ITS#8704 add MDB_PREVMETA flag to mdb_env_open

used to open the previous meta page, in case the latest one
is corrupted

From https://github.com/LMDB/lmdb/pull/12

libraries/liblmdb/lmdb.h

@@ -354,8 +354,10 @@ typedef void (MDB_enc_func)(const MDB_val *src, MDB_val *dst, const MDB_val *key
 #define MDB_NORDAHEAD	0x800000
 	/** don't initialize malloc'd memory before writing to datafile */
 #define MDB_NOMEMINIT	0x1000000
+	/** use the previous meta page rather than the latest one */
+#define MDB_PREVMETA	0x2000000
 	/** don't use a single mmap, remap individual chunks (needs MDB_RPAGE_CACHE) */
-#define MDB_REMAP_CHUNKS	0x2000000
+#define MDB_REMAP_CHUNKS	0x4000000
 /** @} */
 
 /**	@defgroup	mdb_dbi_open	Database Flags
@@ -670,6 +672,10 @@ int  mdb_env_create(MDB_env **env);
 	 *		caller is expected to overwrite all of the memory that was
 	 *		reserved in that case.
 	 *		This flag may be changed at any time using #mdb_env_set_flags().
+	 *	<li>#MDB_PREVMETA
+	 *		Open the environment with the previous meta page rather than the latest
+	 *		one. This loses the latest transaction, but may help work around some
+	 *		types of corruption.
 	 * </ul>
 	 * @param[in] mode The UNIX permissions to set on created files and semaphores.
 	 * This parameter is ignored on Windows.

libraries/liblmdb/mdb.c

@@ -1697,7 +1697,7 @@ static int	mdb_page_merge(MDB_cursor *csrc, MDB_cursor *cdst);
 static int	mdb_page_split(MDB_cursor *mc, MDB_val *newkey, MDB_val *newdata,
 				pgno_t newpgno, unsigned int nflags);
 
-static int  mdb_env_read_header(MDB_env *env, MDB_meta *meta);
+static int  mdb_env_read_header(MDB_env *env, int prev, MDB_meta *meta);
 static MDB_meta *mdb_env_pick_meta(const MDB_env *env);
 static int  mdb_env_write_meta(MDB_txn *txn);
 #ifdef MDB_USE_POSIX_MUTEX /* Drop unused excl arg */
@@ -4270,11 +4270,12 @@ fail:
 /** Read the environment parameters of a DB environment before
  * mapping it into memory.
  * @param[in] env the environment handle
+ * @param[in] prev whether to read the backup meta page
  * @param[out] meta address of where to store the meta information
  * @return 0 on success, non-zero on failure.
  */
 static int ESECT
-mdb_env_read_header(MDB_env *env, MDB_meta *meta)
+mdb_env_read_header(MDB_env *env, int prev, MDB_meta *meta)
 {
 	MDB_metabuf	pbuf;
 	MDB_page	*p;
@@ -4325,7 +4326,7 @@ mdb_env_read_header(MDB_env *env, MDB_meta *meta)
 			return MDB_VERSION_MISMATCH;
 		}
 
-		if (off == 0 || m->mm_txnid > meta->mm_txnid)
+		if (off == 0 || (prev ? m->mm_txnid < meta->mm_txnid : m->mm_txnid > meta->mm_txnid))
 			*meta = *m;
 	}
 	return 0;
@@ -4965,7 +4966,7 @@ mdb_fopen(const MDB_env *env, MDB_name *fname,
 /** Further setup required for opening an LMDB environment
  */
 static int ESECT
-mdb_env_open2(MDB_env *env)
+mdb_env_open2(MDB_env *env, int prev)
 {
 	unsigned int flags = env->me_flags;
 	int i, newenv = 0, rc;
@@ -5028,7 +5029,7 @@ mdb_env_open2(MDB_env *env)
 	}
 #endif
 
-	if ((i = mdb_env_read_header(env, &meta)) != 0) {
+	if ((i = mdb_env_read_header(env, prev, &meta)) != 0) {
 		if (i != ENOENT)
 			return i;
 		DPUTS("new mdbenv");
@@ -5706,7 +5707,7 @@ mdb_env_envflags(MDB_env *env)
 	 */
 #define	CHANGEABLE	(MDB_NOSYNC|MDB_NOMETASYNC|MDB_MAPASYNC|MDB_NOMEMINIT)
 #define	CHANGELESS	(MDB_FIXEDMAP|MDB_NOSUBDIR|MDB_RDONLY| \
-	MDB_WRITEMAP|MDB_NOTLS|MDB_NOLOCK|MDB_NORDAHEAD|MDB_REMAP_CHUNKS)
+	MDB_WRITEMAP|MDB_NOTLS|MDB_NOLOCK|MDB_NORDAHEAD|MDB_PREVMETA|MDB_REMAP_CHUNKS)
 #define EXPOSED		(CHANGEABLE|CHANGELESS | MDB_ENCRYPT)
 
 #if VALID_FLAGS & PERSISTENT_FLAGS & EXPOSED
@@ -5819,7 +5820,7 @@ mdb_env_open(MDB_env *env, const char *path, unsigned int flags, mdb_mode_t mode
 			goto leave;
 	}
 
-	if ((rc = mdb_env_open2(env)) == MDB_SUCCESS) {
+	if ((rc = mdb_env_open2(env, flags & MDB_PREVMETA)) == MDB_SUCCESS) {
 		if (!(flags & (MDB_RDONLY|MDB_WRITEMAP))) {
 			/* Synchronous fd for meta writes. Needed even with
 			 * MDB_NOSYNC/MDB_NOMETASYNC, in case these get reset.