Why catch unhandled rejections? #7855
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Yeah, I'd love to remove this too. Running our test suite to remember why we have it:
So in both cases an error is handled very awkwardly, and worst no error code is emitted, so tests might miss stuff (which happens on the wasm waterfall, 2 unexpected successes). On the other hand, I don't understand the issue you're seeing, since I do see stack traces emitted, while you say they disappear. Do you have a testcase I can see that with? |
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(and in the emscripten test suite, |
rafaelgieschke
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is there a flag to remove it? |
kripken
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…uring async instantiation may behave oddly (they hang for a few seconds, node warns this behavior will be fixed in the future, and it exits with a 0 returncode), which is a downside, but the upside is that it avoids confusion for developers that get the handler popping up when they didn't expect it. Alter a test to work around this. Fixes #7855, #9028.
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…errors during async instantiation may behave oddly (they hang for a few seconds, node warns this behavior will be fixed in the future, and it exits with a 0 returncode), which is a downside, but the upside is that it avoids confusion for developers that get the handler popping up when they didn't expect it. Alter a test to work around this. Fixes #7855, #9028." This reverts commit f132637.
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#9028 is open to add a flag for this. |
kripken
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…jections in node (#9061) * Add NODEJS_CATCH_REJECTION, a flag that allows disabling the catching of unhandled rejections in node. This is useful if a dev doesn't want it to interfere with their own code. We don't set this to 0 by default because it would mean some wasm errors end up not returning a non-zero return code from node (still true as of 12.4.0) Helps #7855, #9028
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This issue has been automatically marked as stale because there has been no activity in the past year. It will be closed automatically if no further activity occurs in the next 7 days. Feel free to re-open at any time if this issue is still relevant. |
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…jections in node (emscripten-core#9061) * Add NODEJS_CATCH_REJECTION, a flag that allows disabling the catching of unhandled rejections in node. This is useful if a dev doesn't want it to interfere with their own code. We don't set this to 0 by default because it would mean some wasm errors end up not returning a non-zero return code from node (still true as of 12.4.0) Helps emscripten-core#7855, emscripten-core#9028
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When using ttf2woff2 as a library (e.g. inside webfonts-loader), this was previously catching promise rejections from other code that had nothing to do with ttf2woff2. See emscripten-core/emscripten#7855. Signed-off-by: Anders Kaseorg <andersk@mit.edu>
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When using ttf2woff2 as a library (e.g. inside webfonts-loader), this was previously catching promise rejections from other code that had nothing to do with ttf2woff2. See emscripten-core/emscripten#7855. Signed-off-by: Anders Kaseorg <andersk@mit.edu>
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Summary:
Currently, initialising `emhermesc.js` (eg by requiring `metro-hermes-compiler`, or just `metro`) sets an uncaught exception handler on the global NodeJS host process:
```
process.on(
'uncaughtException',
function (ex) {if (!(ex instanceof ExitStatus)) {throw ex;}},
);
```
And similarly, an `unhandledRejection` handler
```
process["on"]("unhandledRejection", function (reason) {
throw reason;
});
```
Because these are persistent, global handlers, they affect the behaviour of the host application or other libraries at any time after `emhermesc.js` is initialised - even if it's never used.
As far as NodeJS is concerned, `uncaughtException` listeners should never throw. Node prints the stack trace starting with the entirety of the emhermesc source, and then exits with [code 7 (Internal exception handler runtime failure)](https://nodejs.org/api/process.html#exit-codes). In cases where this output is truncated (eg, CI), the cause of the original exception is lost.
The `unhandledRejection` listener is [apparently](emscripten-core/emscripten#7855 (comment)) to avoid "awkward" output in old (<=10) Node versions. It [serves no purpose at all since Node 15](https://developer.ibm.com/blogs/nodejs-15-release-blog/), but still messes up stack traces when the `throw` comes from a huge single line of minified code. It's particularly unnecessary in `emhermesc`'s case since I don't think we expose any async API.
The injection of this code is gated in the emscripten build step behind `#ifdef NODEJS_CATCH_EXIT` and `#ifdef NODEJS_CATCH_REJECTION` respectively.
