Better message when a side module needs exceptions

src/parseTools.js

@@ -1320,11 +1320,13 @@ function makeStructuralReturn(values, inAsm) {
 }
 
 function makeThrow(what) {
-  if (ASSERTIONS) {
-    return 'throw ' + what + (DISABLE_EXCEPTION_CATCHING == 1 ? ' + " - Exception catching is disabled, this exception cannot be caught. Compile with -s DISABLE_EXCEPTION_CATCHING=0 or DISABLE_EXCEPTION_CATCHING=2 to catch."' : '') + ';';
-  } else {
-  return 'throw ' + what + ';';
+  if (ASSERTIONS && DISABLE_EXCEPTION_CATCHING == 1) {
+    what += ' + " - Exception catching is disabled, this exception cannot be caught. Compile with -s DISABLE_EXCEPTION_CATCHING=0 or DISABLE_EXCEPTION_CATCHING=2 to catch."';
+    if (MAIN_MODULE) {
+      what += ' + " (note: in dynamic linking, if a side module wants exceptions, the main module must be built with that support)"';
+    }
   }
+  return 'throw ' + what + ';';
 }
 
 function makeSignOp(value, type, op, force, ignore) {

tests/test_other.py

@@ -6497,11 +6497,11 @@ def test_ld_library_path(self):
     self.assertContained('Ok', out)
 
   def test_dlopen_rtld_global(self):
-    # TODO: wasm support. this test checks RTLD_GLOBAL where a module is loaded
-    #       before the module providing a global it needs is. in asm.js we use JS
-    #       to create a redirection function. In wasm we just have wasm, so we
-    #       need to introspect the wasm module. Browsers may add that eventually,
-    #       or we could ship a little library that does it.
+    # This test checks RTLD_GLOBAL where a module is loaded
+    # before the module providing a global it needs is. in asm.js we use JS
+    # to create a redirection function. In wasm we just have wasm, so we
+    # need to introspect the wasm module. Browsers may add that eventually,
+    # or we could ship a little library that does it.
     create_test_file('hello1.c', r'''
 #include <stdio.h>
 
@@ -6565,6 +6565,59 @@ def test_dlopen_rtld_global(self):
     self.assertContained('hello1_val by hello1:3', out)
     self.assertContained('hello1_val by hello2:3', out)
 
+  @no_fastcomp()
+  def test_main_module_without_exceptions_message(self):
+    # A side module that needs exceptions needs a main module with that
+    # support enabled; show a clear message in that case.
+    create_test_file('side.cpp', r'''
+      #include <exception>
+      #include <stdio.h>
+
+      extern "C" void test_throw() {
+        try {
+          throw 42;
+        } catch(int x) {
+          printf("catch %d.\n", x);
+          return;
+        }
+        puts("bad location");
+      }
+    ''')
+    create_test_file('main.cpp', r'''
+      #include <assert.h>
+      #include <stdio.h>
+      #include <stdlib.h>
+      #include <string.h>
+      #include <dlfcn.h>
+
+      typedef void (*voidf)();
+
+      int main() {
+        void* h = dlopen ("libside.wasm", RTLD_NOW|RTLD_GLOBAL);
+        assert(h);
+        voidf f = (voidf)dlsym(h, "test_throw");
+        assert(f);
+        f();
+        return 0;
+      }
+      ''')
+    run_process([PYTHON, EMCC, '-o', 'libside.wasm', 'side.cpp', '-s', 'SIDE_MODULE=1', '-fexceptions'])
+
+    def build_main(args):
+      print(args)
+      with env_modify({'EMCC_FORCE_STDLIBS': 'libc++abi'}):
+        run_process([PYTHON, EMCC, 'main.cpp', '-s', 'MAIN_MODULE=1', '-s', 'EXPORT_ALL',
+                     '--embed-file', 'libside.wasm'] + args)
+
+    build_main([])
+    out = run_js('a.out.js', assert_returncode=None, stderr=STDOUT)
+    self.assertContained('Exception catching is disabled, this exception cannot be caught.', out)
+    self.assertContained('note: in dynamic linking, if a side module wants exceptions, the main module must be built with that support', out)
+
+    build_main(['-fexceptions'])
+    out = run_js('a.out.js')
+    self.assertContained('catch 42', out)
+
   def test_debug_asmLastOpts(self):
     create_test_file('src.c', r'''
 #include <stdio.h>