pthread+dylink: Ensure code is in sync on thread start

We still don't have a great way to keep threads in sync but this
at least means they will be correct at the moment the start.

Part of #3494.

Fixes: #16021

emcc.py

@@ -2044,6 +2044,10 @@ def default_setting(name, new_default):
       'emscripten_sync_run_in_main_thread_2',
       'emscripten_sync_run_in_main_thread_4',
     ]
+
+    if settings.MAIN_MODULE:
+      settings.REQUIRED_EXPORTS += ['_emscripten_thread_sync_code', '__dl_seterr']
+
     settings.DEFAULT_LIBRARY_FUNCS_TO_INCLUDE += [
       '$exitOnMainThread',
     ]

src/library_dylink.js

@@ -115,12 +115,12 @@ var LibraryDylink = {
           err("unhandled export type for `" + symName + "`: " + (typeof value));
         }
 #if DYLINK_DEBUG
-        err("updateGOT:  after: " + symName + ' : ' + GOT[symName].value);
+        err("updateGOT:  after: " + symName + ' : ' + GOT[symName].value + ' (' + value + ')');
 #endif
       }
 #if DYLINK_DEBUG
       else if (GOT[symName].value != value) {
-        err("updateGOT: EXISTING SYMBOL: " + symName + ' : ' + GOT[symName].value + " " + value);
+        err("updateGOT: EXISTING SYMBOL: " + symName + ' : ' + GOT[symName].value + ' (' + value + ')');
       }
 #endif
     }
@@ -427,6 +427,13 @@ var LibraryDylink = {
     }
   },
 
+#if DYLINK_DEBUG
+  $dumpTable: function() {
+    for (var i = 0; i < wasmTable.length; i++)
+      err('table: ' + i + ' : ' + wasmTable.get(i));
+  },
+#endif
+
   // Loads a side module from binary data or compiled Module. Returns the module's exports or a
   // promise that resolves to its exports if the loadAsync flag is set.
   $loadWebAssemblyModule__deps: [
@@ -649,6 +656,7 @@ var LibraryDylink = {
   $loadDynamicLibrary: function(lib, flags, handle) {
 #if DYLINK_DEBUG
     err('loadDynamicLibrary: ' + lib + ' handle:' + handle);
+    err('existing: ' + Object.keys(LDSO.loadedLibsByName));
 #endif
     if (lib == '__main__' && !LDSO.loadedLibsByName[lib]) {
       LDSO.loadedLibsByName[lib] = {
@@ -798,7 +806,7 @@ var LibraryDylink = {
   },
 
   // void* dlopen(const char* filename, int flags);
-  $dlopenInternal__deps: ['$FS', '$ENV', '$dlSetError'],
+  $dlopenInternal__deps: ['$FS', '$ENV', '$dlSetError', '$PATH'],
   $dlopenInternal: function(handle, jsflags) {
     // void *dlopen(const char *file, int mode);
     // http://pubs.opengroup.org/onlinepubs/009695399/functions/dlopen.html
@@ -807,6 +815,7 @@ var LibraryDylink = {
 #if DYLINK_DEBUG
     err('dlopenInternal: ' + filename);
 #endif
+    filename = PATH.normalize(filename);
     var searchpaths = [];
 
     var isValidFile = (filename) => {
@@ -933,14 +942,19 @@ var LibraryDylink = {
     }
 
     if (typeof result === 'function') {
+#if DYLINK_DEBUG
+      err('dlsym: ' + symbol + ' getting table slot for: ' + result);
+#endif
       // Insert the function into the wasm table.  If its a direct wasm function
       // the second argument will not be needed.  If its a JS function we rely
       // on the `sig` attribute being set based on the `<func>__sig` specified
       // in library JS file.
-      return addFunction(result, result.sig);
-    } else {
-      return result;
+      result = addFunction(result, result.sig);
     }
+#if DYLINK_DEBUG
+    err('dlsym: ' + symbol + ' -> ' + result);
+#endif
+    return result;
   },
 #endif // MAIN_MODULE != 0
 };

src/library_pthread.js

@@ -996,6 +996,12 @@ var LibraryPThread = {
   $invokeEntryPoint: function(ptr, arg) {
 #if PTHREADS_DEBUG
     err('invokeEntryPoint: ' + ptrToString(ptr));
+#endif
+#if MAIN_MODULE
+    // Before we call the thread entry point, make sure any shared libraries
+    // have been loaded on this there.  Otherwise our table migth be not be
+    // in sync and might not contain the function pointer `ptr` at all.
+    __emscripten_thread_sync_code();
 #endif
     return {{{ makeDynCall('ii', 'ptr') }}}(arg);
   },

tests/core/pthread/test_pthread_dylink_entry_point.c

@@ -0,0 +1,30 @@
+#include <stdio.h>
+#include <dlfcn.h>
+#include <pthread.h>
+
+typedef void* (*thread_main_t)(void*);
+
+int main() {
+  puts("hello from main");
+
+  void *lib_handle = dlopen("./liblib.so", RTLD_NOW);
+  if (!lib_handle) {
+    puts("cannot load side module");
+    puts(dlerror());
+    return 1;
+  }
+
+  thread_main_t tmain = (thread_main_t)dlsym(lib_handle, "side_module_thread_main");
+  if (!tmain) {
+    puts("cannot find side function");
+    return 1;
+  }
+
+  printf("starting thread with %p\n", tmain);
+  pthread_t t;
+  pthread_create(&t, NULL, tmain, (void*)"hello");
+  pthread_join(t, NULL);
+
+  printf("done\n");
+  return 0;
+}

tests/core/pthread/test_pthread_dylink_entry_point.out

@@ -0,0 +1 @@
+call thread fn in side: hello

tests/core/pthread/test_pthread_dylink_entry_point_side.c

@@ -0,0 +1,8 @@
+#include <stdio.h>
+#include <emscripten.h>
+#include <pthread.h>
+
+EMSCRIPTEN_KEEPALIVE void* side_module_thread_main(void* arg) {
+  printf("call thread fn in side: %s\n", (char*)arg);
+  return NULL;
+}

tests/test_core.py

@@ -8681,6 +8681,20 @@ def test_pthread_dylink(self):
     self.dylink_testf(main, so_name=very_long_name,
                       need_reverse=False)
 
+  @parameterized({
+    '': (['-sNO_AUTOLOAD_DYLIBS'],),
+    'autoload': ([],)
+  })
+  @needs_dylink
+  @node_pthreads
+  def test_pthread_dylink_entry_point(self, args):
+    self.emcc_args.append('-Wno-experimental')
+    self.set_setting('EXIT_RUNTIME')
+    self.set_setting('USE_PTHREADS')
+    self.set_setting('PTHREAD_POOL_SIZE', 1)
+    main = test_file('core/pthread/test_pthread_dylink_entry_point.c')
+    self.dylink_testf(main, need_reverse=False, emcc_args=args)
+
   @needs_dylink
   @node_pthreads
   def test_pthread_dlopen(self):