gh-133089: Use original timeout value for TimeoutExpired when the func subprocess.run is called with a timeout

Doc/library/subprocess.rst

@@ -1525,6 +1525,30 @@
 Notes
 -----
 
+.. _subprocess-timeout-behavior:
+
+Timeout Behavior
+---------------
+
+When using the ``timeout`` parameter in functions like :func:`run`,
+:meth:`Popen.wait`, or :meth:`Popen.communicate`,
+users should be aware of the following behaviors:
+
+Process Creation Delay
+~~~~~~~~~~~~~~~~~~~~~
+
+The initial process creation itself cannot be interrupted on many platform APIs.
+This means that even when specifying a timeout, you are not guaranteed to see a timeout exception
+until at least after however long process creation takes.
+
+Extremely Small Timeout Values
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Setting very small timeout values (such as a few milliseconds) may result
+in almost immediate :exc:`TimeoutExpired` exceptions because
+process creation and system scheduling inherently require time.
+
+
 .. _converting-argument-sequence:
 
 Converting an argument sequence to a string on Windows

Lib/subprocess.py

@@ -2031,7 +2031,7 @@ def _wait(self, timeout):
             if self.returncode is not None:
                 return self.returncode
 
-            if timeout is not None:
+            if timeout is not None and timeout > 0:
                 endtime = _time() + timeout
                 # Enter a busy loop if we have a timeout.  This busy loop was
                 # cribbed from Lib/threading.py in Thread.wait() at r71065.
@@ -2049,7 +2049,7 @@ def _wait(self, timeout):
                         finally:
                             self._waitpid_lock.release()
                     remaining = self._remaining_time(endtime)
-                    if remaining <= 0:
+                    if remaining < 0:
                         raise TimeoutExpired(self.args, timeout)
                     delay = min(delay * 2, remaining, .05)
                     time.sleep(delay)

Lib/test/test_subprocess.py

@@ -1660,6 +1660,8 @@ def test_timeout(self):
         # child.
         with self.assertRaises(subprocess.TimeoutExpired):
             self.run_python("while True: pass", timeout=0.0001)
+    def test_timeout_zero(self):
+        self.run_python("import time; time.sleep(0.1)", timeout=0)
 
     def test_capture_stdout(self):
         # capture stdout with zero return code

Misc/NEWS.d/next/Library/2025-04-29-02-23-04.gh-issue-133089.8Jy1ZS.rst

@@ -0,0 +1,2 @@
+Make :func:`subprocess.run`'s behavior is same with 'timeout=None' when
+the timeout is zero