tdb2: allow transaction to nest.

This is definitely a bad idea in general, but SAMBA uses nested transactions
in many and varied ways (some of them probably reflect real bugs) and it's
far easier to support them inside tdb2 with a flag.

We already have part of the TDB1 infrastructure in place, so this patch
just completes it and fixes one place where I'd messed it up.

ccan/tdb2/open.c

@@ -383,7 +383,7 @@ struct tdb_context *tdb_open(const char *name, int tdb_flags,
 	}
 
 	if (tdb_flags & ~(TDB_INTERNAL | TDB_NOLOCK | TDB_NOMMAP | TDB_CONVERT
-			  | TDB_NOSYNC | TDB_SEQNUM)) {
+			  | TDB_NOSYNC | TDB_SEQNUM | TDB_ALLOW_NESTING)) {
 		ecode = tdb_logerr(tdb, TDB_ERR_EINVAL, TDB_LOG_USE_ERROR,
 				   "tdb_open: unknown flags %u", tdb_flags);
 		goto fail;

ccan/tdb2/tdb.c

@@ -327,6 +327,9 @@ void tdb_add_flag(struct tdb_context *tdb, unsigned flag)
 	case TDB_SEQNUM:
 		tdb->flags |= TDB_SEQNUM;
 		break;
+	case TDB_ALLOW_NESTING:
+		tdb->flags |= TDB_ALLOW_NESTING;
+		break;
 	default:
 		tdb->last_error = tdb_logerr(tdb, TDB_ERR_EINVAL,
 					     TDB_LOG_USE_ERROR,
@@ -357,6 +360,9 @@ void tdb_remove_flag(struct tdb_context *tdb, unsigned flag)
 	case TDB_SEQNUM:
 		tdb->flags &= ~TDB_SEQNUM;
 		break;
+	case TDB_ALLOW_NESTING:
+		tdb->flags &= ~TDB_ALLOW_NESTING;
+		break;
 	default:
 		tdb->last_error = tdb_logerr(tdb, TDB_ERR_EINVAL,
 					     TDB_LOG_USE_ERROR,

ccan/tdb2/tdb2.h

@@ -80,6 +80,7 @@ struct tdb_context *tdb_open(const char *name, int tdb_flags,
 #define TDB_CONVERT 16 /* convert endian */
 #define TDB_NOSYNC   64 /* don't use synchronous transactions */
 #define TDB_SEQNUM   128 /* maintain a sequence number */
+#define TDB_ALLOW_NESTING   256 /* fake nested transactions */
 
 /**
  * tdb_close - close and free a tdb.
@@ -248,6 +249,15 @@ static inline struct tdb_data tdb_mkdata(const void *p, size_t len)
  * to read the tdb, but not alter it (they will block), nor will they see
  * any changes until tdb_transaction_commit() is called.
  *
+ * Note that if the TDB_ALLOW_NESTING flag is set, a tdb_transaction_start()
+ * within a transaction will succeed, but it's not a real transaction:
+ * (1) An inner transaction which is committed is not actually committed until
+ *     the outer transaction is; if the outer transaction is cancelled, the
+ *     inner ones are discarded.
+ * (2) tdb_transaction_cancel() marks the outer transaction as having an error,
+ *     so the final tdb_transaction_commit() will fail.
+ * (3) the outer transaction will see the results of the inner transaction.
+ *
  * See Also:
  *	tdb_transaction_cancel, tdb_transaction_commit.
  */
@@ -565,7 +575,7 @@ unsigned int tdb_get_flags(struct tdb_context *tdb);
 /**
  * tdb_add_flag - set a flag for a tdb
  * @tdb: the tdb context returned from tdb_open()
- * @flag: one of TDB_NOLOCK, TDB_NOMMAP or TDB_NOSYNC.
+ * @flag: one of TDB_NOLOCK, TDB_NOMMAP, TDB_NOSYNC or TDB_ALLOW_NESTING.
  *
  * You can use this to set a flag on the TDB.  You cannot set these flags
  * on a TDB_INTERNAL tdb.
@@ -575,7 +585,7 @@ void tdb_add_flag(struct tdb_context *tdb, unsigned flag);
 /**
  * tdb_remove_flag - unset a flag for a tdb
  * @tdb: the tdb context returned from tdb_open()
- * @flag: one of TDB_NOLOCK, TDB_NOMMAP or TDB_NOSYNC.
+ * @flag: one of TDB_NOLOCK, TDB_NOMMAP, TDB_NOSYNC or TDB_ALLOW_NESTING.
  *
  * You can use this to clear a flag on the TDB.  You cannot clear flags
  * on a TDB_INTERNAL tdb.

ccan/tdb2/transaction.c

@@ -110,7 +110,7 @@ struct tdb_transaction {
 	/* when inside a transaction we need to keep track of any
 	   nested tdb_transaction_start() calls, as these are allowed,
 	   but don't create a new transaction */
-	int nesting;
+	unsigned int nesting;
 
 	/* set when a prepare has already occurred */
 	bool prepared;
@@ -531,11 +531,15 @@ enum TDB_ERROR tdb_transaction_start(struct tdb_context *tdb)
 
 	/* cope with nested tdb_transaction_start() calls */
 	if (tdb->transaction != NULL) {
-		return tdb->last_error = tdb_logerr(tdb, TDB_ERR_IO,
+		if (!(tdb->flags & TDB_ALLOW_NESTING)) {
+			return tdb->last_error
+				= tdb_logerr(tdb, TDB_ERR_IO,
 					     TDB_LOG_USE_ERROR,
 					     "tdb_transaction_start:"
-						    " already inside"
-						    " transaction");
+					     " already inside transaction");
+		}
+		tdb->transaction->nesting++;
+		return 0;
 	}
 
 	if (tdb_has_hash_locks(tdb)) {
@@ -907,7 +911,6 @@ static enum TDB_ERROR _tdb_transaction_prepare_commit(struct tdb_context *tdb)
 
 
 	if (tdb->transaction->nesting != 0) {
-		tdb->transaction->nesting--;
 		return TDB_SUCCESS;
 	}