spawnFunction needs to use a script snapshot of the parent isolate when starting

[a-siva]

Currently when isolate.spawnFunction is called it reloads
the same script that the parent isolate loaded but we don't
have a way of ensuring that the same versions of the script
got loaded in the child isolate.

Instead of loading scripts the spawnFunction should use
a script snapshot of the parent isolate.

[iposva-google]

Added Area-VM label.

[iposva-google]

Removed Priority-Medium label.
Added Priority-Unassigned label.

[iposva-google]

Issue #19228 has been merged into this issue.

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cc @dgrove.
cc @munificent.

[dgrove]

Nathan and I believe that this is slowing down the spawning of transformer isolates in pub. Can this be addressed in the relatively near future?

[nex3]

Marked this as blocking #23105.

[a-siva]

When this issue was first created we did not have the 'hot reload' feature, I think it is worth discussing the implications of hot-reloading and using the 'Isolate.spawn' API to spawn child isolates

• If we spawn child isolates using the Isolate.spawn API and then hot reload the parent isolate or a child isolate we could potentially have isolates running different versions of the app and any messages passed consisting non primitive types could result in weird behavior
• if we spawn a child isolate after a hot reload should it use the newly modified app script or the original one?

Getting this right in all situations would require having some kind of check pointing and getting all the isolates in the group reloaded with the new modified source. I don't believe it is worth spending engineering cycles trying to implement this corner case.

Here is what I propose

• if we spawn child isolates and then hot reload an isolate in the group then only primitive values (null, num, bool, double, String), instances of SendPort, and lists and maps whose elements are any of these will be allowed in the messages to that isolate
• if a spawn is done after a hot reload is done we will spawn the child isolate using the original app script an informational message could be produced recommending a restart in this case. Again in this case only the restricted set of messages would be allowed.

[a-siva]

Opening a new issue #35302 for the hot reload issue per Liam's comment.