gh-114911: Add CPUStopwatch test helper (GH-114912)

A few of our tests measure the time of CPU-bound operation, mainly
to avoid quadratic or worse behaviour.
Add a helper to ignore GC and time spent in other processes.

1 files changed, 40 insertions, 0 deletions

diff --git a/Lib/test/support/__init__.py b/Lib/test/support/__init__.py
index 1d03ec0..401b2ce 100644
--- a/Lib/test/support/__init__.py
+++ b/Lib/test/support/__init__.py
@@ -2381,6 +2381,46 @@ def sleeping_retry(timeout, err_msg=None, /,
         delay = min(delay * 2, max_delay)
 
 
+class CPUStopwatch:
+    """Context manager to roughly time a CPU-bound operation.
+
+    Disables GC. Uses CPU time if it can (i.e. excludes sleeps & time of
+    other processes).
+
+    N.B.:
+    - This *includes* time spent in other threads.
+    - Some systems only have a coarse resolution; check
+      stopwatch.clock_info.rseolution if.
+
+    Usage:
+
+    with ProcessStopwatch() as stopwatch:
+        ...
+    elapsed = stopwatch.seconds
+    resolution = stopwatch.clock_info.resolution
+    """
+    def __enter__(self):
+        get_time = time.process_time
+        clock_info = time.get_clock_info('process_time')
+        if get_time() <= 0:  # some platforms like WASM lack process_time()
+            get_time = time.monotonic
+            clock_info = time.get_clock_info('monotonic')
+        self.context = disable_gc()
+        self.context.__enter__()
+        self.get_time = get_time
+        self.clock_info = clock_info
+        self.start_time = get_time()
+        return self
+
+    def __exit__(self, *exc):
+        try:
+            end_time = self.get_time()
+        finally:
+            result = self.context.__exit__(*exc)
+        self.seconds = end_time - self.start_time
+        return result
+
+
 @contextlib.contextmanager
 def adjust_int_max_str_digits(max_digits):
     """Temporarily change the integer string conversion length limit."""