`check_extension_modules.py` hangs build due to SIGTTIN

[haampie]

Bug description:

When building Python in a zsh terminal on macOS with stdin a tty (edit: and GNU readline), the build hangs indefinitely.

This is because during the build, python.exe -E check_extension_modules.py is run, which attempts to import the readline module.

At import time of the readline module

• it runs PyInit_readline
• which calls get_tty_settings from libreadline.dylib,
• which ultimately runs an ioctl syscall
• which immediately suspends the Python process thanks to a SIGTTIN signal.

The Python process isn't supposed to run ioctl syscalls from the background.

I don't know where the fix should go, but I would say it's two separate issues:

• importing modules during the build sounds like a bad idea due to possible side-effects
• the readline module running an ioctl call during import risking the process to be suspended when backgrounded sounds bad too.

CPython versions tested on:

3.14, main branch at commit 2c8f26c

Operating systems tested on:

macOS

[MazinSharaf]

Just checking, does this apply to the main branch of CPython as well?

[haampie]

Yes, using 2c8f26c, it's stuck on

$ ps aux | grep python
USER     PID  %CPU %MEM      VSZ    RSS   TT  STAT STARTED      TIME COMMAND
user   70034   0.0  0.2 411441968  41024 s000  T     3:05PM   0:00.68 ./python.exe -E ./Tools/build/check_extension_modules.py --generate-missing-stdlib-info

the T stands for "stopped".

A minimal reproducer outside of the build is the following:

% python3 -c 'import readline' &
[1] 70136
% 
[1]  + suspended (tty output)  python3 -c 'import readline'
% fg
[1]  + continued  python3 -c 'import readline'
% 

The dependencies are:

python@develop commit=2c8f26cf5c6ff2398f1e1ed8bfe18e9938bbdebd platform=darwin os=sequoia target=m4 %c,cxx=apple-clang@17.0.0
    ^apple-clang@17.0.0
    ^bzip2@1.0.8
    ^expat@2.7.4
    ^gdbm@1.26
    ^gettext@1.0
        ^libiconv@1.18
        ^libxml2@2.15.1
    ^gmake@4.4.1
    ^libffi@3.5.2
    ^ncurses@6.6
    ^openssl@3.6.1
        ^ca-certificates-mozilla@2025-08-12
    ^pkgconf@2.5.1
    ^readline@8.3   # <---
    ^sqlite@3.51.2
    ^xz@5.8.2
    ^zlib-ng@2.3.3
    ^zstd@1.5.7

That readline that seems to trigger it is GNU readline 8.3 from https://tiswww.case.edu/php/chet/readline/rltop.html.

[haampie]

From macos's utility sample output, it shows it's stuck in the ioctl syscall:

Call graph:
    8755 Thread_45988728   DispatchQueue_1: com.apple.main-thread  (serial)
      8755 start  (in dyld) + 6076  [0x18253ab98]
        8755 Py_BytesMain  (in libpython3.15.dylib) + 56  [0x101f8b7dc]  main.c:850
          8755 pymain_main  (in libpython3.15.dylib) + 524  [0x101f8b740]  main.c:826
            8755 Py_RunMain  (in libpython3.15.dylib) + 3652  [0x101f8b098]  main.c:796
              8755 pymain_run_file  (in libpython3.15.dylib) + 620  [0x101f8bef4]  main.c:430
                8755 _PyRun_AnyFileObject  (in libpython3.15.dylib) + 224  [0x101f3b69c]  pythonrun.c:81
                  8755 _PyRun_SimpleFileObject  (in libpython3.15.dylib) + 1244  [0x101f3c1f8]  pythonrun.c:518
                    8755 pyrun_file  (in libpython3.15.dylib) + 212  [0x101f3db68]  pythonrun.c:1296
                      8755 run_mod  (in libpython3.15.dylib) + 1788  [0x101f423ec]  pythonrun.c:1472
                        8755 PyEval_EvalCode  (in libpython3.15.dylib) + 304  [0x101e43738]  ceval.c:686
                          8755 _PyEval_Vector  (in libpython3.15.dylib) + 1072  [0x101e43cd4]  ceval.c:2174
                            8755 _PyEval_EvalFrameDefault  (in libpython3.15.dylib) + 544  [0x101e46348]  ceval.c:1334
                              8755 _TAIL_CALL_CALL_EX_NON_PY_GENERAL  (in libpython3.15.dylib) + 584  [0x101e654c8]  generated_cases.c.h:2647
                                8755 cfunction_vectorcall_FASTCALL  (in libpython3.15.dylib) + 680  [0x101ca433c]  methodobject.c:449
                                  8755 _imp_create_dynamic  (in libpython3.15.dylib) + 1092  [0x101ef88a4]  import.c.h:488
                                    8755 import_run_extension  (in libpython3.15.dylib) + 220  [0x101ef57a4]  import.c:2164
                                      8755 _PyImport_RunModInitFunc  (in libpython3.15.dylib) + 84  [0x101efaa34]  importdl.c:436
                                        8755 PyInit_readline  (in readline.cpython-315-darwin.so) + 1264  [0x1055a6344]  readline.c:1679
                                          8755 rl_initialize  (in libreadline.8.3.dylib) + 228  [0x1057f8aa0]
                                            8755 rltty_set_default_bindings  (in libreadline.8.3.dylib) + 60  [0x105801340]
                                              8755 get_tty_settings  (in libreadline.8.3.dylib) + 48  [0x105801110]
                                                8755 _rl_tcsetwinsize  (in libreadline.8.3.dylib) + 28  [0x105817d80]
                                                  8755 ioctl  (in libsystem_kernel.dylib) + 36  [0x18289e0c0]
                                                    8755 __ioctl  (in libsystem_kernel.dylib) + 8  [0x18289e0d4]