musl default pthread stack stack does not match DEFAULT_PTHREAD_STACK_SIZE

[sbc100]

We have a build setting called DEFAULT_PTHREAD_STACK_SIZE (default to 2Mb) which kicks in when a thread is created without any attributes.

However if one create a default set of attributes using pthread_attr_init musl will configure it with its own internal default which is 80k:

emscripten/system/lib/libc/musl/src/internal/pthread_impl.h

Line 182 in 592dec1

#define DEFAULT_STACK_SIZE 81920

Possible solutions:

1. Have musl call back out the JS to know the actual default desired by the user (not know at libc compile time)
2. Remove DEFAULT_PTHREAD_STACK_SIZE and users who want a custom stack size can do so programmatically using pthread_attr_setstacksize
3. Set musl's DEFAULT_STACK_SIZE to match the default DEFAULT_PTHREAD_STACK_SIZE so most users will never notice (only users who override DEFAULT_PTHREAD_STACK_SIZE would see this behaviour).

(3) is the safest thing to do right away which will fix at least one know issue I found in the lsan codebase.

[kleisauke]

Note that with commit http://git.musl-libc.org/cgit/musl/commit/?id=c0058ab465e950c2c3302d2b62e21cc0b494224b musl increased the default thread stack size to 128K, so another option would be to adjust DEFAULT_PTHREAD_STACK_SIZE to this value. See also the earlier discussion in #14177 (comment).