Closed Bug 1124258 Opened 6 years ago Closed 6 years ago

Avoid using `getChildScripts` in `onNewScript`


(DevTools :: Debugger, defect)

Not set


(firefox40 fixed)

Firefox 40
Tracking Status
firefox40 --- fixed


(Reporter: jlong, Assigned: jlong)


(Blocks 1 open bug)



(1 file, 2 obsolete files)

There's no real reason we need to iterate the child scripts at all on `onNewScript`. In fact it's misleading because `script.getChildScripts` does not recursively return all the nested scripts, it only returns the immediate children.

Our breakpoint architecture has evolved over time and I think there used to be a reason we do it, but I talked with ejpbruel on IRC and he agreed that we should do this simple approach now:

* `setBreakpoint` in the SourceActor takes a line/col and has the responsibility to set a breakpoint on ALL scripts at that location. It will appropriately check if a script already has a breakpoint. When it's done we know that all nested scripts have an activated breakpoint.

* `onNewScripts` and `_restoreBreakpoints` only need to call `_addScript` with a top-level script. This approach is faster because we don't need to go through all the sourcemapping machinery (when resolving a script to a SourceActor and a location in that) for each individual script. I'm not entirely sure what's slow in that code path, but from my experiments it's not the fastest.

It's very fast to just give `_addScript` only top-level scripts, and let `setBreakpoint` take care of the nested child scripts.

One question is do we need to do the start/end line filtering in `_addScript` that we do now? Theoretically if we always pass it top-level scripts, we don't need to, we are basically activating all breakpoints for the entire source.

If we do need to do the filtering, we can't do this until we fix an issue with the Debugger.Script `lineCount` property not always being correct. This blocks on bug 979094 if so.

Note that in the case where the debugger is opened on an existing page, top-level scripts may be GCed. The `setBreakpoint` function in the SourceActor will still have to handled possibly "pending" breakpoints and not assume the top-level script is available.

The end result of this bug, though, should be a *lot* faster debugger loading when a large, complex page is refreshed. I've seen rough results going from ~150ms to ~30ms, but we'll need to do more benchmarking here.
Attached patch 1124258.patch (obsolete) — Splinter Review
This gives us the ability to give a top-level script to `_addScript` and set all the breakpoints within it, even though the `lineCount` is buggy (see bug 979094). Basically if the staticLevel of the script passed to `_addScript` is 0, don't do the range checking, as its the whole source.

This should significantly speed up page refreshes on large sites, I think. It's also just simpler. All tests are passing locally, will do some benchmarking and more testing later.
Attachment #8552626 - Flags: review?(ejpbruel)
(this patch depends on bug 1122064)
Depends on: 1122064
Comment on attachment 8552626 [details] [diff] [review]

Review of attachment 8552626 [details] [diff] [review]:

::: toolkit/devtools/server/actors/script.js
@@ +2036,5 @@
>      let source = this.sources.createNonSourceMappedActor(aScript.source);
>      for (let bpActor of this.breakpointActorMap.findActors({ sourceActor: source })) {
> +      // Limit the search to the line numbers contained in the new
> +      // script if it's not the whole top-level script
> +      if (aScript.staticLevel === 0 ||

We only call _addScript for top level scripts now, don't we? So we should be able to remove this check completely.
Attachment #8552626 - Flags: review?(ejpbruel)
(In reply to Eddy Bruel [:ejpbruel] from comment #3)
> We only call _addScript for top level scripts now, don't we? So we should be
> able to remove this check completely.

I took a conservative approach, but we probably should just make sure `_addScript` is called from the top-level. It is also called from `_restoreBreakpoints`, where it can be called from any script I think. But we should change that behavior (and add an assert at the top of `_addScript`).

I'm also not entirely sure when `_restoreBreakpoints` is called. It's called from `onAttach` but as far as I know breakpoints don't survive across detach/attach.
Assignee: nobody → jlong
Blocks: 973879
Attached patch 1124258.patch (obsolete) — Splinter Review
Tests pass locally; I'll do a try run
Attachment #8552626 - Attachment is obsolete: true
Attachment #8598160 - Flags: review?(ejpbruel)
Comment on attachment 8598160 [details] [diff] [review]

Review of attachment 8598160 [details] [diff] [review]:

Great stuff. Glad to see this is getting cleaned up. r+ with comments addressed.

::: toolkit/devtools/server/actors/script.js
@@ +2015,5 @@
>      }
>    },
>    /**
>     * Add the provided script to the server cache.

This comment needs updating. We're not dealing with scripts anymore, we're dealing with sources.

@@ -2050,5 @@
>        } else {
>          promises.push(this.sources.getGeneratedLocation(actor.originalLocation)
>                                    .then((generatedLocation) => {
> -          // Limit the search to the line numbers contained in the new script.
> -          if (generatedLocation.generatedSourceActor.actorID === sourceActor.actorID &&

If I understand this correctly, the range check can now be omitted because addSource is now only called for top-level scripts, correct?
Attachment #8598160 - Flags: review?(ejpbruel) → review+
(In reply to Eddy Bruel [:ejpbruel] from comment #7)
> If I understand this correctly, the range check can now be omitted because
> addSource is now only called for top-level scripts, correct?

Yes. I think the previous behavior was leftover from some old vision of the architecture. Either we (#1) thought that we would call _addScript for every single script (including all nested ones), and we would *only* set the breakpoint for that script if there is one (but not for any nested scripts). Since _addScript would be called for all nested scripts, breakpoints would be set on each script at line N.

Or (#2) there was something about the debugger API that drove us to do the line range checking, I don't know.

The previous code was kind of weird. `getChildScripts` is *not* recursive, it only returns the immediate children. There are some comments in there that we recurse on children 1-level deep because sometimes the line count of the top-level script is wrong. However, it we use the top-level script, we don't need to check line ranges! Or maybe we *thought* that `getChildScripts` was recursive and was attempting to do #1.

Either way, this is a lot clearer and potentially faster. It also paves the way to handle sources introduced multiple times with the same URL, and somehow handling that in the frontend. I'm think in _addSource we could still notify TabSources to send a `newSource` packet even if one already exists.
Attached patch 1124258.patchSplinter Review
tweaked comments
Attachment #8598160 - Attachment is obsolete: true
The try links looks pretty good, are we having problems with the asan builds though?
Keywords: checkin-needed
Closed: 6 years ago
Resolution: --- → FIXED
Whiteboard: [fixed-in-fx-team]
Target Milestone: --- → Firefox 40
Product: Firefox → DevTools
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