`patchcheck.py` hangs infinitely on branches with merges from `main`

[arhadthedev]

Bug report

Tools/patchcheck/patchcheck.py stalls on Getting base branch for PR ... on my Windows system if a processed branch has a moderately large diff relative to main.

It's easy to achieve with merging main into the branch. For example, git diff --name-status origin/main gives 8KB worth of text for gh-103244:

Details

M	.devcontainer/devcontainer.json
M	Doc/howto/enum.rst
M	Doc/library/enum.rst
M	Doc/library/functions.rst
M	Doc/library/http.client.rst
M	Doc/library/multiprocessing.rst
M	Doc/library/shutil.rst
M	Doc/library/socket.rst
M	Doc/library/sys.rst
D	Doc/tools/.nitignore
A	Doc/tools/clean-files.txt
M	Doc/tools/touch-clean-files.py
M	Doc/whatsnew/3.12.rst
M	Include/patchlevel.h
M	Include/pymacro.h
M	Lib/enum.py
M	Lib/http/client.py
M	Lib/inspect.py
M	Lib/pathlib.py
M	Lib/pydoc_data/topics.py
M	Lib/shutil.py
M	Lib/test/test_exceptions.py
M	Lib/test/test_httplib.py
M	Lib/test/test_pathlib.py
M	Lib/test/test_pickle.py
M	Lib/test/test_shutil.py
M	Lib/test/test_sys_settrace.py
M	Lib/test/test_webbrowser.py
M	Lib/test/test_zipfile/test_core.py
M	Lib/typing.py
M	Lib/webbrowser.py
M	Lib/zipfile/__init__.py
M	Mac/BuildScript/resources/Welcome.rtf
M	Makefile.pre.in
M	Misc/ACKS
D	Misc/NEWS.d/3.12.0a7.rst
D	Misc/NEWS.d/next/Build/2023-02-11-05-31-05.gh-issue-99069.X4LDvY.rst
A	Misc/NEWS.d/next/Build/2023-03-15-02-03-39.gh-issue-102711.zTkjts.rst
A	Misc/NEWS.d/next/Build/2023-03-23-20-58-56.gh-issue-102973.EaJUrw.rst
A	Misc/NEWS.d/next/C API/2023-02-18-00-55-14.gh-issue-102013.83mrtI.rst
A	Misc/NEWS.d/next/Core and Builtins/2023-02-21-17-22-06.gh-issue-101865.fwrTOA.rst
A	Misc/NEWS.d/next/Core and Builtins/2023-02-21-23-42-39.gh-issue-102027.fQARG0.rst
A	Misc/NEWS.d/next/Core and Builtins/2023-02-26-11-43-56.gh-issue-102255.cRnI5x.rst
A	Misc/NEWS.d/next/Core and Builtins/2023-02-26-13-12-55.gh-issue-102213.fTH8X7.rst
A	Misc/NEWS.d/next/Core and Builtins/2023-02-27-15-48-31.gh-issue-102300.8o-_Mt.rst
A	Misc/NEWS.d/next/Core and Builtins/2023-03-02-13-49-21.gh-issue-102281.QCuu2N.rst
A	Misc/NEWS.d/next/Core and Builtins/2023-03-03-23-21-16.gh-issue-102406.XLqYO3.rst
A	Misc/NEWS.d/next/Core and Builtins/2023-03-04-06-48-34.gh-issue-102397.ACJaOf.rst
A	Misc/NEWS.d/next/Core and Builtins/2023-03-06-10-02-22.gh-issue-101291.0FT2QS.rst
A	Misc/NEWS.d/next/Core and Builtins/2023-03-08-08-37-36.gh-issue-102491.SFvvsC.rst
A	Misc/NEWS.d/next/Core and Builtins/2023-03-09-13-57-35.gh-issue-90997.J-Yhn2.rst
A	Misc/NEWS.d/next/Core and Builtins/2023-03-14-00-11-46.gh-issue-102594.BjU-m2.rst
A	Misc/NEWS.d/next/Core and Builtins/2023-03-16-14-44-29.gh-issue-102755.j1GxlV.rst
A	Misc/NEWS.d/next/Core and Builtins/2023-03-16-17-24-44.gh-issue-102701.iNGVaS.rst
