From 580d2782f70f8e0bed7ec20abb03d740cb83b5da Mon Sep 17 00:00:00 2001 From: "Gregory P. Smith" Date: Wed, 11 Sep 2019 04:23:05 -0500 Subject: [PATCH] bpo-37424: Avoid a hang in subprocess.run timeout output capture (GH-14490) Fixes a possible hang when using a timeout on subprocess.run() while capturing output. If the child process spawned its own children or otherwise connected its stdout or stderr handles with another process, we could hang after the timeout was reached and our child was killed when attempting to read final output from the pipes. --- Lib/subprocess.py | 36 ++++++++++++++----- Lib/test/test_subprocess.py | 21 +++++++++++ .../2019-07-04-13-00-20.bpo-37424.0i1MR-.rst | 5 +++ 3 files changed, 53 insertions(+), 9 deletions(-) create mode 100644 Misc/NEWS.d/next/Library/2019-07-04-13-00-20.bpo-37424.0i1MR-.rst diff --git a/Lib/subprocess.py b/Lib/subprocess.py index aed7292541..85b9ea0785 100644 --- a/Lib/subprocess.py +++ b/Lib/subprocess.py @@ -489,11 +489,20 @@ def run(*popenargs, with Popen(*popenargs, **kwargs) as process: try: stdout, stderr = process.communicate(input, timeout=timeout) - except TimeoutExpired: + except TimeoutExpired as exc: process.kill() - stdout, stderr = process.communicate() - raise TimeoutExpired(process.args, timeout, output=stdout, - stderr=stderr) + if _mswindows: + # Windows accumulates the output in a single blocking + # read() call run on child threads, with the timeout + # being done in a join() on those threads. communicate() + # _after_ kill() is required to collect that and add it + # to the exception. + exc.stdout, exc.stderr = process.communicate() + else: + # POSIX _communicate already populated the output so + # far into the TimeoutExpired exception. + process.wait() + raise except: # Including KeyboardInterrupt, communicate handled that. process.kill() # We don't call process.wait() as .__exit__ does that for us. @@ -1050,12 +1059,16 @@ class Popen(object): return endtime - _time() - def _check_timeout(self, endtime, orig_timeout): + def _check_timeout(self, endtime, orig_timeout, stdout_seq, stderr_seq, + skip_check_and_raise=False): """Convenience for checking if a timeout has expired.""" if endtime is None: return - if _time() > endtime: - raise TimeoutExpired(self.args, orig_timeout) + if skip_check_and_raise or _time() > endtime: + raise TimeoutExpired( + self.args, orig_timeout, + output=b''.join(stdout_seq) if stdout_seq else None, + stderr=b''.join(stderr_seq) if stderr_seq else None) def wait(self, timeout=None): @@ -1843,10 +1856,15 @@ class Popen(object): while selector.get_map(): timeout = self._remaining_time(endtime) if timeout is not None and timeout < 0: - raise TimeoutExpired(self.args, orig_timeout) + self._check_timeout(endtime, orig_timeout, + stdout, stderr, + skip_check_and_raise=True) + raise RuntimeError( # Impossible :) + '_check_timeout(..., skip_check_and_raise=True) ' + 'failed to raise TimeoutExpired.') ready = selector.select(timeout) - self._check_timeout(endtime, orig_timeout) + self._check_timeout(endtime, orig_timeout, stdout, stderr) # XXX Rewrite these to use non-blocking I/O on the file # objects; they are no longer using C stdio! diff --git a/Lib/test/test_subprocess.py b/Lib/test/test_subprocess.py index 4fe74bf504..91f525df46 100644 --- a/Lib/test/test_subprocess.py +++ b/Lib/test/test_subprocess.py @@ -10,6 +10,7 @@ import os import errno import tempfile import time +import traceback import selectors import sysconfig import select @@ -1557,6 +1558,26 @@ class RunFuncTestCase(BaseTestCase): self.assertIn('stderr', c.exception.args[0]) self.assertIn('capture_output', c.exception.args[0]) + # This test _might_ wind up a bit fragile on loaded build+test machines + # as it depends on the timing with wide enough margins for normal situations + # but does assert that it happened "soon enough" to believe the right thing + # happened. + @unittest.skipIf(mswindows, "requires posix like 'sleep' shell command") + def test_run_with_shell_timeout_and_capture_output(self): + """Output capturing after a timeout mustn't hang forever on open filehandles.""" + before_secs = time.monotonic() + try: + subprocess.run('sleep 3', shell=True, timeout=0.1, + capture_output=True) # New session unspecified. + except subprocess.TimeoutExpired as exc: + after_secs = time.monotonic() + stacks = traceback.format_exc() # assertRaises doesn't give this. + else: + self.fail("TimeoutExpired not raised.") + self.assertLess(after_secs - before_secs, 1.5, + msg="TimeoutExpired was delayed! Bad traceback:\n```\n" + f"{stacks}```") + @unittest.skipIf(mswindows, "POSIX specific tests") class POSIXProcessTestCase(BaseTestCase): diff --git a/Misc/NEWS.d/next/Library/2019-07-04-13-00-20.bpo-37424.0i1MR-.rst b/Misc/NEWS.d/next/Library/2019-07-04-13-00-20.bpo-37424.0i1MR-.rst new file mode 100644 index 0000000000..b98a17e241 --- /dev/null +++ b/Misc/NEWS.d/next/Library/2019-07-04-13-00-20.bpo-37424.0i1MR-.rst @@ -0,0 +1,5 @@ +Fixes a possible hang when using a timeout on `subprocess.run()` while +capturing output. If the child process spawned its own children or +otherwise connected its stdout or stderr handles with another process, we +could hang after the timeout was reached and our child was killed when +attempting to read final output from the pipes. -- 2.50.1