Avoid reference to SharedArrayBuffer in UTF8ArrayToString (#22844)

This allows program built with `SHARED_MEMORY` enabled to run in a
single threaded context where `SharedArrayBuffer` is not defined (i.e.
when crossOriginIsolated is false).

Fixes: #22837

Author: sbc100

src/parseTools.mjs

@@ -1042,9 +1042,11 @@ function getUnsharedTextDecoderView(heap, start, end) {
   // then unconditionally do a .slice() for smallest code size.
   if (SHRINK_LEVEL == 2 || heap == 'HEAPU8') return shared;
 
-  // Otherwise, generate a runtime type check: must do a .slice() if looking at a SAB,
-  // or can use .subarray() otherwise.
-  return `${heap}.buffer instanceof SharedArrayBuffer ? ${shared} : ${unshared}`;
+  // Otherwise, generate a runtime type check: must do a .slice() if looking at
+  // a SAB, or can use .subarray() otherwise.  Note: We compare with
+  // `ArrayBuffer` here to avoid referencing `SharedArrayBuffer` which could be
+  // undefined.
+  return `${heap}.buffer instanceof ArrayBuffer ? ${unshared} : ${shared}`;
 }
 
 function getEntryFunction() {

test/test_other.py

@@ -12193,14 +12193,26 @@ def test_pthread_relocatable(self):
   @node_pthreads
   def test_pthread_unavailable(self):
     # Run a simple hello world program that uses pthreads
-    self.emcc_args += ['-sPROXY_TO_PTHREAD', '-sEXIT_RUNTIME']
+    self.emcc_args += ['-sPROXY_TO_PTHREAD', '-sEXIT_RUNTIME', '-sDEFAULT_LIBRARY_FUNCS_TO_INCLUDE=$stringToNewUTF8,$UTF8ToString']
     self.do_run_in_out_file_test('hello_world.c')
 
     # Now run the same program but with SharedArrayBuffer undefined, it should run
     # fine and then fail on the first call to pthread_create.
-    create_file('pre.js', 'SharedArrayBuffer = undefined\n')
-    expected = 'pthread_create: environment does not support SharedArrayBuffer, pthreads are not available'
-    self.do_runf('hello_world.c', expected, assert_returncode=NON_ZERO, emcc_args=['--pre-js=pre.js'])
+    #
+    # We specifically test that we can call UTF8ToString, which in older emscripten
+    # versions had an instanceof check against SharedArrayBuffer which would cause
+    # a crash when SharedArrayBuffer was undefined.
+    create_file('pre.js', '''
+      SharedArrayBuffer = undefined;
+      Module.onRuntimeInitialized = () => {
+        var addr = stringToNewUTF8("hello world string, longer than 16 chars");
+        assert(addr);
+        var str = UTF8ToString(addr);
+        console.log("got: " + str);
+        assert(str == "hello world string, longer than 16 chars");
+      };''')
+    expected = ['got: hello world string, longer than 16 chars', 'pthread_create: environment does not support SharedArrayBuffer, pthreads are not available']
+    self.do_runf('hello_world.c', expected, assert_all=True, assert_returncode=NON_ZERO, emcc_args=['--pre-js=pre.js'])
 
   def test_stdin_preprocess(self):
     create_file('temp.h', '#include <string>')