[fix](be) Clamp MonotonicStopWatch elapsed time to non-negative

Problem Summary: MonotonicStopWatch::elapsed_time() can return a small
negative value (e.g. -203 ns) due to rare CLOCK_MONOTONIC rollbacks.
When stop() accumulates this into _total_time and _fresh_profile_counter()
sets it on a RuntimeProfile::Counter, the DCHECK asserting value > -1
fires and crashes the process.

Stack trace:
  RuntimeProfile::Counter::set() at runtime_profile.h:222
  PipelineTask::close() at pipeline_task.cpp:925
  close_task() at task_scheduler.cpp:86

The fix clamps the running-case delta in elapsed_time() and
elapsed_time_seconds() to max(0, delta). Since stop() calls
elapsed_time() while _running is still true, _total_time can never
accumulate a negative value, preventing the crash for all downstream
callers.

None

- Test: No need to test (clock rollback is non-deterministic hardware behavior; the fix is a trivial arithmetic clamp)
- Behavior changed: No
- Does this need documentation: No

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jacktengg

be/src/util/stopwatch.hpp

@@ -22,6 +22,8 @@
 
 #include <time.h>
 
+#include "util/time.h"
+
 namespace doris {
 
 // Stop watch for reporting elapsed time in nanosec based on CLOCK_MONOTONIC.
@@ -69,16 +71,21 @@ class CustomStopWatch {
     }
 
     // Returns time in nanosecond.
-    // Sometimes the time will rollback, so that the value will be negative.
+    // Clamped to 0 to guard against rare CLOCK_MONOTONIC rollbacks.
     int64_t elapsed_time() const {
         if (!_running) {
             return _total_time;
         }
 
         timespec end;
         clock_gettime(Clock, &end);
-        return (end.tv_sec - _start.tv_sec) * 1000L * 1000L * 1000L +
-               (end.tv_nsec - _start.tv_nsec);
+        auto start_nanos = _start.tv_sec * NANOS_PER_SEC + _start.tv_nsec;
+        auto end_nanos = end.tv_sec * NANOS_PER_SEC + end.tv_nsec;
+        if (end_nanos < start_nanos) {
+            LOG(INFO) << "WARNING: time went backwards from " << start_nanos << " to " << end_nanos;
+            return 0;
+        }
+        return end_nanos - start_nanos;
     }
 
     // Return time in microseconds
@@ -87,11 +94,18 @@ class CustomStopWatch {
     // Return time in milliseconds
     int64_t elapsed_time_milliseconds() const { return elapsed_time() / 1000 / 1000; }
 
-    // Returns time in nanosecond.
+    // Returns time in seconds.
+    // Clamped to 0 to guard against rare CLOCK_MONOTONIC rollbacks.
     int64_t elapsed_time_seconds(timespec end) const {
         if (!_running) {
             return _total_time / 1000L / 1000L / 1000L;
         }
+        if (end.tv_sec < _start.tv_sec) {
+            auto start_nanos = _start.tv_sec * NANOS_PER_SEC + _start.tv_nsec;
+            auto end_nanos = end.tv_sec * NANOS_PER_SEC + end.tv_nsec;
+            LOG(INFO) << "WARNING: time went backwards from " << start_nanos << " to " << end_nanos;
+            return 0;
+        }
         return end.tv_sec - _start.tv_sec;
     }