Fix for longjmp(.., 0) under wasm EH

Fixes: emscripten-core#21486

Author: sbc100

system/lib/compiler-rt/emscripten_setjmp.c

@@ -90,6 +90,14 @@ thread_local struct __WasmLongjmpArgs __wasm_longjmp_args;
 // TODO Consider switching to throwing two values at the same time later.
 void __wasm_longjmp(void *env, int val) {
   __wasm_longjmp_args.env = env;
+  /*
+  * C standard:
+  *   The longjmp function cannot cause the setjmp macro to return
+  *   the value 0; if val is 0, the setjmp macro returns the value 1.
+  */
+  if (val == 0) {
+    val = 1;
+  }
   __wasm_longjmp_args.val = val;
   __builtin_wasm_throw(C_LONGJMP, &__wasm_longjmp_args);
 }

test/core/test_longjmp_zero.c

@@ -0,0 +1,28 @@
+/*
+ * Copyright 2016 The Emscripten Authors.  All rights reserved.
+ * Emscripten is available under two separate licenses, the MIT license and the
+ * University of Illinois/NCSA Open Source License.  Both these licenses can be
+ * found in the LICENSE file.
+ */
+
+#include <stdio.h>
+#include <setjmp.h>
+
+int main() {
+  printf("start\n");
+  jmp_buf b1;
+  int val = setjmp(b1);
+  if (val) {
+    printf("success\n");
+    return 0;
+  }
+  /*
+   * C standard:
+   * > The longjmp function cannot cause the setjmp macro to return
+   * > the value 0; if val is 0, the setjmp macro returns the value 1.
+   */
+  printf("longjmp\n");
+  longjmp(b1, 0);
+  __builtin_trap();
+  return 0;
+}

test/core/test_longjmp_zero.out

@@ -0,0 +1,3 @@
+start
+longjmp
+success

test/test_core.py

@@ -892,6 +892,10 @@ def test_longjmp_standalone(self):
   def test_longjmp(self):
     self.do_core_test('test_longjmp.c')
 
+  @with_both_sjlj
+  def test_longjmp_zero(self):
+    self.do_core_test('test_longjmp_zero.c')
+
   def test_longjmp_with_and_without_exceptions(self):
     # Emscripten SjLj with and without Emscripten EH support
     self.set_setting('SUPPORT_LONGJMP', 'emscripten')