ZOOKEEPER-1936: Server exits when unable to create data directory due to race

Resurrecting an ancient ticket which could be fixed with a simple patch.

Jira mentions a scenario when auto purging tool is in use and Zookeeper server could have a race condition when creating snapshot and data directories. (directory auto creating is enabled by default)

Double checking the directory existence might help with it.

Author: Andor Molnar <andor@apache.org>

Reviewers: Enrico Olivelli <eolivelli@apache.org>

Closes #1225 from anmolnar/ZOOKEEPER-1936

zookeeper-server/src/main/java/org/apache/zookeeper/server/persistence/FileTxnSnapLog.java

@@ -132,7 +132,7 @@ public class FileTxnSnapLog {
                     ZOOKEEPER_DATADIR_AUTOCREATE));
             }
 
-            if (!this.dataDir.mkdirs()) {
+            if (!this.dataDir.mkdirs() && !this.dataDir.exists()) {
                 throw new DatadirException("Unable to create data directory " + this.dataDir);
             }
         }
@@ -151,7 +151,7 @@ public class FileTxnSnapLog {
                     ZOOKEEPER_DATADIR_AUTOCREATE));
             }
 
-            if (!this.snapDir.mkdirs()) {
+            if (!this.snapDir.mkdirs() && !this.snapDir.exists()) {
                 throw new DatadirException("Unable to create snap directory " + this.snapDir);
             }
         }