SERVER-48070: account for system clock drift when calculating the time since the last isMaster

1 files changed, 4 insertions, 1 deletions

diff --git a/src/mongo/client/server_is_master_monitor.cpp b/src/mongo/client/server_is_master_monitor.cpp
index 321bffca176..7fe68565ca4 100644
--- a/src/mongo/client/server_is_master_monitor.cpp
+++ b/src/mongo/client/server_is_master_monitor.cpp
@@ -166,7 +166,10 @@ boost::optional<Milliseconds> SingleServerIsMasterMonitor::calculateExpeditedDel
 }
 
 boost::optional<Milliseconds> SingleServerIsMasterMonitor::_timeSinceLastCheck() const {
-    return (_lastIsMasterAt) ? boost::optional<Milliseconds>(_executor->now() - *_lastIsMasterAt)
+    // Since the system clock is not monotonic, the returned value can be negative. In this case we
+    // choose a conservative estimate of 0ms as the time we last checked.
+    return (_lastIsMasterAt) ? boost::optional<Milliseconds>(
+                                   std::max(Milliseconds(0), _executor->now() - *_lastIsMasterAt))
                              : boost::none;
 }