[EH] Make Wasm EH rethrow print stack traces

In Wasm EH, even if we set `-sASSERTION` or `-sEXCEPTION_STACK_TRACES`,
if we rethrow an exception in the code, we lose the effect of that
option because previously we called `__throw_exception_with_stack_trace`
only in `__cxa_throw`:
https://github.com/emscripten-core/emscripten/blob/9ce7020632aa6f7578c6e40e197b800a4dd7073f/system/lib/libcxxabi/src/cxa_exception.cpp#L294-L296

This adds the same mechanism to `__cxa_rethrow` (which is used for
C++ `throw;`) and `__cxa_rethrow_primary_exception` (which is used for
`std::rethrow_exception`).

This does not solve the problem of losing stack traces _before_ the
rethrowing. libc++abi's `__cxa_rethrow` and
`__cxa_rethrow_primary_exception` are implemented as throwing a pointer
in the same way we first throw it and they are not aware of any
metadata. This happens even in the native platform with GDB; GDB's
backtrace only shows stack traces after rethrowing. We may try to fix
this later by passing `exnref`
(WebAssembly/exception-handling#281) to the
library, but this is likely to take some time.

Partially fixes emscripten-core#20301.

system/lib/libcxxabi/src/cxa_exception.cpp

@@ -644,6 +644,14 @@ void __cxa_rethrow() {
     }
 #ifdef __USING_SJLJ_EXCEPTIONS__
     _Unwind_SjLj_RaiseException(&exception_header->unwindHeader);
+#elif __USING_WASM_EXCEPTIONS__
+#ifdef NDEBUG
+    _Unwind_RaiseException(&exception_header->unwindHeader);
+#else
+    // In debug mode, call a JS library function to use WebAssembly.Exception JS
+    // API, which enables us to include stack traces
+    __throw_exception_with_stack_trace(&exception_header->unwindHeader);
+#endif
 #else
     _Unwind_RaiseException(&exception_header->unwindHeader);
 #endif
@@ -769,6 +777,14 @@ __cxa_rethrow_primary_exception(void* thrown_object)
         dep_exception_header->unwindHeader.exception_cleanup = dependent_exception_cleanup;
 #ifdef __USING_SJLJ_EXCEPTIONS__
         _Unwind_SjLj_RaiseException(&dep_exception_header->unwindHeader);
+#elif __USING_WASM_EXCEPTIONS__
+#ifdef NDEBUG
+    _Unwind_RaiseException(&exception_header->unwindHeader);
+#else
+    // In debug mode, call a JS library function to use WebAssembly.Exception JS
+    // API, which enables us to include stack traces
+    __throw_exception_with_stack_trace(&exception_header->unwindHeader);
+#endif
 #else
         _Unwind_RaiseException(&dep_exception_header->unwindHeader);
 #endif

test/test_other.py

@@ -8593,6 +8593,64 @@ def test_exceptions_stack_trace_and_message(self, wasm_eh):
     for check in stack_trace_checks:
       self.assertFalse(re.search(check, err), 'Expected regex "%s" to not match on:\n%s' % (check, err))
 
+    # Rethrowing exception currently loses the stack trace before the
+    # rethrowing due to how rethrowing is implemented. So in the examples below
+    # we don't print 'bar' at the moment.
+    # TODO Make rethrow preserve stack traces before rethrowing?
+    rethrow_src1 = r'''
+      #include <stdexcept>
+
+      void bar() {
+        throw std::runtime_error("my message");
+      }
+      void foo() {
+        try {
+          bar();
+        } catch (...) {
+          throw; // rethrowing by throw;
+        }
+      }
+      int main() {
+        foo();
+        return 0;
+      }
+    '''
+    rethrow_src2 = r'''
+      #include <stdexcept>
+
+      void bar() {
+        throw std::runtime_error("my message");
+      }
+      void foo() {
+        try {
+          bar();
+        } catch (...) {
+          auto e = std::current_exception();
+          std::rethrow_exception(e); // rethrowing by std::rethrow_exception
+        }
+      }
+      int main() {
+        foo();
+        return 0;
+      }
+    '''
+    rethrow_stack_trace_checks = [
+      'std::runtime_error[:,][ ]?my message',  # 'std::runtime_error: my message' for Emscripten EH
+      'at ((src.wasm.)?_?__cxa_rethrow|___resumeException)',  # '___resumeException' (JS symbol) for Emscripten EH
+      'at (src.wasm.)?foo',
+      'at (src.wasm.)?main']
+
+    self.set_setting('ASSERTIONS', 1)
+    self.clear_setting('EXCEPTION_STACK_TRACES')
+    err = self.do_run(rethrow_src1, emcc_args=emcc_args, assert_all=True,
+                      assert_returncode=NON_ZERO,
+                      expected_output=rethrow_stack_trace_checks, regex=True)
+    self.assertNotContained('bar', err)
+    err = self.do_run(rethrow_src2, emcc_args=emcc_args, assert_all=True,
+                      assert_returncode=NON_ZERO,
+                      expected_output=rethrow_stack_trace_checks, regex=True)
+    self.assertNotContained('bar', err)
+
   @requires_node
   def test_jsrun(self):
     print(config.NODE_JS)