Writable stack (#5999)

* fix writable stack in ErrnoError to work in strict mode

* add testing for strict mode

Author: saschanaz

src/library_fs.js

@@ -1368,7 +1368,7 @@ mergeInto(LibraryManager.library, {
         this.setErrno(errno);
         this.message = ERRNO_MESSAGES[errno];
         // Node.js compatibility: assigning on this.stack fails on Node 4 (but fixed on Node 8)
-        if (this.stack) Object.defineProperty(this, "stack", { value: (new Error).stack });
+        if (this.stack) Object.defineProperty(this, "stack", { value: (new Error).stack, writable: true });
 #if ASSERTIONS
         if (this.stack) this.stack = demangleAll(this.stack);
 #endif

tests/test_core.py

@@ -4553,16 +4553,31 @@ def test_fs_mmap(self):
       self.do_run_from_file(src, out)
 
   def test_fs_errorstack(self):
+    # Enables strict mode, which may catch some strict-mode-only errors
+    # so that users can safely work with strict JavaScript if enabled.
+    post = '''
+def process(filename):
+  src = open(filename, 'r').read()
+  open(filename, 'w').write('"use strict";\\n' + src)
+'''
+
     Settings.FORCE_FILESYSTEM = 1
     self.do_run(r'''
       #include <emscripten.h>
+      #include <iostream>
       int main(void) {
+        std::cout << "hello world\n"; // should work with strict mode
         EM_ASM(
-          FS.write('/dummy.txt', 'homu');
+          try {
+            FS.write('/dummy.txt', 'homu');
+          } catch (err) {
+            err.stack = err.stack; // should be writable
+            throw err;
+          }
         );
         return 0;
       }
-    ''', 'at new ErrnoError', js_engines=[NODE_JS]) # engines has different error stack format
+    ''', 'at Object.write', js_engines=[NODE_JS], post_build=post) # engines has different error stack format
 
   def test_unistd_access(self):
     self.clear()