Separate translog from index deletion conditions (#52556)

Separates the translog from the index deletion conditions (allowing the translog to be cleaned
up more eagerly), and avoids taking the write lock on the translog if no clean-up is actually
necessary.

server/src/main/java/org/elasticsearch/index/engine/InternalEngine.java

@@ -539,8 +539,8 @@ public Translog.Location getTranslogLastWriteLocation() {
     private void revisitIndexDeletionPolicyOnTranslogSynced() throws IOException {
         if (combinedDeletionPolicy.hasUnreferencedCommits()) {
             indexWriter.deleteUnusedFiles();
-            translog.trimUnreferencedReaders();
         }
+        translog.trimUnreferencedReaders();
     }
 
     @Override

server/src/main/java/org/elasticsearch/index/translog/Translog.java

@@ -1646,24 +1646,25 @@ public void rollGeneration() throws IOException {
      * required generation
      */
     public void trimUnreferencedReaders() throws IOException {
-        // move most of the data to disk to reduce the time the lock is held
+        // first check under read lock if any readers can be trimmed
+        try (ReleasableLock ignored = readLock.acquire()) {
+            if (closed.get()) {
+                // we're shutdown potentially on some tragic event, don't delete anything
+                return;
+            }
+            if (getMinReferencedGen() == getMinFileGeneration()) {
+                return;
+            }
+        }
+
+        // move most of the data to disk to reduce the time the write lock is held
         sync();
         try (ReleasableLock ignored = writeLock.acquire()) {
             if (closed.get()) {
                 // we're shutdown potentially on some tragic event, don't delete anything
                 return;
             }
-            long minReferencedGen = Math.min(
-                deletionPolicy.getMinTranslogGenRequiredByLocks(),
-                minGenerationForSeqNo(deletionPolicy.getLocalCheckpointOfSafeCommit() + 1, current, readers));
-            assert minReferencedGen >= getMinFileGeneration() :
-                "deletion policy requires a minReferenceGen of [" + minReferencedGen + "] but the lowest gen available is ["
-                    + getMinFileGeneration() + "]";
-            assert minReferencedGen <= currentFileGeneration() :
-                "deletion policy requires a minReferenceGen of [" + minReferencedGen + "] which is higher than the current generation ["
-                    + currentFileGeneration() + "]";
-
-
+            final long minReferencedGen = getMinReferencedGen();
             for (Iterator<TranslogReader> iterator = readers.iterator(); iterator.hasNext(); ) {
                 TranslogReader reader = iterator.next();
                 if (reader.getGeneration() >= minReferencedGen) {
@@ -1690,6 +1691,20 @@ public void trimUnreferencedReaders() throws IOException {
         }
     }
 
+    private long getMinReferencedGen() {
+        assert readLock.isHeldByCurrentThread() || writeLock.isHeldByCurrentThread();
+        long minReferencedGen = Math.min(
+            deletionPolicy.getMinTranslogGenRequiredByLocks(),
+            minGenerationForSeqNo(deletionPolicy.getLocalCheckpointOfSafeCommit() + 1, current, readers));
+        assert minReferencedGen >= getMinFileGeneration() :
+            "deletion policy requires a minReferenceGen of [" + minReferencedGen + "] but the lowest gen available is ["
+                + getMinFileGeneration() + "]";
+        assert minReferencedGen <= currentFileGeneration() :
+            "deletion policy requires a minReferenceGen of [" + minReferencedGen + "] which is higher than the current generation ["
+                + currentFileGeneration() + "]";
+        return minReferencedGen;
+    }
+
     /**
      * deletes all files associated with a reader. package-private to be able to simulate node failures at this point
      */