "delta" is really xdiff with adler32 and all the
         * overheads but it is a quick and dirty approximation.
         *
         * Now we will give some score to it.  100% edit gets
         * 0 points and 0% edit gets MAX_SCORE points.  That is, every
         * 1/MAX_SCORE edit gets 1 point penalty.  The amount of penalty is:
         *
         * (delta_size * 2 / (src->size + dst->size)) * MAX_SCORE
         *
         */
        score = MAX_SCORE-(MAX_SCORE*2*delta_size/(src->size+dst->size));
        if (score < 0) return 0;
        if (MAX_SCORE < score) return MAX_SCORE;
        return score;
}

struct diff_score {
        struct diff_spec_hold *src;
        struct diff_spec_hold *dst;
        int score;
};

static int score_compare(const void *a_, const void *b_)
{
        const struct diff_score *a = a_, *b = b_;
        return b->score - a->score;
}

static void flush_rename_pair(struct diff_spec_hold *src,
                              struct diff_spec_hold *dst,
                              int rename_score)
{
        src->flags |= MATCHED;
        dst->flags |= MATCHED;
        free_data(src);
        free_data(dst);
        run_external_diff(src->path, dst->path,
                          &src->it, &dst->it, rename_score);
}

static void free_held_diff(struct diff_spec_hold *list)
{
        struct diff_spec_hold *h;
        for (h = list; list; list = h) {
                h = list->next;
                free_data(list);
                free(list);
        }
}

void diff_flush(void)
{
        int num_create, num_delete, c, d;
        struct diff_spec_hold *elem, *src, *dst;
        struct diff_score *mx;

        /* We really want to cull the candidates list early
         * with cheap tests in order to avoid doing deltas.
         *
         * With the current callers, we should not have already
         * matched entries at this point, but it is nonetheless
         * checked for sanity.
         */
        for (dst = createdfile; dst; dst = dst->next) {
                if (dst->flags & MATCHED)
                        continue;
                for (src = deletedfile; src; src = src->next) {
                        if (src->flags & MATCHED)
                                continue;
                        if (! is_exact_match(src, dst))
                                continue;
                        flush_rename_pair(src, dst, MAX_SCORE);
                        break;
                }
        }

        /* Count surviving candidates */
        for (num_create = 0, elem = createdfile; elem; elem = elem->next)
                if (!(elem->flags & MATCHED))
                        num_create++;

        for (num_delete = 0, elem = deletedfile; elem; elem = elem->next)
                if (!(elem->flags & MATCHED))
                        num_delete++;

        if (num_create == 0 ||  num_delete == 0)
                goto exit_path;

        mx = xmalloc(sizeof(*mx) * num_create * num_delete);
        for (c = 0, dst = createdfile; dst; dst = dst->next) {
                int base = c * num_delete;
                if (dst->flags & MATCHED)
                        continue;
                for (d = 0, src = deletedfile; src; src = src->next) {
                        struct diff_score *m = &mx[base+d];
                        if (src->flags & MATCHED)
                                continue;
                        m->src = src;
                        m->dst = dst;
                        m->score = estimate_similarity(src, dst);
                        d++;
                }
                c++;
        }
        qsort(mx, num_create*num_delete, sizeof(*mx), score_compare);

#if 0
        for (c = 0; c < num_create * num_delete; c++) {
                src = mx[c].src;
                dst = mx[c].dst;
                if ((src->flags & MATCHED) || (dst->flags & MATCHED))
                        continue;
                fprintf(stderr,
                        "**score ** %d %s %s\n",
                        mx[c].score, src->path, dst->path);
        }
#endif

        for (c = 0; c < num_create * num_delete; c++) {
                src = mx[c].src;
                dst = mx[c].dst;
                if ((src->flags & MATCHED) || (dst->flags & MATCHED))
                        continue;
                if (mx[c].score < minimum_score)
                        break;
                flush_rename_pair(src, dst, mx[c].score);
        }
        free(mx);

 exit_path:
        flush_remaining_diff(createdfile, 1);
        flush_remaining_diff(deletedfile, 0);
        free_held_diff(createdfile);
        free_held_diff(deletedfile);
        createdfile = deletedfile = NULL;
}

int diff_scoreopt_parse(const char *opt)
{
        int diglen, num, scale, i;
        if (opt[0] != '-' || opt[1] != 'M')
                return -1; /* that is not -M option */
        diglen = strspn(opt+2, "0123456789");
        if (diglen == 0 || strlen(opt+2) != diglen)
                return 0; /* use default */
        sscanf(opt+2, "%d", &num);
        for (i = 0, scale = 1; i < diglen; i++)
                scale *= 10;

