Subscription prematurely closed w/ useSubscription + webpack + react-fast-refresh (plus possible simple fix)

[papercuptech]

Intended outcome:
When changing JSX of func component that uses useSubscription, while webpack-dev-server --hot is running, underlying subscription should not be lost; i.e. source pushes should still result in useSubscription causing re-render.

Actual outcome:
After first change to func component, source changes pushed no longer cause useSubscription to cause re-render.

How to reproduce the issue:
Minimal setup of webpack, @pmmmwh/react-refresh-webpack-plugin, react-refresh

Possible fix
I changed this:

apollo-client/src/react/data/SubscriptionData.ts

Line 145 in 0340c48

if (this.currentObservable.subscription) {

to this (quick hack in node_modules/@apollo... to see if would work)

    if (this.currentObservable.subscription && !this.isMounted) {

Simple breakpoint testing for a component fast-refreshed vs. one that has been unmounted appears to indicate adding && !this.isMounted solves the problem;i.e. this.isMounted === true when endSubscription called as part of fast-refresh processing, and this.isMounted === false when endSubsctiption called for a component no longer in (i.e. unmounting /ed or 'leaving') render tree.

Versions
System:
OS: Windows 10 10.0.18363
Binaries:
Node: 14.2.0 - ~\AppData\Local\nvs\default\node.EXE
npm: 6.14.4 - ~\AppData\Local\nvs\default\npm.CMD
Browsers:
Edge: 44.18362.449.0
npmPackages:
@apollo/client: ^3.0.0-rc.2 => 3.0.0-rc.2
@apollo/link-ws: ^2.0.0-beta.3 => 2.0.0-beta.3

[nodabladam]

On my apollo client 2.6 dev machine when I am doing development unrelated to apollo (css/jsx changes), I seem to occasionally get some errors about subscribeToMore being undefined. I haven't been able to figure out why or how to stop it. It feels related to this issue and it would certainly be nice for this to be fixed in 3.

[hwillson]

Let us know if this is still a concern with @apollo/client@latest - thanks!