From f6516ccf023228f286c9018af92990405db2aad7 Mon Sep 17 00:00:00 2001
From: Ti Chi Robot <ti-community-prow-bot@tidb.io>
Date: Thu, 9 Nov 2023 20:20:12 +0800
Subject: [PATCH] *: Fix time step backward for
 `Stopwatch::elapsedFromLastTime` (#8338) (#8347)

close pingcap/tiflash#8336
---
 dbms/src/Common/Stopwatch.h | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/dbms/src/Common/Stopwatch.h b/dbms/src/Common/Stopwatch.h
index 853d53f0db8..af972610810 100644
--- a/dbms/src/Common/Stopwatch.h
+++ b/dbms/src/Common/Stopwatch.h
@@ -30,7 +30,7 @@ inline UInt64 clock_gettime_ns(clockid_t clock_type = CLOCK_MONOTONIC)
     {
     };
     clock_gettime(clock_type, &ts);
-    return UInt64(ts.tv_sec * 1000000000ULL + ts.tv_nsec);
+    return static_cast<UInt64>(ts.tv_sec * 1000000000ULL + ts.tv_nsec);
 }
 
 /// Sometimes monotonic clock may not be monotonic (due to bug in kernel?).
@@ -64,14 +64,14 @@ class Stopwatch
 
     void start()
     {
-        start_ns = nanoseconds();
+        start_ns = nanosecondsWithBound(start_ns);
         last_ns = start_ns;
         is_running = true;
     }
 
     void stop()
     {
-        stop_ns = nanoseconds();
+        stop_ns = nanosecondsWithBound(start_ns);
         is_running = false;
     }
 
@@ -82,14 +82,16 @@ class Stopwatch
         last_ns = 0;
         is_running = false;
     }
+
     void restart() { start(); }
-    UInt64 elapsed() const { return is_running ? nanoseconds() - start_ns : stop_ns - start_ns; }
+
+    UInt64 elapsed() const { return is_running ? nanosecondsWithBound(start_ns) - start_ns : stop_ns - start_ns; }
     UInt64 elapsedMilliseconds() const { return elapsed() / 1000000UL; }
     double elapsedSeconds() const { return static_cast<double>(elapsed()) / 1000000000ULL; }
 
     UInt64 elapsedFromLastTime()
     {
-        const auto now_ns = nanoseconds();
+        const auto now_ns = nanosecondsWithBound(last_ns);
         if (is_running)
         {
             auto rc = now_ns - last_ns;
@@ -100,7 +102,7 @@ class Stopwatch
         {
             return stop_ns - last_ns;
         }
-    };
+    }
 
     UInt64 elapsedMillisecondsFromLastTime() { return elapsedFromLastTime() / 1000000UL; }
     double elapsedSecondsFromLastTime() { return static_cast<double>(elapsedFromLastTime()) / 1000000000ULL; }
@@ -112,7 +114,9 @@ class Stopwatch
     clockid_t clock_type;
     bool is_running = false;
 
-    UInt64 nanoseconds() const { return clock_gettime_ns_adjusted(start_ns, clock_type); }
+    // Get current nano seconds, ensuring the return value is not
+    // less than `lower_bound`.
+    UInt64 nanosecondsWithBound(UInt64 lower_bound) const { return clock_gettime_ns_adjusted(lower_bound, clock_type); }
 };