Debugger instances are never notified via their onNewScript handlers of
code compiled using JS::CompileOffThread. This makes it impossible for
debuggers to insert breakpoints in such code before it runs. Bug 912321
adds a shell function that makes it possible to test this behavior; bug
912719 includes a test case that shows the bug.
https://bugzilla.mozilla.org/show_bug.cgi?id=912321
https://bugzilla.mozilla.org/show_bug.cgi?id=912719
It's not acceptable to land changes that break Debugger, any more than
any other major piece of SpiderMonkey functionality. Just getting
through the test suite is not enough, if you know the suite doesn't
cover such functionality. As 912321 shows, there is no JS test suite
coverage of off-main-thread compilation. And the comment here indicates
that the authors (and reviewers!) did know about the problem, but didn't
follow up:
void
BytecodeEmitter::tellDebuggerAboutCompiledScript(ExclusiveContext *cx)
{
// Note: when parsing off thread the resulting scripts need to be
handed to
// the debugger after rejoining to the main thread.
if (!cx->isJSContext())
return;
Maybe it's just a dropped detail, but it still upsets me. We worked
really hard to make Debugger a trustworthy foundation for our tools, and
to give it enough test coverage to keep it that way. We've also worked
hard on the browser-level tools built on top of that. There's no reason
FinishOffThreadScript couldn't have fired the notifications; the patch
should have been held up until that was addressed.
Off-main-thread compilation is currently only used for XUL elements, but
we have chrome debuggers and add-on debuggers, so this problem is
potentially visible. Debugger is not only for content.
If you know that your work will break Debugger, treat that as a blocker.
Ask me for help, if you need to, or delegate.
With 912321, we have what we need to cover Debugger + off-main-thread
compilation in the tests.
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