[dylink] Match behaviour between sync and async versions of dlopen. NFC (#20207)

In particular honoer LD_LIBRARY_PATH, and check for already loaded
libraries.

Author: sbc100

system/lib/libc/dynlink.c

@@ -514,6 +514,29 @@ static int path_find(const char *name, const char *s, char *buf, size_t buf_size
   }
 }
 
+// Resolve filename using LD_LIBRARY_PATH
+static const char* resolve_path(char* buf, const char* file, size_t buflen) {
+  if (!strchr(file, '/')) {
+    const char* env_path = getenv("LD_LIBRARY_PATH");
+    if (env_path && path_find(file, env_path, buf, buflen) == 0) {
+      dbg("dlopen: found in LD_LIBRARY_PATH: %s", buf);
+      return buf;
+    }
+  }
+  return file;
+}
+
+// Search for library name to see if it's already loaded
+static struct dso* find_existing(const char* file) {
+  for (struct dlevent* e = head; e; e = e->next) {
+    if (e->sym_index == -1 && !strcmp(e->dso->name, file)) {
+      dbg("dlopen: already opened: %p", e->dso);
+      return e->dso;
+    }
+  }
+  return NULL;
+}
+
 // Internal version of dlopen with typed return value.
 // Without this, the compiler won't tell us if we have the wrong return type.
 static struct dso* _dlopen(const char* file, int flags) {
@@ -533,25 +556,12 @@ static struct dso* _dlopen(const char* file, int flags) {
   dlsync();
 #endif
 
-  struct dso* p;
-
-  /* Resolve filename using LD_LIBRARY_PATH */
   char buf[2*NAME_MAX+2];
-  if (!strchr(file, '/')) {
-    const char* env_path = getenv("LD_LIBRARY_PATH");
-    if (env_path && path_find(file, env_path, buf, sizeof buf) == 0) {
-      dbg("dlopen: found in LD_LIBRARY_PATH: %s", buf);
-      file = buf;
-    }
-  }
+  file = resolve_path(buf, file, sizeof buf);
 
-  /* Search for the name to see if it's already loaded */
-  for (struct dlevent* e = head; e; e = e->next) {
-    if (e->sym_index == -1 && !strcmp(e->dso->name, file)) {
-      dbg("dlopen: already opened: %p", e->dso);
-      p = e->dso;
-      goto end;
-    }
+  struct dso* p = find_existing(file);
+  if (p) {
+    goto end;
   }
 
   p = load_library_start(file, flags);
@@ -580,16 +590,24 @@ void* dlopen(const char* file, int flags) {
 
 void emscripten_dlopen(const char* filename, int flags, void* user_data,
                        em_dlopen_callback onsuccess, em_arg_callback_func onerror) {
+  dbg("emscripten_dlopen: %s", filename);
   if (!filename) {
-    onsuccess(user_data, head);
+    onsuccess(user_data, head->dso);
     return;
   }
   do_write_lock();
 #ifdef _REENTRANT
   // Make sure we are in sync before performing any write operations.
   dlsync();
 #endif
-  struct dso* p = load_library_start(filename, flags);
+  char buf[2*NAME_MAX+2];
+  filename = resolve_path(buf, filename, sizeof buf);
+  struct dso* p = find_existing(filename);
+  if (p) {
+    onsuccess(user_data, p);
+    return;
+  }
+  p = load_library_start(filename, flags);
   if (!p) {
     do_write_unlock();
     onerror(user_data);

test/test_other.py

@@ -6777,10 +6777,15 @@ def test_dlopen_rtld_global(self):
 
   def test_dlopen_async(self):
     create_file('side.c', 'int foo = 42;\n')
-    self.run_process([EMCC, 'side.c', '-o', 'libside.so', '-sSIDE_MODULE'])
+    create_file('pre.js', r'''
+Module.preRun = () => {
+  ENV['LD_LIBRARY_PATH']='/usr/lib';
+};
+''')
+    self.run_process([EMCC, 'side.c', '-o', 'tmp.so', '-sSIDE_MODULE'])
     self.set_setting('MAIN_MODULE', 2)
     self.set_setting('EXIT_RUNTIME')
-    self.do_other_test('test_dlopen_async.c')
+    self.do_other_test('test_dlopen_async.c', ['--pre-js=pre.js', '--embed-file', 'tmp.so@/usr/lib/libside.so'])
 
   def test_dlopen_promise(self):
     create_file('side.c', 'int foo = 42;\n')