s3:dbwrap_ctdb: update (C)

[Samba/gebeck_regimport.git] / source3 / lib / dbwrap_ctdb.c

/* 
   Unix SMB/CIFS implementation.
   Database interface wrapper around ctdbd
   Copyright (C) Volker Lendecke 2007-2009
   Copyright (C) Michael Adam 2009

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

#include "includes.h"
#ifdef CLUSTER_SUPPORT
#include "ctdb.h"
#include "ctdb_private.h"
#include "ctdbd_conn.h"
#include "g_lock.h"

struct db_ctdb_transaction_handle {
        struct db_ctdb_ctx *ctx;
        /*
         * we store the reads and writes done under a transaction:
         * - one list stores both reads and writes (m_all),
         * - the other just writes (m_write)
         */
        struct ctdb_marshall_buffer *m_all;
        struct ctdb_marshall_buffer *m_write;
        uint32_t nesting;
        bool nested_cancel;
        char *lock_name;
};

struct db_ctdb_ctx {
        struct db_context *db;
        struct tdb_wrap *wtdb;
        uint32 db_id;
        struct db_ctdb_transaction_handle *transaction;
        struct g_lock_ctx *lock_ctx;
};

struct db_ctdb_rec {
        struct db_ctdb_ctx *ctdb_ctx;
        struct ctdb_ltdb_header header;
};

static struct db_record *fetch_locked_internal(struct db_ctdb_ctx *ctx,
                                               TALLOC_CTX *mem_ctx,
                                               TDB_DATA key,
                                               bool persistent);

static NTSTATUS tdb_error_to_ntstatus(struct tdb_context *tdb)
{
        NTSTATUS status;
        enum TDB_ERROR tret = tdb_error(tdb);

        switch (tret) {
        case TDB_ERR_EXISTS:
                status = NT_STATUS_OBJECT_NAME_COLLISION;
                break;
        case TDB_ERR_NOEXIST:
                status = NT_STATUS_OBJECT_NAME_NOT_FOUND;
                break;
        default:
                status = NT_STATUS_INTERNAL_DB_CORRUPTION;
                break;
        }

        return status;
}


/**
 * fetch a record from the tdb, separating out the header
 * information and returning the body of the record.
 */
static NTSTATUS db_ctdb_ltdb_fetch(struct db_ctdb_ctx *db,
                                   TDB_DATA key,
                                   struct ctdb_ltdb_header *header,
                                   TALLOC_CTX *mem_ctx,
                                   TDB_DATA *data)
{
        TDB_DATA rec;
        NTSTATUS status;

        rec = tdb_fetch(db->wtdb->tdb, key);
        if (rec.dsize < sizeof(struct ctdb_ltdb_header)) {
                status = NT_STATUS_NOT_FOUND;
                if (data) {
                        ZERO_STRUCTP(data);
                }
                if (header) {
                        header->dmaster = (uint32_t)-1;
                        header->rsn = 0;
                }
                goto done;
        }

        if (header) {
                *header = *(struct ctdb_ltdb_header *)rec.dptr;
        }

        if (data) {
                data->dsize = rec.dsize - sizeof(struct ctdb_ltdb_header);
                if (data->dsize == 0) {
                        data->dptr = NULL;
                } else {
                        data->dptr = (unsigned char *)talloc_memdup(mem_ctx,
                                        rec.dptr
                                         + sizeof(struct ctdb_ltdb_header),
                                        data->dsize);
                        if (data->dptr == NULL) {
                                status = NT_STATUS_NO_MEMORY;
                                goto done;
                        }
                }
        }

        status = NT_STATUS_OK;

done:
        SAFE_FREE(rec.dptr);
        return status;
}

/*
 * Store a record together with the ctdb record header
 * in the local copy of the database.
 */
static NTSTATUS db_ctdb_ltdb_store(struct db_ctdb_ctx *db,
                                   TDB_DATA key,
                                   struct ctdb_ltdb_header *header,
                                   TDB_DATA data)
{
        TALLOC_CTX *tmp_ctx = talloc_stackframe();
        TDB_DATA rec;
        int ret;

        rec.dsize = data.dsize + sizeof(struct ctdb_ltdb_header);
        rec.dptr = (uint8_t *)talloc_size(tmp_ctx, rec.dsize);

        if (rec.dptr == NULL) {
                talloc_free(tmp_ctx);
                return NT_STATUS_NO_MEMORY;
        }

        memcpy(rec.dptr, header, sizeof(struct ctdb_ltdb_header));
        memcpy(sizeof(struct ctdb_ltdb_header) + (uint8_t *)rec.dptr, data.dptr, data.dsize);

        ret = tdb_store(db->wtdb->tdb, key, rec, TDB_REPLACE);

        talloc_free(tmp_ctx);

        return (ret == 0) ? NT_STATUS_OK
                          : tdb_error_to_ntstatus(db->wtdb->tdb);