        /* user says num divided by scale and we say internally that
         * is MAX_SCORE * num / scale.
         */
        return MAX_SCORE * num / scale;
}

void diff_setup(int detect_rename_, int minimum_score_, int reverse_diff_,
                int diff_raw_output_,
                const char **pathspec_, int speccnt_)
{
        free_held_diff(createdfile);
        free_held_diff(deletedfile);
        createdfile = deletedfile = NULL;

        detect_rename = detect_rename_;
        reverse_diff = reverse_diff_;
        pathspec = pathspec_;
        diff_raw_output = diff_raw_output_;
        speccnt = speccnt_;
        minimum_score = minimum_score_ ? : DEFAULT_MINIMUM_SCORE;
}

static const char *git_object_type(unsigned mode)
{
        return S_ISDIR(mode) ? "tree" : "blob";
}

void diff_addremove(int addremove, unsigned mode,
                    const unsigned char *sha1,
                    const char *base, const char *path)
{
        char concatpath[PATH_MAX];
        struct diff_spec spec[2], *one, *two;

        if (reverse_diff)
                addremove = (addremove == '+' ? '-' : '+');

        if (0 <= diff_raw_output) {
                if (!path)
                        path = "";
                printf("%c%06o %s %s %s%s%c",
                       addremove,
                       mode,
                       git_object_type(mode), sha1_to_hex(sha1),
                       base, path, diff_raw_output);
                return;
        }
        if (S_ISDIR(mode))
                return;

        memcpy(spec[0].blob_sha1, sha1, 20);
        spec[0].mode = mode;
        spec[0].sha1_valid = !!memcmp(sha1, null_sha1, 20);
        spec[0].file_valid = 1;
        spec[1].file_valid = 0;

        if (addremove == '+') {
                one = spec + 1; two = spec;
        } else {
                one = spec; two = one + 1;
        }

        if (path) {
                strcpy(concatpath, base);
                strcat(concatpath, path);
        }
        hold_diff(path ? concatpath : base, one, two);
}

void diff_change(unsigned old_mode, unsigned new_mode,
                 const unsigned char *old_sha1,
                 const unsigned char *new_sha1,
                 const char *base, const char *path) {
        char concatpath[PATH_MAX];
        struct diff_spec spec[2];

        if (reverse_diff) {
                unsigned tmp;
                const unsigned char *tmp_c;
                tmp = old_mode; old_mode = new_mode; new_mode = tmp;
                tmp_c = old_sha1; old_sha1 = new_sha1; new_sha1 = tmp_c;
        }

        if (0 <= diff_raw_output) {
                char old_hex[41];
                strcpy(old_hex, sha1_to_hex(old_sha1));

                if (!path)
                        path = "";
                printf("*%06o->%06o %s %s->%s %s%s%c",
                       old_mode, new_mode,
                       git_object_type(new_mode),
                       old_hex, sha1_to_hex(new_sha1),
                       base, path, diff_raw_output);
                return;
        }
        if (S_ISDIR(new_mode))
                return;

        if (path) {
                strcpy(concatpath, base);
                strcat(concatpath, path);
        }

        memcpy(spec[0].blob_sha1, old_sha1, 20);
        spec[0].mode = old_mode;
        memcpy(spec[1].blob_sha1, new_sha1, 20);
        spec[1].mode = new_mode;
        spec[0].sha1_valid = !!memcmp(old_sha1, null_sha1, 20);
        spec[1].sha1_valid = !!memcmp(new_sha1, null_sha1, 20);
        spec[1].file_valid = spec[0].file_valid = 1;

        /* We do not look at changed files as candidate for
         * rename detection ever.
         */
        run_external_diff(path ? concatpath : base, NULL,
                          &spec[0], &spec[1], -1);
}

void diff_unmerge(const char *path)
{
        if (0 <= diff_raw_output) {
                printf("U %s%c", path, diff_raw_output);
                return;
        }
        run_external_diff(path, NULL, NULL, NULL, -1);
}