devtools: Show clients where they can set breakpoints #37667

delan · 2025-06-24T07:49:13Z

devtools clients query source actors to determine where the user can set breakpoints in a source. there are two relevant requests here: getBreakableLines controls which line numbers can be clicked in the margin, and once a line number is clicked, getBreakpointPositionsCompressed controls where to show breakpoint buttons within that line.

this patch handles those requests by querying the SpiderMonkey Debugger API for that information:

devtools sends its script thread a GetPossibleBreakpoints message for the source’s spidermonkey_id
the script thread fires a getPossibleBreakpoints event into its debugger global
the debugger script looks up the root Debugger.Script for that source, calls getPossibleBreakpoints(), and returns the result via DebuggerGlobalScope#getPossibleBreakpointsResult()
that method takes the pending result sender, and sends the result back to devtools
devtools massages the result into the format required by the request, and replies to the client

as a result, users of the Firefox devtools client can now set breakpoints, though they don’t have any effect.

Testing: this patch adds new devtools tests
Fixes: part of #36027

…8236) to use the [SpiderMonkey Debugger API](https://firefox-source-docs.mozilla.org/js/Debugger/), we need to call it from an internal debugger script that we will supply. this script must run in the same runtime as the debuggee(s), but in a separate [compartment](https://udn.realityripple.com/docs/Mozilla/Projects/SpiderMonkey/Compartments) ([more details](https://hacks.mozilla.org/2020/03/future-proofing-firefoxs-javascript-debugger-implementation/)). when creating two globals in the same runtime, Servo will generally try to reuse an existing compartment for the second global, and there are other places in script where we rely on this reuse. but for the debugger script, we can’t do this. this patch adds a `use_system_compartment` flag to global object descriptors, which isolates the global in its own compartment in both possible directions: - it forces the global to create a new compartment, preventing the reuse of any debuggee’s compartment - it fails the check in select_compartment(), preventing future debuggees from reusing the global’s compartment Testing: manually tested working in devtools debugger patch (#37667), where it will undergo automated tests --------- Signed-off-by: Delan Azabani <dazabani@igalia.com> Co-authored-by: atbrakhi <atbrakhi@igalia.com>

in the [SpiderMonkey Debugger API](https://firefox-source-docs.mozilla.org/js/Debugger/), hooks like [onNewGlobalObject()](https://firefox-source-docs.mozilla.org/js/Debugger/Debugger.html#onnewglobalobject-global) use an AutoDebuggerJobQueueInterruption to [switch to a new microtask queue](https://github.com/servo/mozjs/blob/b14aebff23ac4d5b0652060ef949334bda08b22f/mozjs-sys/mozjs/js/src/debugger/Debugger.cpp#L2834-L2841) and avoid clobbering the debuggee’s microtask queue. this in turn relies on JobQueue::saveJobQueue(), which is [not yet implemented in RustJobQueue](https://github.com/servo/mozjs/blob/b14aebff23ac4d5b0652060ef949334bda08b22f/mozjs-sys/src/jsglue.cpp#L83-L86). this patch bumps mozjs to servo/mozjs#595, which implements [saveJobQueue() and SavedJobQueue](https://github.com/servo/mozjs/blob/b14aebff23ac4d5b0652060ef949334bda08b22f/mozjs-sys/mozjs/js/public/Promise.h#L92-L114) for RustJobQueue by calling into Servo via two new JobQueueTraps that create and destroy extra “interrupt” queues for use by the debugger. SpiderMonkey [does not own external job queues](https://github.com/servo/mozjs/blob/b14aebff23ac4d5b0652060ef949334bda08b22f/mozjs-sys/mozjs/js/public/Promise.h#L117-L123), so the lifetime of these queues is managed in Servo, where they are stored in a Vec-based stack. stack-like behaviour is adequate for SpiderMonkey’s save and restore patterns, as far as we can tell, but we’ve added an assertion just in case. Testing: manually tested working in devtools debugger patch (#37667), where it will undergo automated tests Fixes: #38311 --------- Signed-off-by: Delan Azabani <dazabani@igalia.com> Co-authored-by: atbrakhi <atbrakhi@igalia.com>