}

/*
  form a ctdb_rec_data record from a key/data pair

  note that header may be NULL. If not NULL then it is included in the data portion
  of the record
 */
static struct ctdb_rec_data *db_ctdb_marshall_record(TALLOC_CTX *mem_ctx, uint32_t reqid,       
                                                  TDB_DATA key, 
                                                  struct ctdb_ltdb_header *header,
                                                  TDB_DATA data)
{
        size_t length;
        struct ctdb_rec_data *d;

        length = offsetof(struct ctdb_rec_data, data) + key.dsize + 
                data.dsize + (header?sizeof(*header):0);
        d = (struct ctdb_rec_data *)talloc_size(mem_ctx, length);
        if (d == NULL) {
                return NULL;
        }
        d->length = length;
        d->reqid = reqid;
        d->keylen = key.dsize;
        memcpy(&d->data[0], key.dptr, key.dsize);
        if (header) {
                d->datalen = data.dsize + sizeof(*header);
                memcpy(&d->data[key.dsize], header, sizeof(*header));
                memcpy(&d->data[key.dsize+sizeof(*header)], data.dptr, data.dsize);
        } else {
                d->datalen = data.dsize;
                memcpy(&d->data[key.dsize], data.dptr, data.dsize);
        }
        return d;
}


/* helper function for marshalling multiple records */
static struct ctdb_marshall_buffer *db_ctdb_marshall_add(TALLOC_CTX *mem_ctx, 
                                               struct ctdb_marshall_buffer *m,
                                               uint64_t db_id,
                                               uint32_t reqid,
                                               TDB_DATA key,
                                               struct ctdb_ltdb_header *header,
                                               TDB_DATA data)
{
        struct ctdb_rec_data *r;
        size_t m_size, r_size;
        struct ctdb_marshall_buffer *m2 = NULL;

        r = db_ctdb_marshall_record(talloc_tos(), reqid, key, header, data);
        if (r == NULL) {
                talloc_free(m);
                return NULL;
        }

        if (m == NULL) {
                m = (struct ctdb_marshall_buffer *)talloc_zero_size(
                        mem_ctx, offsetof(struct ctdb_marshall_buffer, data));
                if (m == NULL) {
                        goto done;
                }
                m->db_id = db_id;
        }

        m_size = talloc_get_size(m);
        r_size = talloc_get_size(r);

        m2 = (struct ctdb_marshall_buffer *)talloc_realloc_size(
                mem_ctx, m,  m_size + r_size);
        if (m2 == NULL) {
                talloc_free(m);
                goto done;
        }

        memcpy(m_size + (uint8_t *)m2, r, r_size);

        m2->count++;

done:
        talloc_free(r);
        return m2;
}

/* we've finished marshalling, return a data blob with the marshalled records */
static TDB_DATA db_ctdb_marshall_finish(struct ctdb_marshall_buffer *m)
{
        TDB_DATA data;
        data.dptr = (uint8_t *)m;
        data.dsize = talloc_get_size(m);
        return data;
}

/* 
   loop over a marshalling buffer 

     - pass r==NULL to start
     - loop the number of times indicated by m->count
*/
static struct ctdb_rec_data *db_ctdb_marshall_loop_next(struct ctdb_marshall_buffer *m, struct ctdb_rec_data *r,
                                                     uint32_t *reqid,
                                                     struct ctdb_ltdb_header *header,
                                                     TDB_DATA *key, TDB_DATA *data)
{
        if (r == NULL) {
                r = (struct ctdb_rec_data *)&m->data[0];
        } else {
                r = (struct ctdb_rec_data *)(r->length + (uint8_t *)r);
        }

        if (reqid != NULL) {
                *reqid = r->reqid;
        }

        if (key != NULL) {
                key->dptr   = &r->data[0];
                key->dsize  = r->keylen;
        }
        if (data != NULL) {
                data->dptr  = &r->data[r->keylen];
                data->dsize = r->datalen;
                if (header != NULL) {
                        data->dptr += sizeof(*header);
                        data->dsize -= sizeof(*header);
                }
        }

        if (header != NULL) {
                if (r->datalen < sizeof(*header)) {
                        return NULL;
                }
                *header = *(struct ctdb_ltdb_header *)&r->data[r->keylen];
        }

        return r;
}

/**
 * CTDB transaction destructor
 */
static int db_ctdb_transaction_destructor(struct db_ctdb_transaction_handle *h)
{
        NTSTATUS status;

        status = g_lock_unlock(h->ctx->lock_ctx, h->lock_name);
        if (!NT_STATUS_IS_OK(status)) {
                DEBUG(0, ("g_lock_unlock failed: %s\n", nt_errstr(status)));
                return -1;
        }
        return 0;
}

