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[js-api] Add stack trace support and preserve identity of JS exception values passing through WebAssembly. #218

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Nov 9, 2022
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[js-api] Add stack trace support and preserve identity of JS exception values passing through WebAssembly. #218

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[js-api] Add stack trace support.
Ms2ger Aug 25, 2022
b15d635
fixes
Ms2ger Sep 16, 2022
8018474
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Ms2ger Sep 20, 2022
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47 changes: 32 additions & 15 deletions document/js-api/index.bs
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Original file line number Diff line number Diff line change
Expand Up @@ -1040,12 +1040,12 @@ This slot holds a [=function address=] relative to the [=surrounding agent=]'s [
1. [=list/Append=] [=ToWebAssemblyValue=](|arg|, |t|) to |args|.
1. Set |i| to |i| + 1.
1. Let (|store|, |ret|) be the result of [=func_invoke=](|store|, |funcaddr|, |args|).
1. Note: The expectation is that [=func_invoke=] will be updated to return (|store|, <var ignore>val</var>* | [=error=] | (exception |exntag| |payload|)).
1. Note: The expectation is that [=func_invoke=] will be updated to return (|store|, <var ignore>val</var>* | [=error=] | (exception |exntag| |payload| |opaqueData|)).
1. Set the [=surrounding agent=]'s [=associated store=] to |store|.
1. If |ret| is [=error=], throw an exception. This exception should be a WebAssembly {{RuntimeError}} exception, unless otherwise indicated by <a href="#errors">the WebAssembly error mapping</a>.
1. If |ret| is exception |exntag| |payload|, then
1. If |exntag| is the [=JavaScript exception tag=], then
1. Let « [=ref.extern=] |externaddr| » be |payload|.
1. If |ret| is exception |exntag| |payload| |opaqueData|, then
1. If |opaqueData| is not [=ref.null=] [=externref=],
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In which case is this ref.null? When it is thrown from wasm?

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Indeed.

1. Let « [=ref.extern=] |externaddr| » be |opaqueData|.
1. Throw the result of [=retrieving an extern value=] from |externaddr|.
1. Let |exception| be [=create an Exception object|a new Exception=] for |exntag| and |payload|.
1. Throw |exception|.
Expand Down Expand Up @@ -1107,8 +1107,9 @@ Note: Exported Functions do not have a \[[Construct]] method and thus it is not
1. Let |payload| be |v|.\[[Payload]].
1. Otherwise,
1. Let |type| be the [=JavaScript exception tag=].
1. Let |payload| be [=ToWebAssemblyValue=](|v|, [=externref=]).
1. [=WebAssembly/Throw=] with |type| and |payload|.
1. Let |payload| be « ».
Comment on lines 1109 to +1110
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@aheejin aheejin Sep 21, 2022 •
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Do we not need to handle |v|.\[[Stack]] in here and also in line 1106-1107?

1. Let |opaqueData| be [=ToWebAssemblyValue=](|v|, [=externref=])
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It looks this wraps the whole value itself as opaqueData, correct? Where does the stack trace belong then?

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The stack trace just stays in a slot on the Exception JS object

1. [=WebAssembly/Throw=] with |type|, |payload| and |opaqueData|.
1. Otherwise, return |result|.\[[Value]].
1. Let |store| be the [=surrounding agent=]'s [=associated store=].
1. Let (|store|, |funcaddr|) be [=func_alloc=](|store|, |functype|, |hostfunc|).
Expand Down Expand Up @@ -1258,20 +1259,25 @@ Advisement: This method is only expected to be implemented or shipped when both
<h4 id="runtime-exceptions">Runtime exceptions</h4>

<pre class="idl">
dictionary ExceptionOptions {
boolean traceStack = false;
};

[LegacyNamespace=WebAssembly, Exposed=(Window,Worker,Worklet)]
interface Exception {
constructor(Tag exceptionTag, sequence&lt;any> payload);
constructor(Tag exceptionTag, sequence&lt;any> payload, optional ExceptionOptions options = {});
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any getArg(Tag exceptionTag, unsigned long index);
boolean is(Tag exceptionTag);
readonly attribute (DOMString or undefined) stack;
};
</pre>

An {{Exception}} value represents an exception.

<div algorithm>

To <dfn>create an Exception object</dfn> from a [=tag address=] |tagAddress| and [=list=]
of WebAssembly values |payload|, perform the following steps:
To <dfn>create an Exception object</dfn> from a [=tag address=] |tagAddress| and a [=list=] of
WebAssembly values |payload|, perform the following steps:

1. Let |store| be the [=surrounding agent=]'s [=associated store=].
1. Let |types| be [=tag_parameters=](|store|, |tagAddress|).
Expand All @@ -1281,14 +1287,15 @@ of WebAssembly values |payload|, perform the following steps:
1. Let |exception| be a [=new=] {{Exception}}.
1. Set |exception|.\[[Type]] to |tagAddress|.
1. Set |exception|.\[[Payload]] to |payload|.
1. Set |exception|.\[[Stack]] to undefined.
1. Return |exception|.

</div>

<div algorithm>

The <dfn constructor for=Exception
lt="Exception(exceptionTag, payload)">new Exception(|exceptionTag|, |payload|)</dfn>
lt="Exception(exceptionTag, payload, options)">new Exception(|exceptionTag|, |payload|, |options|)</dfn>
constructor steps are:

1. Let |store| be the [=surrounding agent=]'s [=associated store=].
Expand All @@ -1300,6 +1307,10 @@ constructor steps are:
1. [=list/Append=] ? [=ToWebAssemblyValue=](|value|, |resultType|) to |wasmPayload|.
1. Set **this**.\[[Type]] to |exceptionTag|.\[[Address]].
1. Set **this**.\[[Payload]] to |wasmPayload|.
1. If |options|["traceStack"] is true,
1. Set **this**.\[[Stack]] to either a {{DOMString}} representation of the current call stack or undefined.
1. Otherwise,
1. Set **this**.\[[Stack]] to undefined.

</div>

Expand All @@ -1326,22 +1337,28 @@ The <dfn method for="Exception">is(|exceptionTag|)</dfn> method steps are:

</div>

<div algorithm>

The <dfn attribute for="Exception">stack</dfn> getter steps are:

1. Return **this**.\[[Stack]].

</div>

<h4 id="js-exceptions">JavaScript exceptions</h4>

The <dfn>JavaScript exception tag</dfn> is a [=tag address=] reserved by this
specification to distinguish exceptions originating from JavaScript.

For any [=associated store=] |store|, the result of
[=tag_parameters=](|store|, [=JavaScript exception tag=]) must be « [=externref=] ».

Issue: Should it be possible for `catch <JS-exception-tag>` to extract the payload from an exception with this tag?
[=tag_parameters=](|store|, [=JavaScript exception tag=]) must be « ».

<div algorithm>

To <dfn for=WebAssembly>throw</dfn> with a [=tag address=] |type| and matching [=list=] of WebAssembly values |payload|, perform the following steps:
To <dfn for=WebAssembly>throw</dfn> with a [=tag address=] |type|, a matching [=list=] of WebAssembly values |payload|, and an [=externref=] |opaqueData|, perform the following steps:

1. Unwind the stack until reaching the *catching try block* given |type|.
1. Invoke the catch block with |payload|.
1. Invoke the catch block with |payload| and |opaqueData|.

Note: This algorithm is expected to be moved into the core specification.

Expand Down