8 files changed, 52 insertions, 18 deletions
diff --git a/src/mongo/util/background_thread_clock_source.cpp b/src/mongo/util/background_thread_clock_source.cpp
index 96dececff9c..3a2d43c1f82 100644
--- a/src/mongo/util/background_thread_clock_source.cpp
+++ b/src/mongo/util/background_thread_clock_source.cpp
@@ -56,6 +56,10 @@ BackgroundThreadClockSource::~BackgroundThreadClockSource() {
     _timer.join();
 }
 
+Milliseconds BackgroundThreadClockSource::getPrecision() {
+    return _granularity;
+}
+
 Date_t BackgroundThreadClockSource::now() {
     return Date_t::fromMillisSinceEpoch(_current.load());
 }
diff --git a/src/mongo/util/background_thread_clock_source.h b/src/mongo/util/background_thread_clock_source.h
index eecd75c0f4c..c532efd3817 100644
--- a/src/mongo/util/background_thread_clock_source.h
+++ b/src/mongo/util/background_thread_clock_source.h
@@ -53,6 +53,7 @@ class BackgroundThreadClockSource final : public ClockSource {
 public:
     BackgroundThreadClockSource(std::unique_ptr<ClockSource> clockSource, Milliseconds granularity);
     ~BackgroundThreadClockSource() override;
+    Milliseconds getPrecision() override;
     Date_t now() override;
 
 private:
diff --git a/src/mongo/util/background_thread_clock_source_test.cpp b/src/mongo/util/background_thread_clock_source_test.cpp
index 5bd82e4186e..69bb997ea56 100644
--- a/src/mongo/util/background_thread_clock_source_test.cpp
+++ b/src/mongo/util/background_thread_clock_source_test.cpp
@@ -39,29 +39,39 @@ namespace {
 
 using namespace mongo;
 
-TEST(BackgroundThreadClockSource, CreateAndTerminate) {
-    auto clockSource = stdx::make_unique<ClockSourceMock>();
-    auto btClockSource =
-        stdx::make_unique<BackgroundThreadClockSource>(std::move(clockSource), Milliseconds(1));
-    btClockSource.reset();  // destroys the clock source
+class BTCSTest : public mongo::unittest::Test {
+public:
+    void setUpClocks(Milliseconds granularity) {
+        auto csMock = stdx::make_unique<ClockSourceMock>();
+        _csMock = csMock.get();
+        _btcs = stdx::make_unique<BackgroundThreadClockSource>(std::move(csMock), granularity);
+    }
 
-    clockSource = stdx::make_unique<ClockSourceMock>();
-    btClockSource =
-        stdx::make_unique<BackgroundThreadClockSource>(std::move(clockSource), Hours(48));
-    btClockSource.reset();  // destroys the clock source
+protected:
+    std::unique_ptr<ClockSource> _btcs;
+    ClockSourceMock* _csMock;
+};
+
+TEST_F(BTCSTest, CreateAndTerminate) {
+    setUpClocks(Milliseconds(1));
+    _btcs.reset();  // destroys the clock source
+
+    setUpClocks(Hours(48));
+    _btcs.reset();
 }
 
-TEST(BackgroundThreadClockSource, TimeKeeping) {
-    auto clockSource = stdx::make_unique<ClockSourceMock>();
-    ClockSourceMock* clockSourceMock = clockSource.get();
+TEST_F(BTCSTest, TimeKeeping) {
+    setUpClocks(Milliseconds(1));
+    ASSERT_EQUALS(_btcs->now(), _csMock->now());
 
-    auto btClockSource =
-        stdx::make_unique<BackgroundThreadClockSource>(std::move(clockSource), Milliseconds(1));
-    ASSERT_EQUALS(btClockSource->now(), clockSourceMock->now());
+    _csMock->advance(Milliseconds(100));
+    sleepFor(Seconds(1));  // give the _btcs opportunity to read the new time
+    ASSERT_EQUALS(_btcs->now(), _csMock->now());
+}
 
-    clockSourceMock->advance(Milliseconds(100));
-    sleepFor(Milliseconds(10));  // give the btClockSource opportunity to read the new time
-    ASSERT_EQUALS(btClockSource->now(), clockSourceMock->now());
+TEST_F(BTCSTest, GetPrecision) {
+    setUpClocks(Milliseconds(1));
+    ASSERT_EQUALS(_btcs->getPrecision(), Milliseconds(1));
 }
 
 }  // namespace
diff --git a/src/mongo/util/clock_source.h b/src/mongo/util/clock_source.h
index 6ba47e6fce3..2c6ad1b6da3 100644
--- a/src/mongo/util/clock_source.h
+++ b/src/mongo/util/clock_source.h
@@ -28,6 +28,8 @@
 
 #pragma once
 
+#include "mongo/util/time_support.h"
+
 namespace mongo {
 
 class Date_t;
@@ -40,6 +42,11 @@ public:
     virtual ~ClockSource() = default;
 
     /**
+     * Returns the minimum time change that the clock can describe.
+     */
+    virtual Milliseconds getPrecision() = 0;
+
+    /**
      * Returns the current wall clock time, as defined by this source.
      */
     virtual Date_t now() = 0;
diff --git a/src/mongo/util/clock_source_mock.cpp b/src/mongo/util/clock_source_mock.cpp
index e18ac5a823e..a85b3045a0f 100644
--- a/src/mongo/util/clock_source_mock.cpp
+++ b/src/mongo/util/clock_source_mock.cpp
@@ -29,9 +29,14 @@
 #include "mongo/platform/basic.h"
 
 #include "mongo/util/clock_source_mock.h"
+#include "mongo/util/time_support.h"
 
 namespace mongo {
 
+Milliseconds ClockSourceMock::getPrecision() {
+    return Milliseconds(1);
+}
+
 Date_t ClockSourceMock::now() {
     return _now;
 }
diff --git a/src/mongo/util/clock_source_mock.h b/src/mongo/util/clock_source_mock.h
index 3a15c52085b..2ee372da48c 100644
--- a/src/mongo/util/clock_source_mock.h
+++ b/src/mongo/util/clock_source_mock.h
@@ -43,6 +43,7 @@ public:
      */
     ClockSourceMock() = default;
 
+    Milliseconds getPrecision() override;
     Date_t now() final;
 
     /**
diff --git a/src/mongo/util/system_clock_source.cpp b/src/mongo/util/system_clock_source.cpp
index ac64e49ca62..0d5974c46b9 100644
--- a/src/mongo/util/system_clock_source.cpp
+++ b/src/mongo/util/system_clock_source.cpp
@@ -44,4 +44,8 @@ SystemClockSource* SystemClockSource::get() {
     return globalSystemClockSource.get();
 }
 
+Milliseconds SystemClockSource::getPrecision() {
+    return Milliseconds(1);
+}
+
 }  // namespace mongo
diff --git a/src/mongo/util/system_clock_source.h b/src/mongo/util/system_clock_source.h
index 69f2b661d86..160eea95ee7 100644
--- a/src/mongo/util/system_clock_source.h
+++ b/src/mongo/util/system_clock_source.h
@@ -29,6 +29,7 @@
 #pragma once
 
 #include "mongo/util/clock_source.h"
+#include "mongo/util/time_support.h"
 
 namespace mongo {
 
@@ -43,6 +44,7 @@ public:
      * Returns the singleton instance of SystemClockSource.
      */
     static SystemClockSource* get();
+    Milliseconds getPrecision() override;
 };
 
 }  // namespace mongo