SERVER-63651 Support non-monotonic clocks in PrepareConflictTracker

3 files changed, 29 insertions, 7 deletions

diff --git a/src/mongo/db/prepare_conflict_tracker.cpp b/src/mongo/db/prepare_conflict_tracker.cpp
index 7dc8123e282..e8e031a1405 100644
--- a/src/mongo/db/prepare_conflict_tracker.cpp
+++ b/src/mongo/db/prepare_conflict_tracker.cpp
@@ -51,14 +51,8 @@ void PrepareConflictTracker::endPrepareConflict(OperationContext* opCtx) {
         auto tickSource = opCtx->getServiceContext()->getTickSource();
         auto curTick = tickSource->getTicks();
 
-        invariant(_prepareConflictStartTime <= curTick,
-                  str::stream() << "Prepare conflict start time ("
-                                << tickSource->ticksTo<Microseconds>(_prepareConflictStartTime)
-                                << ") is somehow greater than current time ("
-                                << tickSource->ticksTo<Microseconds>(curTick) << ")");
-
         auto curConflictDuration =
-            tickSource->ticksTo<Microseconds>(curTick - _prepareConflictStartTime);
+            tickSource->spanTo<Microseconds>(_prepareConflictStartTime, curTick);
         _prepareConflictDuration.store(_prepareConflictDuration.load() + curConflictDuration);
         _prepareConflictStartTime = 0;
 
diff --git a/src/mongo/util/tick_source.h b/src/mongo/util/tick_source.h
index f77ea9d1d6f..5b0abc88aca 100644
--- a/src/mongo/util/tick_source.h
+++ b/src/mongo/util/tick_source.h
@@ -29,6 +29,7 @@
 
 #pragma once
 
+#include <algorithm>
 #include <cstdint>
 
 namespace mongo {
@@ -64,5 +65,18 @@ public:
             static_cast<double>(getTicksPerSecond()) * D::period::num / D::period::den;
         return D(static_cast<int64_t>(ticks / ticksPerD));
     }
+
+    /**
+     * Measures the length of the span from the start tick to the end tick and returns the result
+     * using duration type D.
+     * If the start tick is after (greater than) the end tick, returns a duration equivalent to 0
+     * ticks.
+     *
+     * e.g. tickSource->spanTo<Milliseconds>(start, end);
+     */
+    template <typename D>
+    D spanTo(Tick start, Tick end) {
+        return ticksTo<D>(std::max((end - start), Tick{0}));
+    }
 };
 }  // namespace mongo
diff --git a/src/mongo/util/tick_source_test.cpp b/src/mongo/util/tick_source_test.cpp
index aef28a7e97c..cf08b69cbdc 100644
--- a/src/mongo/util/tick_source_test.cpp
+++ b/src/mongo/util/tick_source_test.cpp
@@ -52,5 +52,19 @@ TEST(TickSourceTest, TicksToDurationConversion) {
     tsMicros.reset(1);
     ASSERT_EQ(tsMicros.ticksTo<Microseconds>(tsMicros.getTicks()).count(), 1);
 }
+
+TEST(TickSourceTest, SpansToDurationConversion) {
+    TickSourceMock<Seconds> tsSecs;
+    tsSecs.reset(0);
+    TickSource::Tick zero = tsSecs.getTicks();
+    tsSecs.reset(10);
+    TickSource::Tick ten = tsSecs.getTicks();
+    ASSERT_EQ(tsSecs.spanTo<Seconds>(zero, ten).count(), 10);
+    ASSERT_EQ(tsSecs.spanTo<Seconds>(ten, zero).count(), 0);
+    ASSERT_EQ(tsSecs.spanTo<Milliseconds>(zero, ten).count(), 10 * 1000);
+    ASSERT_EQ(tsSecs.spanTo<Milliseconds>(ten, zero).count(), 0);
+    ASSERT_EQ(tsSecs.spanTo<Microseconds>(zero, ten).count(), 10 * 1000 * 1000);
+    ASSERT_EQ(tsSecs.spanTo<Microseconds>(ten, zero).count(), 0);
+}
 }  // namespace
 }  // namespace mongo