[GitHub] [zookeeper] enixon commented on a change in pull request #1069: ZOOKEEPER-3056: Fails to load database with missing snapshot file but with valid transaction log file.

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enixon commented on a change in pull request #1069: ZOOKEEPER-3056: Fails to load database with missing snapshot file but with valid transaction log file.
URL: https://github.com/apache/zookeeper/pull/1069#discussion_r319670645
 
 

 ##########
 File path: zookeeper-server/src/main/java/org/apache/zookeeper/server/persistence/FileTxnSnapLog.java
 ##########
 @@ -232,11 +240,19 @@ public long restore(DataTree dt, Map<Long, Integer> sessions, PlayBackListener l
         } else {
             trustEmptyDB = autoCreateDB;
         }
+
         if (-1L == deserializeResult) {
             /* this means that we couldn't find any snapshot, so we need to
              * initialize an empty database (reported in ZOOKEEPER-2325) */
             if (txnLog.getLastLoggedZxid() != -1) {
-                throw new IOException("No snapshot found, but there are log entries. " + "Something is broken!");
+                // ZOOKEEPER-3056: provides an escape hatch for users upgrading
+                // from old versions of zookeeper (3.4.x, pre 3.5.3).
+                if (!trustEmptySnapshot) {
+                    throw new IOException(EMPTY_SNAPSHOT_WARNING + "Something is broken!");
+                } else {
+                    LOG.warn(EMPTY_SNAPSHOT_WARNING + "This should only be allowed during upgrading.");
+                    trustEmptySnapshot = false;
+                }
             }
 
             if (trustEmptyDB) {
 
 Review comment:
   If `trustEmptyDB` is not set does the code (1) load the transaction log and (2) create a snapshot? With the second question, can we have a scenario where clearing `trustEmptySnapshot` causes the Peer to shutdown because it cannot find a snapshot on a second pass?

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