/**
 * CTDB dbwrap API: transaction_start function
 * starts a transaction on a persistent database
 */
static int db_ctdb_transaction_start(struct db_context *db)
{
        struct db_ctdb_transaction_handle *h;
        NTSTATUS status;
        struct db_ctdb_ctx *ctx = talloc_get_type_abort(db->private_data,
                                                        struct db_ctdb_ctx);

        if (!db->persistent) {
                DEBUG(0,("transactions not supported on non-persistent database 0x%08x\n", 
                         ctx->db_id));
                return -1;
        }

        if (ctx->transaction) {
                ctx->transaction->nesting++;
                return 0;
        }

        h = talloc_zero(db, struct db_ctdb_transaction_handle);
        if (h == NULL) {
                DEBUG(0,(__location__ " oom for transaction handle\n"));                
                return -1;
        }

        h->ctx = ctx;

        h->lock_name = talloc_asprintf(h, "transaction_db_0x%08x",
                                       (unsigned int)ctx->db_id);
        if (h->lock_name == NULL) {
                DEBUG(0, ("talloc_asprintf failed\n"));
                TALLOC_FREE(h);
                return -1;
        }

        /*
         * Wait a day, i.e. forever...
         */
        status = g_lock_lock(ctx->lock_ctx, h->lock_name, G_LOCK_WRITE,
                             timeval_set(86400, 0));
        if (!NT_STATUS_IS_OK(status)) {
                DEBUG(0, ("g_lock_lock failed: %s\n", nt_errstr(status)));
                TALLOC_FREE(h);
                return -1;
        }

        talloc_set_destructor(h, db_ctdb_transaction_destructor);

        ctx->transaction = h;

        DEBUG(5,(__location__ " Started transaction on db 0x%08x\n", ctx->db_id));

        return 0;
}

static bool pull_newest_from_marshall_buffer(struct ctdb_marshall_buffer *buf,
                                             TDB_DATA key,
                                             struct ctdb_ltdb_header *pheader,
                                             TALLOC_CTX *mem_ctx,
                                             TDB_DATA *pdata)
{
        struct ctdb_rec_data *rec = NULL;
        struct ctdb_ltdb_header h;
        bool found;
        TDB_DATA data;
        int i;

        if (buf == NULL) {
                return false;
        }

        /*
         * Walk the list of records written during this
         * transaction. If we want to read one we have already
         * written, return the last written sample. Thus we do not do
         * a "break;" for the first hit, this record might have been
         * overwritten later.
         */

        for (i=0; i<buf->count; i++) {
                TDB_DATA tkey, tdata;
                uint32_t reqid;

                rec = db_ctdb_marshall_loop_next(buf, rec, &reqid, &h, &tkey,
                                                 &tdata);
                if (rec == NULL) {
                        return false;
                }

                if (tdb_data_equal(key, tkey)) {
                        found = true;
                        data = tdata;
                }
        }

        if (!found) {
                return false;
        }

        if (pdata != NULL) {
                data.dptr = (uint8_t *)talloc_memdup(mem_ctx, data.dptr,
                                                     data.dsize);
                if ((data.dsize != 0) && (data.dptr == NULL)) {
                        return false;
                }
                *pdata = data;
        }

        if (pheader != NULL) {
                *pheader = h;
        }

        return true;
}

/*
  fetch a record inside a transaction
 */
static int db_ctdb_transaction_fetch(struct db_ctdb_ctx *db, 
                                     TALLOC_CTX *mem_ctx, 
                                     TDB_DATA key, TDB_DATA *data)
{
        struct db_ctdb_transaction_handle *h = db->transaction;
        NTSTATUS status;
        bool found;

        found = pull_newest_from_marshall_buffer(h->m_write, key, NULL,
                                                 mem_ctx, data);
        if (found) {
                return 0;
        }

        status = db_ctdb_ltdb_fetch(h->ctx, key, NULL, mem_ctx, data);

        if (NT_STATUS_EQUAL(status, NT_STATUS_NOT_FOUND)) {
                *data = tdb_null;
        } else if (!NT_STATUS_IS_OK(status)) {
                return -1;
        }

        h->m_all = db_ctdb_marshall_add(h, h->m_all, h->ctx->db_id, 1, key,
                                        NULL, *data);
        if (h->m_all == NULL) {
                DEBUG(0,(__location__ " Failed to add to marshalling "
                         "record\n"));
                data->dsize = 0;
                talloc_free(data->dptr);
                return -1;
        }

        return 0;
}


static NTSTATUS db_ctdb_store_transaction(struct db_record *rec, TDB_DATA data, int flag);
static NTSTATUS db_ctdb_delete_transaction(struct db_record *rec);

static struct db_record *db_ctdb_fetch_locked_transaction(struct db_ctdb_ctx *ctx,
                                                          TALLOC_CTX *mem_ctx,
                                                          TDB_DATA key)
{
        struct db_record *result;
        TDB_DATA ctdb_data;

        if (!(result = talloc(mem_ctx, struct db_record))) {
                DEBUG(0, ("talloc failed\n"));
                return NULL;
        }

        result->private_data = ctx->transaction;

