[EH] Add __cxa_init_primary_exception to cxa_noexception.cpp

tl;dr:

This adds `__cxa_init_primary_exception` to `cxa_noexception.cpp`.

Other targets build `cxa_exception.cpp` in `-fignore-exceptions` mode
and build `cxa_noexception.cpp` only in `-fno-exeptions` mode. But we
build `cxa_noexception.cpp` in `-fignore-exceptions` mode, which means
it needs the definition for `__cxa_init_primary_exception` for linking
to succeed when no EH argument is given (which means
`-fignore-exceptions`).

---

Long version (Feel free to skip):

Background:

After emscripten-core#21638, `__cxa_init_primary_exception` was added in libcxxabi:
https://github.com/emscripten-core/emscripten/blob/28c10a1c5e2862edadd68ab627478204ae96d134/system/lib/libcxxabi/src/cxa_exception.cpp#L209-L226
https://github.com/emscripten-core/emscripten/blob/28c10a1c5e2862edadd68ab627478204ae96d134/system/lib/libcxxabi/src/cxa_exception_emscripten.cpp#L155-L162

Currently the files containing `__cxa_init_primary_exception`,
`cxa_exception.cpp` and `cxa_exception_emscripten.cpp`, are only
compiled when any of the EH mode is specified. `cxa_exception.cpp` is
compiled when Wasm EH is selected, and `cxa_exception_emscripten.cpp` is
compiled when Emscripten EH is selected:
https://github.com/emscripten-core/emscripten/blob/28c10a1c5e2862edadd68ab627478204ae96d134/tools/system_libs.py#L1599-L1608

and this function is called from `make_exception_ptr` in libcxx:
https://github.com/emscripten-core/emscripten/blob/28c10a1c5e2862edadd68ab627478204ae96d134/system/lib/libcxx/include/__exception/exception_ptr.h#L87-L99

And `make_exception_ptr` is called from `std::promise`'s destructor:
https://github.com/emscripten-core/emscripten/blob/28c10a1c5e2862edadd68ab627478204ae96d134/system/lib/libcxx/include/future#L1161-L1168

---

Bug:

Currently any program that calls `std::promise`'s destructor without
specifying any exception-related arguments fails, saying `undefined
symbol: __cxa_init_primary_exception`. Not specifying any exception
arguments, meaning not specifying any among `-fno-exceptions`,
`-fwasm-exceptions`, `-fexceptions`, or
`-sDISABLE_EXCEPTION_CATCHING=0`, defaults to `-fignore-exceptions`,
which allows throwing but not catching.

---

Analysis:

The callsite of `__cxa_init_primary_exception` in `make_exception_ptr`
is guarded with `#ifndef _LIBCPP_HAS_NO_EXCEPTIONS`, so it seems it is
supposed to be included only when exceptions are enabled. This
`_LIBCPP_HAS_NO_EXCEPTIONS` is defined when `__cpp_exceptions` is
defined:
https://github.com/emscripten-core/emscripten/blob/28c10a1c5e2862edadd68ab627478204ae96d134/system/lib/libcxx/include/__config#L644-L646

And that `__cpp_exceptions` is defined in clang, when `-fcxx-exceptions`
is given:
https://github.com/llvm/llvm-project/blob/be566d2eacdaed972b90d2eeb1e66d732c9fe7c1/clang/lib/Frontend/InitPreprocessor.cpp#L638-L639

And that `-fcxx-exceptions` can be specified in command line, but it is
also programmatically added here:
https://github.com/llvm/llvm-project/blob/be566d2eacdaed972b90d2eeb1e66d732c9fe7c1/clang/lib/Driver/ToolChains/Clang.cpp#L371-L388

You can see it is added by default unless the arch is XCore or PS4/PS5,
which means for Wasm it has been always added so far, unless
`-fno-exceptions` is explicitly specified.

So I tried checking the arguments there in Clang and adding
`-fcxx-exceptions` only if either of `-fwasm-exceptions` or
`-enable-emscripten-cxx-exceptions` is given. But this fails when none
of EH is selected (which means `-fignore-exceptions`), because if
`-fcxx-exceptions` is not added, we can't use keywords like `throw`.

So basically the problem is, other targets build `cxa_exception.cpp`
in `-fignore-exceptions` mode and build `cxa_noexception.cpp` only in
`-fno-exeptions` mode. But we build `cxa_noexception.cpp` in
`-fignore-exceptions` mode, which means it needs the definition for
`__cxa_init_primary_exception` for linking to succeed, because
`_LIBCPP_HAS_NO_EXCEPTIONS` cannot be defined in `-fignore-exceptions`
(because we couldn't disable `-fcxx-exceptions` above in Clang to use
 `throw`)

So this adds `__cxa_init_primary_exception` to `cxa_noexception.cpp`.

system/lib/libcxxabi/src/cxa_noexception.cpp

@@ -74,6 +74,12 @@ void __cxa_free_exception(void *thrown_object) throw() {
         ((char *)cxa_exception_from_thrown_object(thrown_object));
     free((void *)raw_buffer);
 }
+
+__cxa_exception* __cxa_init_primary_exception(void* object, std::type_info* tinfo,
+                                              void *(_LIBCXXABI_DTOR_FUNC* dest)(void*)) throw() {
+  __cxa_exception* exception_header = cxa_exception_from_thrown_object(object);
+  return exception_header;
+}
 #endif
 
 } // extern "C"

test/test_other.py

@@ -14859,3 +14859,23 @@ def test_mimalloc_headers(self):
   def test_SUPPORT_BIG_ENDIAN(self):
     # Just a simple build-only test for now
     self.run_process([EMCC, '-sSUPPORT_BIG_ENDIAN', test_file('hello_world.c')])
+
+  @parameterized({
+    'noexcept': ['-fno-exceptions'],
+    'default': [],
+    'except': ['-sDISABLE_EXCEPTION_CATCHING=0'],
+    'except_wasm': ['-fwasm-exceptions'],
+    'except_wasm_exnref': ['-fwasm-exceptions', '-sWASM_EXNREF']
+  })
+  def test_std_promise(self, *args):
+    # Regression test for the bug where std::promise's destructor caused a link
+    # error with __cxa_init_primary_exception when no exception argument is
+    # given (which defaults to -fignore-exceptions)
+    create_file('src.cpp', r'''
+      #include <future>
+      int main() {
+        std::promise<int> p;
+        return 0;
+      }
+    ''')
+    self.run_process([EMXX, 'src.cpp', '-pthread'] + list(args))