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doc: clarify synchronous blocking of Worker stdio
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Fixes: nodejs#25630
Signed-off-by: James M Snell <jasnell@gmail.com>
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jasnell committed May 13, 2021
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Expand Up @@ -1191,6 +1191,30 @@ active handle in the event system. If the worker is already `unref()`ed calling

## Notes

### Synchronous blocking of stdio

`Worker`s utilize message passing via {MessagePort} to implement interactions
with `stdio`. This means that `stdio` output originating from a `Worker` can
get blocked by synchronous code on the receiving end that is blocking the
Node.js event loop.

```js
'use strict';

const {
Worker,
isMainThread,
} = require('worker_threads');

if (isMainThread) {
new Worker(__filename);
for (let n = 0; n < 1e10; n++) {}
} else {
// This output will be blocked by the for loop in the main thread
console.log('foo');
}
```

### Launching worker threads from preload scripts

Take care when launching worker threads from preload scripts (scripts loaded
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