Separate script and layout messages, issue #8843

[jkachmar]

Separated layout-specific messages to the constellation out from the ScriptMsg enum into a LayoutMsg enum within script_traits/script_msg.rs, addresses #8843.

I initially tried to move LayoutMsg into layout_traits/lib.rs, but this introduced a cyclic dependency: layout_traits depends on script_traits for the LayoutTaskFactory implementation, and script_traits/script_task.rs now depends on LayoutMsg for new layout channels in InitialScriptState and ScriptTask.

[highfive]

Thanks for the pull request, and welcome! The Servo team is excited to review your changes, and you should hear from @larsbergstrom (or someone else) soon.

[highfive]

Warning

• These commits modify layout code, but no reftests are modified. Please consider adding a reftest!

[bors-servo]

☔ The latest upstream changes (presumably #8979) made this pull request unmergeable. Please resolve the merge conflicts.

[Manishearth]

r? @KiChjang

[Manishearth]

I think we should just use no_jsmanaged_fields! on ConstellationChan<T>?

[Manishearth]

@bors-servo delegate=KiChjang

Looks good to me (modulo nits) at a glance; Keith can do a more thorough review

[bors-servo]

✌️ @KiChjang can now approve this pull request

[KiChjang]

-S-awaiting-review -S-needs-rebase +S-needs-code-changes

Great job so far! I've left comments in places where care must be taken when dealing with passing messages to the constellation.

The most important idea here is to recognize that the script objects should only send ScriptMsgs to the constellation - it should not, in anyway, need a ConstellationChan<LayoutMsg>. That's reserved for layout objects.

Secondly, both script and layout objects should never need to know what kind of messages (i.e. ScriptMsgand LayoutMsg) that the underlying channel accepts - all they need to know is that they are, in fact, communicating with the constellation, hence the ScriptMsg as ConstellationMsg in script files and LayoutMsg as ConstellationMsg in layout files.

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Reviewed 11 of 19 files at r1, 8 of 8 files at r2.
Review status: all files reviewed at latest revision, 11 unresolved discussions, some commit checks failed.

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components/compositing/constellation.rs, line 102 [r2] (raw file):
nit: Should follow the comment style of previous receivers, i.e. "Receives messages from the layout task".

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components/compositing/pipeline.rs, line 85 [r2] (raw file):
This is not actually true; it's a channel for the layout task to send messages to the constellation.

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components/script/dom/document.rs, line 1163 [r2] (raw file):
This sounds very wrong to me. A document is a script object, and yet it is sending a LayoutMsg to the constellation. We should instead make two copies of the ChangeRunningAnimationsState message: one for LayoutMsgs, and the other for ScriptMsgs.

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components/script/dom/window.rs, line 199 [r2] (raw file):
Again, wrong idea here. It's a channel to communicate with the constellation, but given the comment I left on document.rs, we really should remove this channel altogether.

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components/script/layout_interface.rs, line 21 [r2] (raw file):
LayoutMsg as ConstellationMsg

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components/script/lib.rs, line 51 [r2] (raw file):
Shouldn't be necessary anymore, since the original plan of moving LayoutMsg to layout_traits was scrapped.

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components/script_traits/lib.rs, line 262 [r2] (raw file):
"from layout"; but we really should remove this, since this is from the script, and scripts should only ever send ScriptMsgs to the constellation.

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Comments from the review on Reviewable.io

[bors-servo]

☔ The latest upstream changes (presumably #8612) made this pull request unmergeable. Please resolve the merge conflicts.

[KiChjang]

-S-awaiting-review -S-needs-rebase +S-needs-code-changes

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Reviewed 13 of 16 files at r3.
Review status: all files reviewed at latest revision, 8 unresolved discussions.

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components/layout/layout_task.rs, line 692 [r3] (raw file):
Since there's no other type of ConstellationChan, we can just leave it named as constellation_chan.

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components/script/layout_interface.rs, line 259 [r3] (raw file):
I'm confused, why is there a ConstellationChan<ScriptMsg> here?

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components/script/script_task.rs, line 1203 [r3] (raw file):
There's absolutely no need for the layout to have a ConstellationChan<ScriptMsg> at all. I don't find it necessary to add a new layout_to_constellation_chan field. You can just change the message type of constellation_chan to be LayoutMsg.

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Comments from the review on Reviewable.io

[KiChjang]

-S-awaiting-review

@bors-servo r+

Thanks for doing this!

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Reviewed 5 of 11 files at r4.
Review status: all files reviewed at latest revision, 6 unresolved discussions.

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Comments from the review on Reviewable.io

[bors-servo]

📌 Commit 655268d has been approved by KiChjang

[bors-servo]

⌛ Testing commit 655268d with merge 941653d...

[bors-servo]

☀️ Test successful - android, gonk, linux-dev, linux-rel, mac-dev-ref-unit, mac-rel-css, mac-rel-wpt