        result->key.dsize = key.dsize;
        result->key.dptr = (uint8 *)talloc_memdup(result, key.dptr, key.dsize);
        if (result->key.dptr == NULL) {
                DEBUG(0, ("talloc failed\n"));
                TALLOC_FREE(result);
                return NULL;
        }

        result->store = db_ctdb_store_transaction;
        result->delete_rec = db_ctdb_delete_transaction;

        if (pull_newest_from_marshall_buffer(ctx->transaction->m_write, key,
                                             NULL, result, &result->value)) {
                return result;
        }

        ctdb_data = tdb_fetch(ctx->wtdb->tdb, key);
        if (ctdb_data.dptr == NULL) {
                /* create the record */
                result->value = tdb_null;
                return result;
        }

        result->value.dsize = ctdb_data.dsize - sizeof(struct ctdb_ltdb_header);
        result->value.dptr = NULL;

        if ((result->value.dsize != 0)
            && !(result->value.dptr = (uint8 *)talloc_memdup(
                         result, ctdb_data.dptr + sizeof(struct ctdb_ltdb_header),
                         result->value.dsize))) {
                DEBUG(0, ("talloc failed\n"));
                TALLOC_FREE(result);
        }

        SAFE_FREE(ctdb_data.dptr);

        return result;
}

static int db_ctdb_record_destructor(struct db_record **recp)
{
        struct db_record *rec = talloc_get_type_abort(*recp, struct db_record);
        struct db_ctdb_transaction_handle *h = talloc_get_type_abort(
                rec->private_data, struct db_ctdb_transaction_handle);
        int ret = h->ctx->db->transaction_commit(h->ctx->db);
        if (ret != 0) {
                DEBUG(0,(__location__ " transaction_commit failed\n"));
        }
        return 0;
}

/*
  auto-create a transaction for persistent databases
 */
static struct db_record *db_ctdb_fetch_locked_persistent(struct db_ctdb_ctx *ctx,
                                                         TALLOC_CTX *mem_ctx,
                                                         TDB_DATA key)
{
        int res;
        struct db_record *rec, **recp;

        res = db_ctdb_transaction_start(ctx->db);
        if (res == -1) {
                return NULL;
        }

        rec = db_ctdb_fetch_locked_transaction(ctx, mem_ctx, key);
        if (rec == NULL) {
                ctx->db->transaction_cancel(ctx->db);           
                return NULL;
        }

        /* destroy this transaction when we release the lock */
        recp = talloc(rec, struct db_record *);
        if (recp == NULL) {
                ctx->db->transaction_cancel(ctx->db);
                talloc_free(rec);
                return NULL;
        }
        *recp = rec;
        talloc_set_destructor(recp, db_ctdb_record_destructor);
        return rec;
}


/*
  stores a record inside a transaction
 */
static int db_ctdb_transaction_store(struct db_ctdb_transaction_handle *h, 
                                     TDB_DATA key, TDB_DATA data)
{
        TALLOC_CTX *tmp_ctx = talloc_new(h);
        TDB_DATA rec;
        struct ctdb_ltdb_header header;

        ZERO_STRUCT(header);

        /* we need the header so we can update the RSN */

        if (!pull_newest_from_marshall_buffer(h->m_write, key, &header,
                                              NULL, NULL)) {

                rec = tdb_fetch(h->ctx->wtdb->tdb, key);

                if (rec.dptr != NULL) {
                        memcpy(&header, rec.dptr,
                               sizeof(struct ctdb_ltdb_header));
                        rec.dsize -= sizeof(struct ctdb_ltdb_header);

                        /*
                         * a special case, we are writing the same
                         * data that is there now
                         */
                        if (data.dsize == rec.dsize &&
                            memcmp(data.dptr,
                                   rec.dptr + sizeof(struct ctdb_ltdb_header),
                                   data.dsize) == 0) {
                                SAFE_FREE(rec.dptr);
                                talloc_free(tmp_ctx);
                                return 0;
                        }
                }
                SAFE_FREE(rec.dptr);
        }

        header.dmaster = get_my_vnn();
        header.rsn++;

        h->m_all = db_ctdb_marshall_add(h, h->m_all, h->ctx->db_id, 0, key,
                                        NULL, data);
        if (h->m_all == NULL) {
                DEBUG(0,(__location__ " Failed to add to marshalling "
                         "record\n"));
                talloc_free(tmp_ctx);
                return -1;
        }

        h->m_write = db_ctdb_marshall_add(h, h->m_write, h->ctx->db_id, 0, key, &header, data);
        if (h->m_write == NULL) {
                DEBUG(0,(__location__ " Failed to add to marshalling record\n"));
                talloc_free(tmp_ctx);
                return -1;
        }

        talloc_free(tmp_ctx);
        return 0;
}


/* 
   a record store inside a transaction
 */
static NTSTATUS db_ctdb_store_transaction(struct db_record *rec, TDB_DATA data, int flag)
{
        struct db_ctdb_transaction_handle *h = talloc_get_type_abort(
                rec->private_data, struct db_ctdb_transaction_handle);
        int ret;

        ret = db_ctdb_transaction_store(h, rec->key, data);
        if (ret != 0) {
                return tdb_error_to_ntstatus(h->ctx->wtdb->tdb);
        }
        return NT_STATUS_OK;
}

