unix/mpthreadport: Fix type / comparison of PTHREAD_STACK_MIN.

[andrewleech]

Summary

When compiling the unix port on my recently updated ubuntu machine I ran into this error:

mpthreadport.c: In function ‘mp_thread_create’:
mpthreadport.c:253:21: error: comparison of integer expressions of different signedness: ‘size_t’ {aka ‘long unsigned int’} and ‘long int’ [-Werror=sign-compare]
  253 |     if (*stack_size < PTHREAD_STACK_MIN) {
      |                     ^
cc1: all warnings being treated as errors
See https://github.com/micropython/micropython/wiki/Build-Troubleshooting
make: *** [../../py/mkrules.mk:101: build-standard/mpthreadport.o] Error 1
make: *** Waiting for unfinished jobs....
make: Leaving directory '/home/corona/micropython/ports/unix'

It looks like my gcc was updated:

[corona@Telie micropython]$ gcc --version
gcc (GCC) 14.2.1 20250207
Copyright (C) 2024 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

The variable PTHREAD_STACK_MIN is defined by the pthread libraries:

/usr/include/bits/pthread_stack_min.h
1:/* Definition of PTHREAD_STACK_MIN.  Linux version.
20:#define PTHREAD_STACK_MIN    16384

It seems GCC 14 got stricter with warn/errs like -Wsign-compare and types a "bare number" as a long int that can't be compared to a (unsigned) size_t ?

Testing

With this change the unix port compiled again. The thread unit tests pass:

[corona@Telie tests]$ ./run-tests.py ./thread/thread*.py
platform=linux arch=x64
pass  ./thread/thread_heap_lock.py
pass  ./thread/thread_ident1.py
pass  ./thread/thread_lock1.py
pass  ./thread/thread_exc2.py
pass  ./thread/thread_exit2.py
pass  ./thread/thread_lock2.py
pass  ./thread/thread_gc1.py
pass  ./thread/thread_lock5.py
pass  ./thread/thread_qstr1.py
pass  ./thread/thread_shared1.py
pass  ./thread/thread_shared2.py
pass  ./thread/thread_sleep1.py
pass  ./thread/thread_lock3.py
pass  ./thread/thread_lock4.py
pass  ./thread/thread_stacksize1.py
pass  ./thread/thread_exit1.py
pass  ./thread/thread_coop.py
pass  ./thread/thread_exc1.py
pass  ./thread/thread_sleep2.py
pass  ./thread/thread_start1.py
pass  ./thread/thread_start2.py
pass  ./thread/thread_stdin.py
22 tests performed (127 individual testcases)
22 tests passed

Trade-offs and Alternatives

The function could accept a ssize_t instead but that's arguably a broader change.
Perhaps there's a compiler flag to work around this default signing of integers a different way?

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.38%. Comparing base (7729e80) to head (bc77b27).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #17440   +/-   ##
=======================================
  Coverage   98.38%   98.38%           
=======================================
  Files         171      171           
  Lines       22238    22238           
=======================================
  Hits        21879    21879           
  Misses        359      359           

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[github-actions[bot]]

Code size report:

   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:    +0 +0.000% standard
      stm32:    +0 +0.000% PYBV10
     mimxrt:    +0 +0.000% TEENSY40
        rp2:    +0 +0.000% RPI_PICO_W
       samd:    +0 +0.000% ADAFRUIT_ITSYBITSY_M4_EXPRESS
  qemu rv32:    +0 +0.000% VIRT_RV32