A	Misc/NEWS.d/next/Core and Builtins/2023-03-17-12-09-45.gh-issue-100982.Pf_BI6.rst
A	Misc/NEWS.d/next/Core and Builtins/2023-03-17-13-43-34.gh-issue-102778.ANDv8I.rst
A	Misc/NEWS.d/next/Core and Builtins/2023-03-18-02-36-39.gh-issue-101975.HwMR1d.rst
A	Misc/NEWS.d/next/Core and Builtins/2023-03-21-00-46-36.gh-issue-102859.PRkGca.rst
A	Misc/NEWS.d/next/Core and Builtins/2023-03-24-02-50-33.gh-issue-89987.oraTzh.rst
A	Misc/NEWS.d/next/Core and Builtins/2023-03-31-12-22-25.gh-issue-102192.gYxJP_.rst
D	Misc/NEWS.d/next/Documentation/2023-03-10-04-59-35.gh-issue-86094.zOYdy8.rst
A	Misc/NEWS.d/next/Documentation/2023-03-29-14-51-39.gh-issue-103112.XgGSEO.rst
D	Misc/NEWS.d/next/Library/2018-07-16-14-10-29.bpo-22708.592iRR.rst
A	Misc/NEWS.d/next/Library/2019-03-15-22-50-27.bpo-36305.Pbkv6u.rst
D	Misc/NEWS.d/next/Library/2021-12-03-23-00-56.bpo-44844.tvg2VY.rst
A	Misc/NEWS.d/next/Library/2022-04-11-18-34-33.gh-issue-72346.pC7gnM.rst
A	Misc/NEWS.d/next/Library/2022-06-30-21-28-41.gh-issue-94440.LtgX0d.rst
A	Misc/NEWS.d/next/Library/2022-07-09-13-07-30.gh-issue-94684.nV5yno.rst
A	Misc/NEWS.d/next/Library/2022-07-30-23-01-43.gh-issue-95495.RA-q1d.rst
A	Misc/NEWS.d/next/Library/2022-09-19-08-12-58.gh-issue-96931.x0WQhh.rst
A	Misc/NEWS.d/next/Library/2022-10-10-19-14-51.gh-issue-98169.DBWIxL.rst
A	Misc/NEWS.d/next/Library/2022-11-24-13-23-07.gh-issue-48330.6uAX9F.rst
A	Misc/NEWS.d/next/Library/2022-12-09-11-21-38.gh-issue-100131.v863yR.rst
A	Misc/NEWS.d/next/Library/2022-12-16-10-27-58.gh-issue-89727.y64ZLM.rst
A	Misc/NEWS.d/next/Library/2022-12-20-10-55-14.gh-issue-100372.utfP65.rst
A	Misc/NEWS.d/next/Library/2023-01-27-14-51-07.gh-issue-101313.10AEXh.rst
A	Misc/NEWS.d/next/Library/2023-02-09-19-40-41.gh-issue-101673.mX-Ppq.rst
A	Misc/NEWS.d/next/Library/2023-02-18-23-03-50.gh-issue-98886.LkKGWv.rst
A	Misc/NEWS.d/next/Library/2023-02-19-01-49-46.gh-issue-102038.n3if3D.rst
A	Misc/NEWS.d/next/Library/2023-02-20-16-47-56.gh-issue-102069.FS7f1j.rst
A	Misc/NEWS.d/next/Library/2023-02-21-11-56-16.gh-issue-102103.Dj0WEj.rst
A	Misc/NEWS.d/next/Library/2023-02-26-17-29-57.gh-issue-79940.SAfmAy.rst
A	Misc/NEWS.d/next/Library/2023-03-03-19-53-08.gh-issue-102378.kRdOZc.rst
A	Misc/NEWS.d/next/Library/2023-03-04-20-58-29.gh-issue-74468.Ac5Ew_.rst
A	Misc/NEWS.d/next/Library/2023-03-08-23-08-38.gh-issue-102519.wlcsFI.rst
A	Misc/NEWS.d/next/Library/2023-03-10-13-21-16.gh-issue-102578.-gujoI.rst
A	Misc/NEWS.d/next/Library/2023-03-10-13-51-21.gh-issue-100112.VHh4mw.rst
A	Misc/NEWS.d/next/Library/2023-03-13-12-05-55.gh-issue-102615.NcA_ZL.rst
A	Misc/NEWS.d/next/Library/2023-03-13-18-27-00.gh-issue-102670.GyoThv.rst
A	Misc/NEWS.d/next/Library/2023-03-16-08-17-29.gh-issue-102748.WNACpI.rst
A	Misc/NEWS.d/next/Library/2023-03-16-16-43-04.gh-issue-78530.Lr8eq_.rst
A	Misc/NEWS.d/next/Library/2023-03-18-14-59-21.gh-issue-88965.kA70Km.rst
