SERVER-36554 Make computeAtClusterTimeForOneShard return latest clusterTime if there is no cached last committed opTime

3 files changed, 36 insertions, 2 deletions

diff --git a/src/mongo/s/commands/cluster_commands_helpers.cpp b/src/mongo/s/commands/cluster_commands_helpers.cpp
index dedd07a3acc..6535ff03d4d 100644
--- a/src/mongo/s/commands/cluster_commands_helpers.cpp
+++ b/src/mongo/s/commands/cluster_commands_helpers.cpp
@@ -616,7 +616,16 @@ boost::optional<LogicalTime> computeAtClusterTimeForOneShard(OperationContext* o
 
     auto shardRegistry = Grid::get(opCtx)->shardRegistry();
     invariant(shardRegistry);
-    return shardRegistry->getShardNoReload(shardId)->getLastCommittedOpTime();
+
+    auto shard = shardRegistry->getShardNoReload(shardId);
+    uassert(ErrorCodes::ShardNotFound, str::stream() << "Could not find shard " << shardId, shard);
+
+    // Return the cached last committed opTime for the shard if there is one, otherwise return the
+    // lastest cluster time from the logical clock.
+    auto lastCommittedOpTime = shard->getLastCommittedOpTime();
+    return lastCommittedOpTime != LogicalTime::kUninitialized
+        ? lastCommittedOpTime
+        : LogicalClock::get(opCtx)->getClusterTime();
 }
 
 namespace {
diff --git a/src/mongo/s/commands/cluster_commands_helpers.h b/src/mongo/s/commands/cluster_commands_helpers.h
index bc848435357..6c5e89af0f0 100644
--- a/src/mongo/s/commands/cluster_commands_helpers.h
+++ b/src/mongo/s/commands/cluster_commands_helpers.h
@@ -215,7 +215,8 @@ std::set<ShardId> getTargetedShardsForQuery(OperationContext* opCtx,
                                             const BSONObj& collation);
 
 /**
- * Returns the latest known lastCommittedOpTime for the targeted shard.
+ * Returns the latest known lastCommittedOpTime for the targeted shard, or the latest in-memory
+ * cluster time if there is none.
  *
  * A null logical time is returned if the readConcern on the OperationContext is not snapshot.
  */
diff --git a/src/mongo/s/commands/compute_at_cluster_time_test.cpp b/src/mongo/s/commands/compute_at_cluster_time_test.cpp
index 4d34d390326..8dfccf5754a 100644
--- a/src/mongo/s/commands/compute_at_cluster_time_test.cpp
+++ b/src/mongo/s/commands/compute_at_cluster_time_test.cpp
@@ -123,6 +123,30 @@ TEST_F(AtClusterTimeTest, ComputeInvalidInvalid) {
     ASSERT_EQ(*maxTime, kInMemoryLogicalTime);
 }
 
+TEST_F(AtClusterTimeTest, ComputeForOneShard) {
+    auto shardOne = shardRegistry()->getShardNoReload(shardOneId);
+
+    LogicalTime timeOne(Timestamp(10, 2));
+    shardOne->updateLastCommittedOpTime(timeOne);
+    ASSERT_EQ(timeOne, shardOne->getLastCommittedOpTime());
+
+    auto atClusterTime = computeAtClusterTimeForOneShard(operationContext(), shardOneId);
+    ASSERT_EQ(*atClusterTime, timeOne);
+}
+
+TEST_F(AtClusterTimeTest, ComputeForOneShardNoCachedOpTime) {
+    auto shardOne = shardRegistry()->getShardNoReload(shardOneId);
+    ASSERT_EQ(LogicalTime(), shardOne->getLastCommittedOpTime());
+
+    auto atClusterTime = computeAtClusterTimeForOneShard(operationContext(), shardOneId);
+    ASSERT_EQ(*atClusterTime, kInMemoryLogicalTime);
+}
+
+TEST_F(AtClusterTimeTest, ComputeForOneShardNoShard) {
+    ASSERT_THROWS_CODE(computeAtClusterTimeForOneShard(operationContext(), ShardId("fakeShard")),
+                       AssertionException,
+                       ErrorCodes::ShardNotFound);
+}
 
 class AtClusterTimeTargetingTest : public CatalogCacheTestFixture {
 protected: