s3:registry: fix seqnum race in fetch_values_internal

This prevents race between fetching seqnum and key content.

Because there is currently no way to atomically fetch the
record along with the seqnum, I use a loop.
This is far from optimal and should should ideally be done
differently. But for now it fixes the race.

Signed-off-by: Andreas Schneider <asn@samba.org>

diff --git a/source3/registry/reg_backend_db.c b/source3/registry/reg_backend_db.c
index 65355aec2370eff994bc9d9e767bf69877a81c91..abe9f7bdca5c3866f41f262178f1301a05a6b28a 100644 (file)
--- a/source3/registry/reg_backend_db.c
+++ b/source3/registry/reg_backend_db.c
@@ -1862,6 +1862,7 @@ static int regdb_fetch_values_internal(struct db_context *db, const char* key,
        int ret = 0;
        TDB_DATA value;
        WERROR werr;
+       int seqnum[2], count;
 
        DEBUG(10,("regdb_fetch_values: Looking for values of key [%s]\n", key));
 
@@ -1874,10 +1875,27 @@ static int regdb_fetch_values_internal(struct db_context *db, const char* key,
                goto done;
        }
 
-       werr = regval_ctr_set_seqnum(values, dbwrap_get_seqnum(db));
-       W_ERROR_NOT_OK_GOTO_DONE(werr);
+       ZERO_STRUCT(value);
+       count = 0;
+       seqnum[0] = dbwrap_get_seqnum(db);
+
+       do {
+               count++;
+               TALLOC_FREE(value.dptr);
+               value = regdb_fetch_key_internal(db, ctx, keystr);
+               seqnum[count % 2] = dbwrap_get_seqnum(db);
+       } while (seqnum[0] != seqnum[1]);
+
+       if (count > 1) {
+               DEBUG(5, ("regdb_fetch_values_internal: it took %d attempts "
+                         "to fetch key '%s' with constant seqnum\n",
+                         count, key));
+       }
 
-       value = regdb_fetch_key_internal(db, ctx, keystr);
+       werr = regval_ctr_set_seqnum(values, seqnum[0]);
+       if (!W_ERROR_IS_OK(werr)) {
+               goto done;
+       }
 
        if (!value.dptr) {
                /* all keys have zero values by default */