/* 
   a record delete inside a transaction
 */
static NTSTATUS db_ctdb_delete_transaction(struct db_record *rec)
{
        struct db_ctdb_transaction_handle *h = talloc_get_type_abort(
                rec->private_data, struct db_ctdb_transaction_handle);
        int ret;

        ret = db_ctdb_transaction_store(h, rec->key, tdb_null);
        if (ret != 0) {
                return tdb_error_to_ntstatus(h->ctx->wtdb->tdb);
        }
        return NT_STATUS_OK;
}

/*
  commit a transaction
 */
static int db_ctdb_transaction_commit(struct db_context *db)
{
        struct db_ctdb_ctx *ctx = talloc_get_type_abort(db->private_data,
                                                        struct db_ctdb_ctx);
        NTSTATUS rets;
        int status;
        struct db_ctdb_transaction_handle *h = ctx->transaction;

        if (h == NULL) {
                DEBUG(0,(__location__ " transaction commit with no open transaction on db 0x%08x\n", ctx->db_id));
                return -1;
        }

        if (h->nested_cancel) {
                db->transaction_cancel(db);
                DEBUG(5,(__location__ " Failed transaction commit after nested cancel\n"));
                return -1;
        }

        if (h->nesting != 0) {
                h->nesting--;
                return 0;
        }

        DEBUG(5,(__location__ " Commit transaction on db 0x%08x\n", ctx->db_id));

again:
        if (h->m_write == NULL) {
                /* no changes were made, potentially after a retry */
                goto done;
        }

        /* tell ctdbd to commit to the other nodes */
        rets = ctdbd_control_local(messaging_ctdbd_connection(),
                                   CTDB_CONTROL_TRANS3_COMMIT,
                                   h->ctx->db_id, 0,
                                   db_ctdb_marshall_finish(h->m_write),
                                   NULL, NULL, &status);
        if (!NT_STATUS_IS_OK(rets) || status != 0) {
                /*
                 * TODO:
                 * check the database sequence number and
                 * compare it to the seqnum after applying the
                 * marshall buffer. If it is the same: return success.
                 */
                goto again;
        }

done:
        h->ctx->transaction = NULL;
        talloc_free(h);
        return 0;
}


/*
  cancel a transaction
 */
static int db_ctdb_transaction_cancel(struct db_context *db)
{
        struct db_ctdb_ctx *ctx = talloc_get_type_abort(db->private_data,
                                                        struct db_ctdb_ctx);
        struct db_ctdb_transaction_handle *h = ctx->transaction;

        if (h == NULL) {
                DEBUG(0,(__location__ " transaction cancel with no open transaction on db 0x%08x\n", ctx->db_id));
                return -1;
        }

        if (h->nesting != 0) {
                h->nesting--;
                h->nested_cancel = true;
                return 0;
        }

        DEBUG(5,(__location__ " Cancel transaction on db 0x%08x\n", ctx->db_id));

        ctx->transaction = NULL;
        talloc_free(h);
        return 0;
}


static NTSTATUS db_ctdb_store(struct db_record *rec, TDB_DATA data, int flag)
{
        struct db_ctdb_rec *crec = talloc_get_type_abort(
                rec->private_data, struct db_ctdb_rec);

        return db_ctdb_ltdb_store(crec->ctdb_ctx, rec->key, &(crec->header), data);
}



static NTSTATUS db_ctdb_delete(struct db_record *rec)
{
        TDB_DATA data;

        /*
         * We have to store the header with empty data. TODO: Fix the
         * tdb-level cleanup
         */

        ZERO_STRUCT(data);

        return db_ctdb_store(rec, data, 0);

}

static int db_ctdb_record_destr(struct db_record* data)
{
        struct db_ctdb_rec *crec = talloc_get_type_abort(
                data->private_data, struct db_ctdb_rec);

        DEBUG(10, (DEBUGLEVEL > 10
                   ? "Unlocking db %u key %s\n"
                   : "Unlocking db %u key %.20s\n",
                   (int)crec->ctdb_ctx->db_id,
                   hex_encode_talloc(data, (unsigned char *)data->key.dptr,
                              data->key.dsize)));

        if (tdb_chainunlock(crec->ctdb_ctx->wtdb->tdb, data->key) != 0) {
                DEBUG(0, ("tdb_chainunlock failed\n"));
                return -1;
        }

        return 0;
}

static struct db_record *fetch_locked_internal(struct db_ctdb_ctx *ctx,
                                               TALLOC_CTX *mem_ctx,
                                               TDB_DATA key,
                                               bool persistent)
{
        struct db_record *result;
        struct db_ctdb_rec *crec;
        NTSTATUS status;
        TDB_DATA ctdb_data;
        int migrate_attempts = 0;

