Account trimAboveSeqNo in committed translog generation (#50205)

Today we do not consider trimAboveSeqNo when calculating the translog 
generation of an index commit. If there is no new indexing after the
primary promotion, then we won't be able to clean up the translog.

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

@@ -106,6 +106,17 @@ private void write(DataOutput out) throws IOException {
         out.writeLong(trimmedAboveSeqNo);
     }
 
+    /**
+     * Returns the maximum sequence number of operations in this checkpoint after applying {@link #trimmedAboveSeqNo}.
+     */
+    long maxEffectiveSeqNo() {
+        if (trimmedAboveSeqNo == SequenceNumbers.UNASSIGNED_SEQ_NO) {
+            return maxSeqNo;
+        } else {
+            return Math.min(trimmedAboveSeqNo, maxSeqNo);
+        }
+    }
+
     static Checkpoint emptyTranslogCheckpoint(final long offset, final long generation, final long globalCheckpoint,
                                               long minTranslogGeneration) {
         final long minSeqNo = SequenceNumbers.NO_OPS_PERFORMED;

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

@@ -696,11 +696,7 @@ private Stream<? extends BaseTranslogReader> readersAboveMinSeqNo(long minSeqNo)
         assert readLock.isHeldByCurrentThread() || writeLock.isHeldByCurrentThread() :
             "callers of readersAboveMinSeqNo must hold a lock: readLock ["
                 + readLock.isHeldByCurrentThread() + "], writeLock [" + readLock.isHeldByCurrentThread() + "]";
-        return Stream.concat(readers.stream(), Stream.of(current))
-            .filter(reader -> {
-                final long maxSeqNo = reader.getCheckpoint().maxSeqNo;
-                return maxSeqNo == SequenceNumbers.UNASSIGNED_SEQ_NO || maxSeqNo >= minSeqNo;
-            });
+        return Stream.concat(readers.stream(), Stream.of(current)).filter(reader -> minSeqNo <= reader.getCheckpoint().maxEffectiveSeqNo());
     }
 
     /**
@@ -1629,7 +1625,7 @@ public TranslogGeneration getMinGenerationForSeqNo(final long seqNo) {
              */
             long minTranslogFileGeneration = this.currentFileGeneration();
             for (final TranslogReader reader : readers) {
-                if (seqNo <= reader.getCheckpoint().maxSeqNo) {
+                if (seqNo <= reader.getCheckpoint().maxEffectiveSeqNo()) {
                     minTranslogFileGeneration = Math.min(minTranslogFileGeneration, reader.getGeneration());
                 }
             }

server/src/test/java/org/elasticsearch/index/replication/RecoveryDuringReplicationTests.java

@@ -800,6 +800,11 @@ public void testRollbackOnPromotion() throws Exception {
             shards.assertAllEqual(initDocs + inFlightOpsOnNewPrimary + moreDocsAfterRollback);
             done.set(true);
             thread.join();
+
+            for (IndexShard shard : shards) {
+                shard.flush(new FlushRequest().force(true).waitIfOngoing(true));
+                assertThat(shard.translogStats().getUncommittedOperations(), equalTo(0));
+            }
         }
     }