atexit callback reads an incorrect value from a TLS variable when `-sPROXY_TO_PTHREAD=1`

[ktock]

In Emscripten, an atexit callback reads an unexpected value from a TLS variable when -sPROXY_TO_PTHREAD=1 is used.

Reproducer

#include <stdio.h>
#include <stdlib.h>

__thread int a = 0;

static void print_a(void)
{
  printf("a=%d\n", a);
}

int main(int argc, char **argv)
{
  a = 1;
  atexit(print_a);
  exit(0);
}

Expected behaviour

When -sPROXY_TO_PTHREAD=1 is not used, atexit callback can read the correct value from the TLS variable.

$ emcc -o ./test.js -sEXIT_RUNTIME=1 ./test.c
$ node ./test.js
a=1

gcc can also read the TLS variable correctly.

$ gcc -o ./a.out ./test.c
$ ./a.out
a=1

Actual behaviour

When -sPROXY_TO_PTHREAD=1 is used, Emscripten's atexit callback reads an incorrect value from the TLS variable.

$ emcc -o ./test.js -pthread -sPROXY_TO_PTHREAD=1 -sEXIT_RUNTIME=1 ./test.c
$ node ./test.js
a=0

emcc -v

emcc -v
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 4.0.23-git (28bcb86466a273859b8adb43cb167b97e05e145d)
clang version 22.0.0git (https:/github.com/llvm/llvm-project 8518d2c4057d9aa4249b8466a4d77771e4f1bf4f)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /emsdk/upstream/bin
Build config: +assertions

[brendandahl]

This appears to be happening because we call __funcs_on_exit() on the main browser thread where TLS is not setup. We should either be calling it from the worker pthread thread that calls main or maybe setup TLS on the main thread. I'm guessing the former would be better, but not sure.