        if (!(result = talloc(mem_ctx, struct db_record))) {
                DEBUG(0, ("talloc failed\n"));
                return NULL;
        }

        if (!(crec = TALLOC_ZERO_P(result, struct db_ctdb_rec))) {
                DEBUG(0, ("talloc failed\n"));
                TALLOC_FREE(result);
                return NULL;
        }

        result->private_data = (void *)crec;
        crec->ctdb_ctx = ctx;

        result->key.dsize = key.dsize;
        result->key.dptr = (uint8 *)talloc_memdup(result, key.dptr, key.dsize);
        if (result->key.dptr == NULL) {
                DEBUG(0, ("talloc failed\n"));
                TALLOC_FREE(result);
                return NULL;
        }

        /*
         * Do a blocking lock on the record
         */
again:

        if (DEBUGLEVEL >= 10) {
                char *keystr = hex_encode_talloc(result, key.dptr, key.dsize);
                DEBUG(10, (DEBUGLEVEL > 10
                           ? "Locking db %u key %s\n"
                           : "Locking db %u key %.20s\n",
                           (int)crec->ctdb_ctx->db_id, keystr));
                TALLOC_FREE(keystr);
        }

        if (tdb_chainlock(ctx->wtdb->tdb, key) != 0) {
                DEBUG(3, ("tdb_chainlock failed\n"));
                TALLOC_FREE(result);
                return NULL;
        }

        result->store = db_ctdb_store;
        result->delete_rec = db_ctdb_delete;
        talloc_set_destructor(result, db_ctdb_record_destr);

        ctdb_data = tdb_fetch(ctx->wtdb->tdb, key);

        /*
         * See if we have a valid record and we are the dmaster. If so, we can
         * take the shortcut and just return it.
         */

        if ((ctdb_data.dptr == NULL) ||
            (ctdb_data.dsize < sizeof(struct ctdb_ltdb_header)) ||
            ((struct ctdb_ltdb_header *)ctdb_data.dptr)->dmaster != get_my_vnn()
#if 0
            || (random() % 2 != 0)
#endif
) {
                SAFE_FREE(ctdb_data.dptr);
                tdb_chainunlock(ctx->wtdb->tdb, key);
                talloc_set_destructor(result, NULL);

                migrate_attempts += 1;

                DEBUG(10, ("ctdb_data.dptr = %p, dmaster = %u (%u)\n",
                           ctdb_data.dptr, ctdb_data.dptr ?
                           ((struct ctdb_ltdb_header *)ctdb_data.dptr)->dmaster : -1,
                           get_my_vnn()));

                status = ctdbd_migrate(messaging_ctdbd_connection(),ctx->db_id, key);
                if (!NT_STATUS_IS_OK(status)) {
                        DEBUG(5, ("ctdb_migrate failed: %s\n",
                                  nt_errstr(status)));
                        TALLOC_FREE(result);
                        return NULL;
                }
                /* now its migrated, try again */
                goto again;
        }

        if (migrate_attempts > 10) {
                DEBUG(0, ("db_ctdb_fetch_locked needed %d attempts\n",
                          migrate_attempts));
        }

        memcpy(&crec->header, ctdb_data.dptr, sizeof(crec->header));

        result->value.dsize = ctdb_data.dsize - sizeof(crec->header);
        result->value.dptr = NULL;

        if ((result->value.dsize != 0)
            && !(result->value.dptr = (uint8 *)talloc_memdup(
                         result, ctdb_data.dptr + sizeof(crec->header),
                         result->value.dsize))) {
                DEBUG(0, ("talloc failed\n"));
                TALLOC_FREE(result);
        }

        SAFE_FREE(ctdb_data.dptr);

        return result;
}

static struct db_record *db_ctdb_fetch_locked(struct db_context *db,
                                              TALLOC_CTX *mem_ctx,
                                              TDB_DATA key)
{
        struct db_ctdb_ctx *ctx = talloc_get_type_abort(db->private_data,
                                                        struct db_ctdb_ctx);

        if (ctx->transaction != NULL) {
                return db_ctdb_fetch_locked_transaction(ctx, mem_ctx, key);
        }

        if (db->persistent) {
                return db_ctdb_fetch_locked_persistent(ctx, mem_ctx, key);
        }

        return fetch_locked_internal(ctx, mem_ctx, key, db->persistent);
}

/*
  fetch (unlocked, no migration) operation on ctdb
 */
static int db_ctdb_fetch(struct db_context *db, TALLOC_CTX *mem_ctx,
                         TDB_DATA key, TDB_DATA *data)
{
        struct db_ctdb_ctx *ctx = talloc_get_type_abort(db->private_data,
                                                        struct db_ctdb_ctx);
        NTSTATUS status;
        TDB_DATA ctdb_data;

        if (ctx->transaction) {
                return db_ctdb_transaction_fetch(ctx, mem_ctx, key, data);
        }

