Bug: unstable_useMutableSource throws error when mutated before subscribe #19948

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1. SreeniIO

   added

   Status: UnconfirmedA potential issue that we haven't yet confirmed as a bugA potential issue that we haven't yet confirmed as a bug

   on Oct 2, 2020
2. bvaughn

   self-assigned this

   on Oct 2, 2020
3. SreeniIO commented on Oct 7, 2020

   

   SreeniIO

   on Oct 7, 2020

   Author

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   I'm using this workaround for now

     try {
       value = useMutableSource(mutableSource, getSnapshot, defaultSubscribe);
     } catch (error) {
       throw new Promise((resolve) => resolve());
     }

   https://codesandbox.io/s/usemutablesource-forked-0suof?file=/src/App.js:1126-1269
4. bvaughn

   added

   Component: Concurrent Features

   Type: Bug

   Type: Needs Investigation

   and removed

   Status: UnconfirmedA potential issue that we haven't yet confirmed as a bugA potential issue that we haven't yet confirmed as a bug

   on Nov 5, 2020
5. bvaughn commented on Jan 26, 2021

   

   bvaughn

   on Jan 26, 2021

   Contributor

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   Proposed solution (really just a better warning and error message) in #20665
6. bvaughn

   closed this as completedin #20665on Jan 29, 2021
7. bvaughn commented on Jan 29, 2021

   

   bvaughn

   on Jan 29, 2021

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   @SreeniIO Thanks for the repro case and discussion. I've landed a solution in #20665 and I'd suggest reconsidering the workaround you mentioned. My reasoning is this (copied from the PR description):

   Mutating a mutable source (or any external variable) during render is not expected to be supported. However this error message misattributes the mutation as a React bug.

   Should this use case be supported?

   This seems very similar lazily initializing a ref so it might be tempting to think it's okay. Mutable sources are external though, so modifying them during render is a side effect which makes it unsafe and unsupported. So while we could "support" this case in a very limited way, we shouldn't.