in the [SpiderMonkey Debugger API](https://firefox-source-docs.mozilla.org/js/Debugger/), hooks like [onNewGlobalObject()](https://firefox-source-docs.mozilla.org/js/Debugger/Debugger.html#onnewglobalobject-global) use an AutoDebuggerJobQueueInterruption to [switch to a new microtask queue](https://github.com/servo/mozjs/blob/b14aebff23ac4d5b0652060ef949334bda08b22f/mozjs-sys/mozjs/js/src/debugger/Debugger.cpp#L2834-L2841) and avoid clobbering the debuggee’s microtask queue. this in turn relies on JobQueue::saveJobQueue(), which is [not yet implemented in RustJobQueue](https://github.com/servo/mozjs/blob/b14aebff23ac4d5b0652060ef949334bda08b22f/mozjs-sys/src/jsglue.cpp#L83-L86). this patch bumps mozjs to servo/mozjs#595, which implements [saveJobQueue() and SavedJobQueue](https://github.com/servo/mozjs/blob/b14aebff23ac4d5b0652060ef949334bda08b22f/mozjs-sys/mozjs/js/public/Promise.h#L92-L114) for RustJobQueue by calling into Servo via two new JobQueueTraps that create and destroy extra “interrupt” queues for use by the debugger. SpiderMonkey [does not own external job queues](https://github.com/servo/mozjs/blob/b14aebff23ac4d5b0652060ef949334bda08b22f/mozjs-sys/mozjs/js/public/Promise.h#L117-L123), so the lifetime of these queues is managed in Servo, where they are stored in a Vec-based stack. stack-like behaviour is adequate for SpiderMonkey’s save and restore patterns, as far as we can tell, but we’ve added an assertion just in case. Testing: manually tested working in devtools debugger patch (servo#37667), where it will undergo automated tests Fixes: servo#38311 --------- Signed-off-by: Delan Azabani <dazabani@igalia.com> Co-authored-by: atbrakhi <atbrakhi@igalia.com>

…38330) This is allowed by the [protocol docs](https://firefox-source-docs.mozilla.org/devtools/backend/protocol.html#loading-script-sources), and eliminates an unwrap(). Testing: does not get exercised yet, but will be used in #37667 Fixes: part of #36027 Signed-off-by: Delan Azabani <dazabani@igalia.com> Co-authored-by: Delan Azabani <dazabani@igalia.com>

in the [SpiderMonkey Debugger API](https://firefox-source-docs.mozilla.org/js/Debugger/), hooks like [onNewGlobalObject()](https://firefox-source-docs.mozilla.org/js/Debugger/Debugger.html#onnewglobalobject-global) use an AutoDebuggerJobQueueInterruption to [switch to a new microtask queue](https://github.com/servo/mozjs/blob/b14aebff23ac4d5b0652060ef949334bda08b22f/mozjs-sys/mozjs/js/src/debugger/Debugger.cpp#L2834-L2841) and avoid clobbering the debuggee’s microtask queue. this in turn relies on JobQueue::runJobs(), which is [not yet implemented in RustJobQueue](https://github.com/servo/mozjs/blob/b14aebff23ac4d5b0652060ef949334bda08b22f/mozjs-sys/src/jsglue.cpp#L76-L78). this patch bumps mozjs to servo/mozjs#597, which implements [runJobs()](https://github.com/servo/mozjs/blob/b14aebff23ac4d5b0652060ef949334bda08b22f/mozjs-sys/mozjs/js/public/Promise.h#L61-L76) for RustJobQueue by calling into Servo’s MicrotaskQueue::checkpoint() via a new function in JobQueueTraps. SpiderMonkey [does not own external job queues](https://github.com/servo/mozjs/blob/b14aebff23ac4d5b0652060ef949334bda08b22f/mozjs-sys/mozjs/js/public/Promise.h#L117-L123), so the lifetime of these queues is managed in Servo, where they are stored in a Vec-based stack. stack-like behaviour is adequate for SpiderMonkey’s save and restore patterns, as far as we can tell, but we’ve added an assertion just in case. Testing: manually tested working in devtools debugger patch (#37667), where it will undergo automated tests Signed-off-by: Delan Azabani <dazabani@igalia.com> Co-authored-by: atbrakhi <atbrakhi@igalia.com>

Co-authored-by: atbrakhi <atbrakhi@igalia.com> Signed-off-by: Delan Azabani <dazabani@igalia.com>

Signed-off-by: Delan Azabani <dazabani@igalia.com>

jdm

Nice work! I appreciate the new unit test, too.

components/script/dom/debuggerglobalscope.rs

Signed-off-by: Delan Azabani <dazabani@igalia.com>

Co-authored-by: atbrakhi <atbrakhi@igalia.com> Signed-off-by: Delan Azabani <dazabani@igalia.com>

delan force-pushed the devtools-set-breakpoint branch 3 times, most recently from 3cbf9c7 to 3ef1980 Compare June 25, 2025 00:52

delan force-pushed the devtools-set-breakpoint branch 8 times, most recently from 84aca55 to a9ec9e5 Compare July 23, 2025 09:05