        /* try a direct fetch */
        ctdb_data = tdb_fetch(ctx->wtdb->tdb, key);

        /*
         * See if we have a valid record and we are the dmaster. If so, we can
         * take the shortcut and just return it.
         * we bypass the dmaster check for persistent databases
         */
        if ((ctdb_data.dptr != NULL) &&
            (ctdb_data.dsize >= sizeof(struct ctdb_ltdb_header)) &&
            (db->persistent ||
             ((struct ctdb_ltdb_header *)ctdb_data.dptr)->dmaster == get_my_vnn())) {
                /* we are the dmaster - avoid the ctdb protocol op */

                data->dsize = ctdb_data.dsize - sizeof(struct ctdb_ltdb_header);
                if (data->dsize == 0) {
                        SAFE_FREE(ctdb_data.dptr);
                        data->dptr = NULL;
                        return 0;
                }

                data->dptr = (uint8 *)talloc_memdup(
                        mem_ctx, ctdb_data.dptr+sizeof(struct ctdb_ltdb_header),
                        data->dsize);

                SAFE_FREE(ctdb_data.dptr);

                if (data->dptr == NULL) {
                        return -1;
                }
                return 0;
        }

        SAFE_FREE(ctdb_data.dptr);

        /* we weren't able to get it locally - ask ctdb to fetch it for us */
        status = ctdbd_fetch(messaging_ctdbd_connection(),ctx->db_id, key, mem_ctx, data);
        if (!NT_STATUS_IS_OK(status)) {
                DEBUG(5, ("ctdbd_fetch failed: %s\n", nt_errstr(status)));
                return -1;
        }

        return 0;
}

struct traverse_state {
        struct db_context *db;
        int (*fn)(struct db_record *rec, void *private_data);
        void *private_data;
};

static void traverse_callback(TDB_DATA key, TDB_DATA data, void *private_data)
{
        struct traverse_state *state = (struct traverse_state *)private_data;
        struct db_record *rec;
        TALLOC_CTX *tmp_ctx = talloc_new(state->db);
        /* we have to give them a locked record to prevent races */
        rec = db_ctdb_fetch_locked(state->db, tmp_ctx, key);
        if (rec && rec->value.dsize > 0) {
                state->fn(rec, state->private_data);
        }
        talloc_free(tmp_ctx);
}

static int traverse_persistent_callback(TDB_CONTEXT *tdb, TDB_DATA kbuf, TDB_DATA dbuf,
                                        void *private_data)
{
        struct traverse_state *state = (struct traverse_state *)private_data;
        struct db_record *rec;
        TALLOC_CTX *tmp_ctx = talloc_new(state->db);
        int ret = 0;
        /* we have to give them a locked record to prevent races */
        rec = db_ctdb_fetch_locked(state->db, tmp_ctx, kbuf);
        if (rec && rec->value.dsize > 0) {
                ret = state->fn(rec, state->private_data);
        }
        talloc_free(tmp_ctx);
        return ret;
}

static int db_ctdb_traverse(struct db_context *db,
                            int (*fn)(struct db_record *rec,
                                      void *private_data),
                            void *private_data)
{
        struct db_ctdb_ctx *ctx = talloc_get_type_abort(db->private_data,
                                                        struct db_ctdb_ctx);
        struct traverse_state state;

        state.db = db;
        state.fn = fn;
        state.private_data = private_data;

        if (db->persistent) {
                /* for persistent databases we don't need to do a ctdb traverse,
                   we can do a faster local traverse */
                return tdb_traverse(ctx->wtdb->tdb, traverse_persistent_callback, &state);
        }


        ctdbd_traverse(ctx->db_id, traverse_callback, &state);
        return 0;
}

static NTSTATUS db_ctdb_store_deny(struct db_record *rec, TDB_DATA data, int flag)
{
        return NT_STATUS_MEDIA_WRITE_PROTECTED;
}

static NTSTATUS db_ctdb_delete_deny(struct db_record *rec)
{
        return NT_STATUS_MEDIA_WRITE_PROTECTED;
}

static void traverse_read_callback(TDB_DATA key, TDB_DATA data, void *private_data)
{
        struct traverse_state *state = (struct traverse_state *)private_data;
        struct db_record rec;
        rec.key = key;
        rec.value = data;
        rec.store = db_ctdb_store_deny;
        rec.delete_rec = db_ctdb_delete_deny;
        rec.private_data = state->db;
        state->fn(&rec, state->private_data);
}

static int traverse_persistent_callback_read(TDB_CONTEXT *tdb, TDB_DATA kbuf, TDB_DATA dbuf,
                                        void *private_data)
{
        struct traverse_state *state = (struct traverse_state *)private_data;
        struct db_record rec;
        rec.key = kbuf;
        rec.value = dbuf;
        rec.store = db_ctdb_store_deny;
        rec.delete_rec = db_ctdb_delete_deny;
        rec.private_data = state->db;

