[issue42888] Not installed “libgcc_s.so.1” causes parser crash.

[Alexey Izbyshev]

Alexey Izbyshev <izbyshev at ispras.ru> added the comment:

I've encountered this issue too. My use case was a 32-bit Python on a 64-bit CentOS system, and my understanding of the issue was that 64-bit libgcc_s is somehow counted as a "provider" of libgcc_s for 32-bit libc by the package manager, so 32-bit libgcc_s is never installed even when "yum install glibc.i686" is used because everything seems "already OK":

$ yum deplist glibc

...
  dependency: libgcc
   provider: libgcc.x86_64 4.1.2-55.el5
   provider: libgcc.i386 4.1.2-55.el5
...

(The above is for CentOS 5, but at the time I tested 6 and 7 as well, with the same result).

But I suggest not to dismiss this issue as a packaging bug. Glibc needs libgcc_s for pthread_exit() because it's implemented in terms of pthread_cancel(), and the latter wants to do stack unwinding (probably to cleanup resources for each stack frame). But the code for unwinding lives in libgcc_s. The non-intuitive thing here is that glibc tried to optimize this dependency by dlopen'ing libgcc_s when pthread_cancel() is called instead of making it a startup dependency. Many people consider this a terrible design choice since your program can now fail at an arbitrary moment due to dlopen() failure (and missing libgcc_s is not the only possible reason[1]).

Since CPython doesn't use pthread_cancel() directly, I propose a simple solution  -- just `return NULL` from thread functions instead of calling pthread_exit(). The last time I looked, pthread_exit() in CPython is only called from top frames of the threads, so a direct `return` should suffice. If the top-level thread function simply returns, no stack unwinding is needed, so glibc never uses its cancellation machinery.

I've tested that this solution worked at the time (for 3.8 I think), and the test suite passed. If there is interest in going this way, I can test again.

[1] https://www.sourceware.org/bugzilla/show_bug.cgi?id=13119

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nosy: +izbyshev

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<https://bugs.python.org/issue42888>
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