A	Misc/NEWS.d/next/Library/2023-03-19-15-30-59.gh-issue-102828.NKClXg.rst
A	Misc/NEWS.d/next/Library/2023-03-20-12-21-19.gh-issue-102839.RjRi12.rst
A	Misc/NEWS.d/next/Library/2023-03-21-15-17-07.gh-issue-102871.U9mchn.rst
A	Misc/NEWS.d/next/Library/2023-03-22-16-15-18.gh-issue-102780.NEcljy.rst
A	Misc/NEWS.d/next/Library/2023-03-23-13-34-33.gh-issue-102947.cTwcpU.rst
A	Misc/NEWS.d/next/Library/2023-03-25-02-08-05.gh-issue-103023.Qfn7Hl.rst
A	Misc/NEWS.d/next/Library/2023-03-25-16-57-18.gh-issue-102433.L-7x2Q.rst
A	Misc/NEWS.d/next/Library/2023-03-26-20-54-57.gh-issue-103046.xBlA2l.rst
A	Misc/NEWS.d/next/Library/2023-03-27-15-01-16.gh-issue-103056.-Efh5Q.rst
A	Misc/NEWS.d/next/Library/2023-03-27-19-21-51.gh-issue-102549.NQ6Nlv.rst
A	Misc/NEWS.d/next/Library/2023-03-28-05-14-59.gh-issue-103068.YQTmrA.rst
A	Misc/NEWS.d/next/Library/2023-03-28-15-12-53.gh-issue-103085.DqNehf.rst
D	Misc/NEWS.d/next/Library/2023-04-02-17-51-08.gh-issue-103193.xrZbM1.rst
D	Misc/NEWS.d/next/Library/2023-04-02-22-04-26.gh-issue-75586.526iJm.rst
A	Misc/NEWS.d/next/Tests/2023-01-27-18-10-40.gh-issue-101377.IJGpqh.rst
A	Misc/NEWS.d/next/Tests/2023-03-08-13-54-20.gh-issue-102537.Vfplpb.rst
A	Misc/NEWS.d/next/Tests/2023-03-23-23-25-18.gh-issue-102980.Zps4QF.rst
A	Misc/NEWS.d/next/Tests/2023-04-05-06-45-20.gh-issue-103186.640Eg-.rst
A	Misc/NEWS.d/next/Tools-Demos/2023-03-21-01-27-07.gh-issue-102809.2F1Byz.rst
A	Misc/NEWS.d/next/Windows/2023-02-22-17-26-10.gh-issue-99726.76t957.rst
A	Misc/NEWS.d/next/Windows/2023-03-14-10-52-43.gh-issue-102690.sbXtqk.rst
D	Misc/NEWS.d/next/macOS/2023-04-04-13-37-28.gh-issue-103207.x0vvQp.rst
M	Modules/_pickle.c
M	Modules/_ssl.c
M	Modules/_ssl.h
M	Modules/_tkinter.c
M	Modules/_winapi.c
M	Modules/_xxsubinterpretersmodule.c
M	Modules/clinic/_pickle.c.h
M	Modules/clinic/_winapi.c.h
M	Modules/posixmodule.c
M	PC/launcher.c
M	PC/launcher2.c
M	Python/ceval_gil.c
M	Python/import.c
M	Python/initconfig.c
M	Python/sysmodule.c
M	README.rst
M	Tools/c-analyzer/cpython/globals-to-fix.tsv

The reason is waiting for Git using Popen.wait(). According to the documentation:

This will deadlock when using stdout=PIPE or stderr=PIPE and the child process generates enough output to a pipe such that it blocks waiting for the OS pipe buffer to accept more data. Use Popen.communicate() when using pipes to avoid that.

As a result, the Azure Pipelines PR buildbot with a larger pipe buffer runs the script with no issue while my desktop edition fails to do so.

Your environment

• CPython versions tested on: Python 3.12.0a7+ (heads/main-dirty:de18267685, Apr 5 2023, 22:28:44) [MSC v.1929 64 bit (AMD64)] on win32
• Operating system and architecture: Windows 10 Home 22H2

Linked PRs

• gh-103300: Fix Popen.wait() deadlock in patchcheck.py #103301