        if (rec.value.dsize <= sizeof(struct ctdb_ltdb_header)) {
                /* a deleted record */
                return 0;
        }
        rec.value.dsize -= sizeof(struct ctdb_ltdb_header);
        rec.value.dptr += sizeof(struct ctdb_ltdb_header);

        return state->fn(&rec, state->private_data);
}

static int db_ctdb_traverse_read(struct db_context *db,
                                 int (*fn)(struct db_record *rec,
                                           void *private_data),
                                 void *private_data)
{
        struct db_ctdb_ctx *ctx = talloc_get_type_abort(db->private_data,
                                                        struct db_ctdb_ctx);
        struct traverse_state state;

        state.db = db;
        state.fn = fn;
        state.private_data = private_data;

        if (db->persistent) {
                /* for persistent databases we don't need to do a ctdb traverse,
                   we can do a faster local traverse */
                return tdb_traverse_read(ctx->wtdb->tdb, traverse_persistent_callback_read, &state);
        }

        ctdbd_traverse(ctx->db_id, traverse_read_callback, &state);
        return 0;
}

static int db_ctdb_get_seqnum(struct db_context *db)
{
        struct db_ctdb_ctx *ctx = talloc_get_type_abort(db->private_data,
                                                        struct db_ctdb_ctx);
        return tdb_get_seqnum(ctx->wtdb->tdb);
}

static int db_ctdb_get_flags(struct db_context *db)
{
        struct db_ctdb_ctx *ctx = talloc_get_type_abort(db->private_data,
                                                        struct db_ctdb_ctx);
        return tdb_get_flags(ctx->wtdb->tdb);
}

struct db_context *db_open_ctdb(TALLOC_CTX *mem_ctx,
                                const char *name,
                                int hash_size, int tdb_flags,
                                int open_flags, mode_t mode)
{
        struct db_context *result;
        struct db_ctdb_ctx *db_ctdb;
        char *db_path;
        struct ctdbd_connection *conn;

        if (!lp_clustering()) {
                DEBUG(10, ("Clustering disabled -- no ctdb\n"));
                return NULL;
        }

        if (!(result = TALLOC_ZERO_P(mem_ctx, struct db_context))) {
                DEBUG(0, ("talloc failed\n"));
                TALLOC_FREE(result);
                return NULL;
        }

        if (!(db_ctdb = TALLOC_P(result, struct db_ctdb_ctx))) {
                DEBUG(0, ("talloc failed\n"));
                TALLOC_FREE(result);
                return NULL;
        }

        db_ctdb->transaction = NULL;
        db_ctdb->db = result;

        conn = messaging_ctdbd_connection();

        if (!NT_STATUS_IS_OK(ctdbd_db_attach(conn, name, &db_ctdb->db_id, tdb_flags))) {
                DEBUG(0, ("ctdbd_db_attach failed for %s\n", name));
                TALLOC_FREE(result);
                return NULL;
        }

        db_path = ctdbd_dbpath(conn, db_ctdb, db_ctdb->db_id);

        result->persistent = ((tdb_flags & TDB_CLEAR_IF_FIRST) == 0);

        /* only pass through specific flags */
        tdb_flags &= TDB_SEQNUM;

        /* honor permissions if user has specified O_CREAT */
        if (open_flags & O_CREAT) {
                chmod(db_path, mode);
        }

        db_ctdb->wtdb = tdb_wrap_open(db_ctdb, db_path, hash_size, tdb_flags, O_RDWR, 0);
        if (db_ctdb->wtdb == NULL) {
                DEBUG(0, ("Could not open tdb %s: %s\n", db_path, strerror(errno)));
                TALLOC_FREE(result);
                return NULL;
        }
        talloc_free(db_path);

        if (result->persistent) {
                db_ctdb->lock_ctx = g_lock_ctx_init(db_ctdb,
                                                    ctdb_conn_msg_ctx(conn));
                if (db_ctdb->lock_ctx == NULL) {
                        DEBUG(0, ("g_lock_ctx_init failed\n"));
                        TALLOC_FREE(result);
                        return NULL;
                }
        }

        result->private_data = (void *)db_ctdb;
        result->fetch_locked = db_ctdb_fetch_locked;
        result->fetch = db_ctdb_fetch;
        result->traverse = db_ctdb_traverse;
        result->traverse_read = db_ctdb_traverse_read;
        result->get_seqnum = db_ctdb_get_seqnum;
        result->get_flags = db_ctdb_get_flags;
        result->transaction_start = db_ctdb_transaction_start;
        result->transaction_commit = db_ctdb_transaction_commit;
        result->transaction_cancel = db_ctdb_transaction_cancel;

        DEBUG(3,("db_open_ctdb: opened database '%s' with dbid 0x%x\n",
                 name, db_ctdb->db_id));

        return result;
}
#endif