ITS#7970 LMDB: Critical Heisenbug

Inconsistent reading & SIGSEGV due to the race condition.

diff --git a/libraries/liblmdb/mdb.c b/libraries/liblmdb/mdb.c
index 89d4a2a2575b1382e77b91939fabf1fede783c86..27dbb9b1ae5857160441681b9de357395b804170 100644 (file)
--- a/libraries/liblmdb/mdb.c
+++ b/libraries/liblmdb/mdb.c
@@ -2539,7 +2539,10 @@ mdb_txn_renew0(MDB_txn *txn)
                                        return rc;
                                }
                        }
-                       txn->mt_txnid = r->mr_txnid = ti->mti_txnid;
+                       do /* LY: Retry on a race, ITS#7970. */
+                               r->mr_txnid = ti->mti_txnid;
+                       while(r->mr_txnid != ti->mti_txnid);
+                       txn->mt_txnid = r->mr_txnid;
                        txn->mt_u.reader = r;
                        meta = env->me_metas[txn->mt_txnid & 1];
                }