This was referenced Jul 23, 2025

Implement saveJobQueue() for the SpiderMonkey Debugger API servo/mozjs#595

Merged

script: Implement jsglue traps for saveJobQueue() #38232

Merged

script: Add support for creating globals in isolated compartments #38236

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delan force-pushed the devtools-set-breakpoint branch 2 times, most recently from 5067308 to 9844f5a Compare July 23, 2025 11:27

delan mentioned this pull request Jul 25, 2025

script: Implement jsglue trap for runJobs() #38265

Merged

delan changed the title ~~Devtools: set breakpoints~~ devtools: set breakpoints Jul 25, 2025

delan changed the title ~~devtools: set breakpoints~~ devtools: Set breakpoints Jul 25, 2025

delan mentioned this pull request Jul 28, 2025

devtools: Implement Debugger tab #36027

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delan force-pushed the devtools-set-breakpoint branch 2 times, most recently from d2bbe18 to a0be1d7 Compare July 29, 2025 10:23

atbrakhi mentioned this pull request Jul 29, 2025

devtools: Make contentType optional in source actor source responses #38330

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delan mentioned this pull request Jul 29, 2025

script: Create a debugger script for the SpiderMonkey Debugger API #38331

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delan force-pushed the devtools-set-breakpoint branch from b4c5e98 to aad368f Compare July 29, 2025 12:17

delan force-pushed the devtools-set-breakpoint branch 4 times, most recently from 385f8e1 to 5f6a0ee Compare August 11, 2025 08:36

delan and others added 4 commits August 11, 2025 17:01

Handle getBreakableLines in source actor

d89bfd5

Co-authored-by: atbrakhi <atbrakhi@igalia.com> Signed-off-by: Delan Azabani <dazabani@igalia.com>

Handle getBreakpointPositionsCompressed in source actor

dc6b2f1

Co-authored-by: atbrakhi <atbrakhi@igalia.com> Signed-off-by: Delan Azabani <dazabani@igalia.com>

Add comment to setBreakpoint handler in breakpoint list actor

83b727f

Co-authored-by: atbrakhi <atbrakhi@igalia.com> Signed-off-by: Delan Azabani <dazabani@igalia.com>

Add new test test_source_breakable_lines_and_positions()

10074a8

Co-authored-by: atbrakhi <atbrakhi@igalia.com> Signed-off-by: Delan Azabani <dazabani@igalia.com>

delan force-pushed the devtools-set-breakpoint branch from caa9754 to 4a1fbaf Compare August 11, 2025 09:05

delan changed the title ~~devtools: Set breakpoints~~ devtools: Show clients where they can set breakpoints Aug 11, 2025

delan requested review from jdm and sagudev August 11, 2025 09:29

delan marked this pull request as ready for review August 11, 2025 09:29

delan requested a review from gterzian as a code owner August 11, 2025 09:29

Plumb in Debugger.Source.prototype.getPossibleBreakpoints()

91f6dc1

Signed-off-by: Delan Azabani <dazabani@igalia.com>

delan force-pushed the devtools-set-breakpoint branch from 4a1fbaf to 91f6dc1 Compare August 11, 2025 09:42

jdm approved these changes Aug 11, 2025

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components/script/dom/debuggerglobalscope.rs Outdated Show resolved Hide resolved

delan added 2 commits August 12, 2025 10:39

Prefix name of GetPossibleBreakpointsEvent

74e2fd7

Signed-off-by: Delan Azabani <dazabani@igalia.com>

Fix expect message in fire_get_possible_breakpoints()

79edffd

Signed-off-by: Delan Azabani <dazabani@igalia.com>

delan force-pushed the devtools-set-breakpoint branch from 2ca0955 to 79edffd Compare August 12, 2025 02:40

delan enabled auto-merge August 12, 2025 02:41

delan added this pull request to the merge queue Aug 12, 2025

github-merge-queue bot removed this pull request from the merge queue due to failed status checks Aug 12, 2025

delan mentioned this pull request Aug 12, 2025

Update expectations for /css/cssom-view/elementsFromPoint.html #38613

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delan added this pull request to the merge queue Aug 12, 2025

delan added a commit that referenced this pull request Aug 12, 2025

devtools: Show clients where they can set breakpoints (#37667)

a8e686b

Co-authored-by: atbrakhi <atbrakhi@igalia.com> Signed-off-by: Delan Azabani <dazabani@igalia.com>

Merged via the queue into main with commit f5b631e Aug 12, 2025
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delan deleted the devtools-set-breakpoint branch August 12, 2025 06:16

atbrakhi mentioned this pull request Sep 2, 2025

devtools: Add scaffolding for Debugger.Script and setbreakpoint in SM #39088

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