HDFS-16642. Moving the selecting inputstream from journalnode in EditLogTailer outof FSNLock (#4497)

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/ha/EditLogTailer.java

@@ -328,34 +328,37 @@ public class EditLogTailer {
   
   @VisibleForTesting
   public long doTailEdits() throws IOException, InterruptedException {
+    Collection<EditLogInputStream> streams;
+    FSImage image = namesystem.getFSImage();
+
+    long lastTxnId = image.getLastAppliedTxId();
+    LOG.debug("lastTxnId: {}", lastTxnId);
+    long startTime = timer.monotonicNow();
+    try {
+      streams = editLog.selectInputStreams(lastTxnId + 1, 0,
+          null, inProgressOk, true);
+    } catch (IOException ioe) {
+      // This is acceptable. If we try to tail edits in the middle of an edits
+      // log roll, i.e. the last one has been finalized but the new inprogress
+      // edits file hasn't been started yet.
+      LOG.warn("Edits tailer failed to find any streams. Will try again " +
+          "later.", ioe);
+      return 0;
+    } finally {
+      NameNode.getNameNodeMetrics().addEditLogFetchTime(
+          timer.monotonicNow() - startTime);
+    }
     // Write lock needs to be interruptible here because the 
     // transitionToActive RPC takes the write lock before calling
     // tailer.stop() -- so if we're not interruptible, it will
     // deadlock.
     namesystem.writeLockInterruptibly();
     try {
-      FSImage image = namesystem.getFSImage();
-
-      long lastTxnId = image.getLastAppliedTxId();
-      
-      if (LOG.isDebugEnabled()) {
-        LOG.debug("lastTxnId: " + lastTxnId);
-      }
-      Collection<EditLogInputStream> streams;
-      long startTime = timer.monotonicNow();
-      try {
-        streams = editLog.selectInputStreams(lastTxnId + 1, 0,
-            null, inProgressOk, true);
-      } catch (IOException ioe) {
-        // This is acceptable. If we try to tail edits in the middle of an edits
-        // log roll, i.e. the last one has been finalized but the new inprogress
-        // edits file hasn't been started yet.
-        LOG.warn("Edits tailer failed to find any streams. Will try again " +
-            "later.", ioe);
+      long currentLastTxnId = image.getLastAppliedTxId();
+      if (lastTxnId != currentLastTxnId) {
+        LOG.warn("The currentLastTxnId({}) is different from preLastTxtId({})",
+            currentLastTxnId, lastTxnId);
         return 0;
-      } finally {
-        NameNode.getNameNodeMetrics().addEditLogFetchTime(
-            timer.monotonicNow() - startTime);
       }
       if (LOG.isDebugEnabled()) {
         LOG.debug("edit streams to load from: " + streams.size());