1445091 - Web Replay: Take temporary snapshots while navigating in the debugger

Status: RESOLVED WONTFIX

(Core :: General, enhancement)

Description

[Brian Hackett [Laid off!]]

When replaying a process, when paused at a breakpoint and running forward to another breakpoint or forward-stepping we have to rewind to the last snapshot and run forward to the point we were paused at, before we can start executing forward in search of a new place to pause (this is done to erase side effects that occurred while processing debugger requests).  When running backwards or backward-stepping we have to rewind and run forward twice: once to find the last point where a breakpoint is hit, and another to get to that execution point.

On simple pages this re-execution is not too expensive, but even on moderately complicated pages like TodoMVC each re-execution can cost hundreds of milliseconds.  The patches in this bug address this by taking temporary snapshots between the normal snapshot points.  These are done internally to the replaying process --- the middleman never knows about them --- in order to reduce the amount of time spent re-executing.  Whenever re-execution time goes above a threshold (10ms currently) a temporary snapshot is taken, either at the point we are paused if we are going forward, or at entry to the frame containing our destination if we are going backward.  Re-executing starting at the temporary snapshot instead of earlier snapshot points saves quite a bit of time when stepping around, especially when stepping backwards.

Comment 1

[Brian Hackett [Laid off!]]

Attachment: Part 1 - Support temporary snapshots in record/replay interface.

Add support for taking temporary snapshots.  This just adds a new public API and adds checks to make sure we never expose information about temporary snapshots to the middleman process.

Comment 2

[Brian Hackett [Laid off!]]

Attachment: Part 2 - Take temporary snapshots when re-execution time exceeds a threshold.

Changes to the debugger to take temporary snapshots and deal with them when rewinding past them or playing forward past the next normal snapshot.

Comment 3

[Brian Hackett [Laid off!]]

https://hg.mozilla.org/projects/ash/rev/d33a604fb0e001c43828a3192cc7a621280de7ef
https://hg.mozilla.org/projects/ash/rev/b18e3812dfe68394c9f74232223b907ff1ee1c33

Comment 4

[Brian Hackett [Laid off!]]

Attachment: Part 3 - Tolerate non-empty tracked hits after traversing some interim snapshots.

Fix a crash seen when rewinding with OnPop breakpoints set.

Comment 5

[Brian Hackett [Laid off!]]

https://hg.mozilla.org/projects/ash/rev/39a434be4dccffcea7940b7c3072e56841f22acd

Comment 6

[Brian Hackett [Laid off!]]

Attachment: Part 4 - Ignore positions hit while paused.

Fix crashes seen when (I think) certain code is evaluated while paused.

Comment 7

[BugBot [:suhaib / :marco/ :calixte]]

The bug assignee didn't login in Bugzilla in the last 7 months, so the assignee is being reset.

Comment 8

[Andrew McCreight [:mccr8]]

WebReplay has been removed.