Fix exception handling (throw) in xeus-cpp-lite.

[anutosh491]

I would expect this to work with what is there as of now but I see the following doesn't work

So out of all the steps (code -> PTU -> LLVM IR -> wasm object -> wasm binary -> loaded on top of main module using dlopen) we need to think which one is going wrong.

1. The linking step through clang-repl is successful here and hence a shared wasm module has been created.

2. But the dlopen step to load it on top of the main module is not working as expected. Hence the trace

[anutosh491]

The trace indicates that the loadDynamicLibrary function call fails from dlopenInternal leading to the error

        var dlopenInternal = (handle, jsflags) => {
            // void *dlopen(const char *file, int mode);
            // http://pubs.opengroup.org/onlinepubs/009695399/functions/dlopen.html
            var filename = UTF8ToString(handle + 36);
            var flags = HEAP32[(((handle) + (4)) >> 2)];
            filename = PATH.normalize(filename);
            var searchpaths = [];

            var global = Boolean(flags & 256);
            var localScope = global ? null : {};

            // We don't care about RTLD_NOW and RTLD_LAZY.
            var combinedFlags = {
                global,
                nodelete: Boolean(flags & 4096),
                loadAsync: jsflags.loadAsync,
            }

            if (jsflags.loadAsync) {
                return loadDynamicLibrary(filename, combinedFlags, localScope, handle);
            }

            try { // THIS TRY BLOCK FAILS !!!!
                return loadDynamicLibrary(filename, combinedFlags, localScope, handle)
            } catch (e) {
                err(`Error in loading dynamic library ${filename}: ${e}`);
                dlSetError(`Could not load dynamic lib: ${filename}\n${e}`);
                return 0;
            }
        }
        

[anutosh491]

Also this appears in the console

[anutosh491]

Going through emscripten's flags they also provide an error like this. We might be missing the flag responsible for enabling exceptions somewhere.

Assertion failed: Exception thrown, but exception catching is not enabled. Compile with -sNO_DISABLE_EXCEPTION_CATCHING or -sEXCEPTION_CATCHING_ALLOWED=[..] to catch. (note: in dynamic linking, if a side module wants exceptions, the main module must be built with that support))

[mcbarton]

@anutosh491 As far as I can tell from the linked repo exception handling isn't currently supported by WebAssembly https://github.com/WebAssembly/exception-handling/tree/master , but the repo contains a proposal on how they hope to add support for it.

[anutosh491]

Hmm, weird. That shouldn't be the case cause as I wrote about the linking step is fine which means we are creating a wasm module out of the "throw" command. It's the loading that is not working as expected. So I am guessing we should be able to do this !

[anutosh491]

This is the wasm module being generated for throw("error")

(module $incr_module_2.wasm
  (memory $env.memory (;0;) (import "env" "memory") 1)
  (table $env.__indirect_function_table (;0;) (import "env" "__indirect_function_table") 0 funcref)
  (global $__memory_base (;0;) (import "env" "__memory_base") i32)
  (global $__table_base (;1;) (import "env" "__table_base") i32)
  (func $__cxa_allocate_exception (;0;) (import "env" "__cxa_allocate_exception") (param i32) (result i32))
  (func $__cxa_throw (;1;) (import "env" "__cxa_throw") (param i32 i32 i32))
  (global $typeinfo for char const* (;2;) (import "GOT.mem" "_ZTIPKc") (mut i32))
  (func $__wasm_call_ctors (;2;) (export "__wasm_call_ctors")
    call $_GLOBAL__sub_I_incr_module_2
  )
  (func $__stmts__0 (;3;)
    (local $var0 i32)
    (local $var1 i32)
    global.get $__memory_base
    local.set $var0
    i32.const 4
    call $__cxa_allocate_exception
    local.tee $var1
    local.get $var0
    i32.const 0
    i32.add
    i32.store
    local.get $var1
    global.get $typeinfo for char const*
    i32.const 0
    call $__cxa_throw
    unreachable
  )
  (func $_GLOBAL__sub_I_incr_module_2 (;4;)
    call $__stmts__0
  )
  (data (global.get $__memory_base) "error\00")
)

[mcbarton]

@anutosh491 The -fexceptions and -fwasm-exceptions flags may be relevant this PR (saw them in this issue emscripten-core/emscripten#22587 )