SERVER-34768 Sync source's optime cannot be behind the syncing node even if chaining is disabled.

(cherry picked from commit 319757ebb72611fb91044a2a81d1b77a6f3729c1)

SERVER-46050 Use getLastAppliedOpTime rather than getHeartbeatAppliedOpTime for checking primary's position.

2 files changed, 25 insertions, 0 deletions

diff --git a/src/mongo/db/repl/topology_coordinator.cpp b/src/mongo/db/repl/topology_coordinator.cpp
index 841b3d42b7b..24abcf15c68 100644
--- a/src/mongo/db/repl/topology_coordinator.cpp
+++ b/src/mongo/db/repl/topology_coordinator.cpp
@@ -287,6 +287,15 @@ HostAndPort TopologyCoordinator::chooseNewSyncSource(Date_t now,
                 << "Cannot select a sync source because chaining is not allowed and we are primary";
             _syncSource = HostAndPort();
             return _syncSource;
+        } else if (_memberData.at(_currentPrimaryIndex).getLastAppliedOpTime() <
+                   lastOpTimeFetched) {
+            LOG(1) << "Cannot select a sync source because chaining is not allowed and the primary "
+                      "is behind me. Last oplog optime of primary "
+                   << _currentPrimaryMember()->getHostAndPort() << ": "
+                   << _memberData.at(_currentPrimaryIndex).getLastAppliedOpTime().toBSON()
+                   << ", my last fetched oplog optime: " << lastOpTimeFetched.toBSON();
+            _syncSource = HostAndPort();
+            return _syncSource;
         } else {
             _syncSource = _currentPrimaryMember()->getHostAndPort();
             log() << "chaining not allowed, choosing primary as sync source candidate: "
diff --git a/src/mongo/db/repl/topology_coordinator_v1_test.cpp b/src/mongo/db/repl/topology_coordinator_v1_test.cpp
index c3d88099458..249b59f1eea 100644
--- a/src/mongo/db/repl/topology_coordinator_v1_test.cpp
+++ b/src/mongo/db/repl/topology_coordinator_v1_test.cpp
@@ -633,6 +633,22 @@ TEST_F(TopoCoordTest, ChooseOnlyPrimaryAsSyncSourceWhenChainingIsDisallowed) {
                         Milliseconds(300));
     ASSERT_EQUALS(2, getCurrentPrimaryIndex());
 
+    // h3 is primary, but its last applied isn't as up-to-date as ours, so it cannot be chosen
+    // as the sync source.
+    ASSERT_EQUALS(HostAndPort(),
+                  getTopoCoord().chooseNewSyncSource(
+                      now()++,
+                      OpTime(Timestamp(10, 0), 0),
+                      TopologyCoordinator::ChainingPreference::kUseConfiguration));
+    ASSERT_EQUALS(HostAndPort(), getTopoCoord().getSyncSourceAddress());
+
+    // Update the primary's position.
+    heartbeatFromMember(HostAndPort("h3"),
+                        "rs0",
+                        MemberState::RS_PRIMARY,
+                        OpTime(Timestamp(10, 0), 0),
+                        Milliseconds(300));
+
     // h3 is primary and should be chosen as the sync source when we are not in catch-up mode,
     // despite being further away than h2 and the primary (h3) being behind our most recently
     // applied optime.