event_data does not have suitable alignment (SAFE_HEAP abort)

[paradust7]

uint8_t event_data[]; in system/lib/html5/callback.c is at offset 12 into the callback_args_t structure. So when the event data contains a double (such as EmscriptenWheelEvent), there will be a misaligned read in the callback. This aborts when SAFE_HEAP is enabled.

Version of emscripten/emsdk:
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 6.0.4 (fe5be6a)
clang version 24.0.0git (https:/github.com/llvm/llvm-project c050c487e9edb97ef44f53cb29fe1d8bcddb8f76)
Target: wasm32-unknown-emscripten
Thread model: posix

Minimal reproducer

//
// Run with:
//
//   emcc -O0 -pthread -sPTHREAD_POOL_SIZE=2 -sSAFE_HEAP=1 repro.c -o repro.js
//   node repro.js
//
// On emcc 6.0.4 with SAFE_HEAP:
//
//   Aborted(alignment fault)
//

#include <emscripten.h>
#include <emscripten/threading.h>
#include <emscripten/html5.h>
#include <pthread.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// Internal API from system/lib/html5/emscripten_internal.h
typedef bool (*event_callback)(int event_type, void *event_data, void *user_data);
extern void _emscripten_run_callback_on_thread(pthread_t t,
                                               event_callback f,
                                               int event_type,
                                               void *event_data,
                                               size_t event_data_size,
                                               void *user_data);

static volatile int g_done = 0;

static bool on_wheel(int event_type, void *event_data, void *user_data) {
  const EmscriptenWheelEvent *e = (const EmscriptenWheelEvent *)event_data;
  printf("%f %f %f\n", e->deltaX, e->deltaY, e->deltaZ);
  g_done = 1;
  return true;
}

static void *sender(void *arg) {
  pthread_t target = (pthread_t)arg;
  EmscriptenWheelEvent ev;
  memset(&ev, 0, sizeof ev);
  ev.deltaX = 1.0;
  ev.deltaY = 2.0;
  ev.deltaZ = 3.0;
  _emscripten_run_callback_on_thread(target, on_wheel, EMSCRIPTEN_EVENT_WHEEL,
                                     &ev, sizeof ev, NULL);
  return NULL;
}

int main(void) {
  pthread_t t;
  pthread_create(&t, NULL, sender, (void *)pthread_self());
  pthread_detach(t);

  // Wait for delivery. "emscripten_thread_sleep" services the mailbox.
  while (!g_done)
    emscripten_thread_sleep(10);
  return 0;
}

Output

$ emcc -O0 -pthread -sPTHREAD_POOL_SIZE=2 -sSAFE_HEAP=1 repro.c -o repro.js
$ node repro.js
Aborted(alignment fault)