test_subprocess hanged on the build 46 of x86 Windows7 3.x:

http://buildbot.python.org/all/#/builders/58/builds/46

(...)
1:29:55 [405/407] test_cmath passed -- running: test_subprocess (1249 sec)
1:29:58 [406/407] test_pstats passed -- running: test_subprocess (1252 sec)

The buildbot is slow which might have caused a timing issue.

Sadly, the AMD64 Windows10 3.x buildbot worker is also sick:

http://buildbot.python.org/all/#/builders/3/builds/117

1:15:30 [405/407] test_urllibnet passed -- running: test_subprocess (3338 sec)
1:15:52 [406/407] test_bytes passed -- running: test_subprocess (3361 sec)
command timed out: 1200 seconds without output running ['Tools\\buildbot\\test.bat', '-x64', '-j2', '--timeout', '900'], attempting to kill
program finished with exit code 1
elapsedTime=5757.133000

Bug occurred on AMD64 Windows8.1 Refleaks 3.6:

http://buildbot.python.org/all/#/builders/64/builds/52

...
running: test_subprocess (134462 sec)
running: test_subprocess (134492 sec)
running: test_subprocess (134522 sec)
running: test_subprocess (134552 sec)
running: test_subprocess (134582 sec)
running: test_subprocess (134612 sec)
running: test_subprocess (134642 sec)
program finished with exit code 1

The bug still occurs randomly:

http://buildbot.python.org/all/#/builders/38/builds/122

1:34:03 [404/406] test_typing passed -- running: test_subprocess (4577 sec)
1:34:22 [405/406] test_winreg passed -- running: test_subprocess (4596 sec)
command timed out: 1200 seconds without output running ['Tools\\buildbot\\test.bat', '-x64', '-j2', '--timeout', '900'], attempting to kill
program finished with exit code 1
elapsedTime=6864.917000

The test still hangs sometimes. Today on AMD64 Windows10 3.x:

http://buildbot.python.org/all/#/builders/3/builds/563

1:24:46 [409/414] test_tokenize passed (489 sec) -- running: test_subprocess (4242 sec)
1:24:50 [410/414] test_codecencodings_cn passed -- running: test_subprocess (4246 sec)
1:25:14 [411/414] test_codecmaps_kr passed -- running: test_subprocess (4270 sec)
1:25:28 [412/414] test_runpy passed -- running: test_subprocess (4284 sec)
1:25:36 [413/414] test_tempfile passed -- running: test_subprocess (4292 sec)
minkernel\crts\ucrt\src\appcrt\lowio\write.cpp(49) : Assertion failed: (_osfile(fh) & FOPEN)
command timed out: 1200 seconds without output running ['Tools\\buildbot\\test.bat', '-x64', '-j2', '--timeout', '900'], attempting to kill
program finished with exit code 1
elapsedTime=6338.784000

It's an assert, so it only affects debug builds - releases are not impacted.

I'm guessing there's a test that spawns a subprocess and doesn't initialize it properly for tests (specifically the part where we disable assertions on Windows).

When ran tests in 3.8 on Windows I got a number of dialpog windows with the following text:

---------------------------
Microsoft Visual C++ Runtime Library
---------------------------
Debug Assertion Failed!

Program: C:\py\cpython3.8\PCBuild\win32\python_d.exe
File: minkernel\crts\ucrt\src\appcrt\lowio\osfinfo.cpp
Line: 258

Expression: _osfile(fh) & FOPEN

For information on how your program can cause an assertion
failure, see the Visual C++ documentation on asserts.

(Press Retry to debug the application)

And tests are hung.

When ran tests in 3.7 on Windows I got multiple messages "minkernel\crts\ucrt\src\appcrt\lowio\write.cpp(49) : Assertion failed: (_osfile(fh) & FOPEN)" on output. This may be related.

I didn't see the failure recently, I close the issue.