Context from emscripten
- [Handler addition code in `shell.js`](https://github.com/emscripten-core/emscripten/blob/main/src/shell.js#L217-L233)
- [Issue: Code generated by emscripten is catching all uncaught exceptions](emscripten-core/emscripten#5957)
- [Issue: Why catch unhandled rejections?](emscripten-core/emscripten#7855)
- [Docs for the flags](https://github.com/emscripten-core/emscripten/blob/main/src/settings.js#L704-L722)
Reviewed By: neildhar
Differential Revision: D34790221
fbshipit-source-id: e8b355e07022f4c377d918aa7dfdefe3d5272639
robhogan
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…N=0 (facebook#698) Summary: Pull Request resolved: facebook#698 Currently, initialising `emhermesc.js` (eg by requiring `metro-hermes-compiler`, or just `metro`) sets an uncaught exception handler on the global NodeJS host process: ``` process.on( 'uncaughtException', function (ex) {if (!(ex instanceof ExitStatus)) {throw ex;}}, ); ``` And similarly, an `unhandledRejection` handler ``` process["on"]("unhandledRejection", function (reason) { throw reason; }); ``` Because these are persistent, global handlers, they affect the behaviour of the host application or other libraries at any time after `emhermesc.js` is initialised - even if it's never used. As far as NodeJS is concerned, `uncaughtException` listeners should never throw. Node prints the stack trace starting with the entirety of the emhermesc source, and then exits with [code 7 (Internal exception handler runtime failure)](https://nodejs.org/api/process.html#exit-codes). In cases where this output is truncated (eg, CI), the cause of the original exception is lost. The `unhandledRejection` listener is [apparently](emscripten-core/emscripten#7855 (comment)) to avoid "awkward" output in old (<=10) Node versions. It [serves no purpose at all since Node 15](https://developer.ibm.com/blogs/nodejs-15-release-blog/), but still messes up stack traces when the `throw` comes from a huge single line of minified code. It's particularly unnecessary in `emhermesc`'s case since I don't think we expose any async API. The injection of this code is gated in the emscripten build step behind `#ifdef NODEJS_CATCH_EXIT` and `#ifdef NODEJS_CATCH_REJECTION` respectively. Context from emscripten - [Handler addition code in `shell.js`](https://github.com/emscripten-core/emscripten/blob/main/src/shell.js#L217-L233) - [Issue: Code generated by emscripten is catching all uncaught exceptions](emscripten-core/emscripten#5957) - [Issue: Why catch unhandled rejections?](emscripten-core/emscripten#7855) - [Docs for the flags](https://github.com/emscripten-core/emscripten/blob/main/src/settings.js#L704-L722) Reviewed By: neildhar Differential Revision: D34790221 fbshipit-source-id: 19782de6af0d4e26a91746443abb528a239932a0
facebook-github-bot
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…N=0 (#698) Summary: Pull Request resolved: #698 Currently, initialising `emhermesc.js` (eg by requiring `metro-hermes-compiler`, or just `metro`) sets an uncaught exception handler on the global NodeJS host process: ``` process.on( 'uncaughtException', function (ex) {if (!(ex instanceof ExitStatus)) {throw ex;}}, ); ``` And similarly, an `unhandledRejection` handler ``` process["on"]("unhandledRejection", function (reason) { throw reason; }); ``` Because these are persistent, global handlers, they affect the behaviour of the host application or other libraries at any time after `emhermesc.js` is initialised - even if it's never used. As far as NodeJS is concerned, `uncaughtException` listeners should never throw. Node prints the stack trace starting with the entirety of the emhermesc source, and then exits with [code 7 (Internal exception handler runtime failure)](https://nodejs.org/api/process.html#exit-codes). In cases where this output is truncated (eg, CI), the cause of the original exception is lost. The `unhandledRejection` listener is [apparently](emscripten-core/emscripten#7855 (comment)) to avoid "awkward" output in old (<=10) Node versions. It [serves no purpose at all since Node 15](https://developer.ibm.com/blogs/nodejs-15-release-blog/), but still messes up stack traces when the `throw` comes from a huge single line of minified code. It's particularly unnecessary in `emhermesc`'s case since I don't think we expose any async API. The injection of this code is gated in the emscripten build step behind `#ifdef NODEJS_CATCH_EXIT` and `#ifdef NODEJS_CATCH_REJECTION` respectively. Context from emscripten - [Handler addition code in `shell.js`](https://github.com/emscripten-core/emscripten/blob/main/src/shell.js#L217-L233) - [Issue: Code generated by emscripten is catching all uncaught exceptions](emscripten-core/emscripten#5957) - [Issue: Why catch unhandled rejections?](emscripten-core/emscripten#7855) - [Docs for the flags](https://github.com/emscripten-core/emscripten/blob/main/src/settings.js#L704-L722) Reviewed By: neildhar Differential Revision: D34790221 fbshipit-source-id: 645e514db4107127b849e3a0baae088bd1ed9b50
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This is a problem I always eventually have when working with Emscripten, so I figured I should open an issue because at the moment it makes for painful debugging sessions and error reports if you forget this detail.
Why is Emscripten's default shell configured to automatically catch unhandled rejections and exit the program? Shouldn't that be a decision taken by the user of the Emscriptened code, rather than Emscripten itself? This is even more problematic when Emscripten is used to compile a library, because then it silently bleeds into the code of the consumer applications - and goes unnoticed until it actually matters.
In particular, this problem causes stacktraces to disappear and to be replaced by an unhelpful generic error message (since Node doesn't print the exception if it sees nothing bound to
unhandledRejection).cc @dschuff @kripken
ref #5948 / #7135
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