1444619 - Web Replay: Don't take snapshots when diverging from recording.

Status: RESOLVED WONTFIX

(Core :: General, enhancement)

Description

[Brian Hackett [Laid off!]]

Currently, Web Replay deals with recording divergence (when we are paused at e.g. a breakpoint and the debugger is trying to interact with the system in ways that differ from the recording) by taking snapshots on every operation that might diverge (certain debugger requests) and restoring those snapshots if an unhandled divergence occurs during the operation.  This is simple but slow: hundreds of snapshots might be taken while paused at a breakpoint, and even though each of these is small in space, they involve a fair amount of overhead to produce (mainly in corralling other threads).

The following patches remove the need to take these snapshots.  If an unhandled divergence occurs while processing a debugger request, we rewind to the most recent normal snapshot, execute forward to the breakpoint, and re-process any old debugger requests before indicating the failed one to the middleman.  This adds a fair amount of bookkeeping requirements on the debugger but simplifies the record/replay infrastructure itself.

Comment 1

[Brian Hackett [Laid off!]]

Attachment: Part 1 - Don't take snapshots when diverging from the recording.

Infrastructure changes to avoid taking snapshots when diverging from the recording.

Comment 2

[Brian Hackett [Laid off!]]

Attachment: Part 2 - Debugger bookkeeping for recovering after an unhandled divergence.

Deal with unhandled divergences in the debugger.

Comment 3

[Brian Hackett [Laid off!]]

Attachment: Part 3 - Tests.

Comment 4

[Brian Hackett [Laid off!]]

https://hg.mozilla.org/projects/ash/rev/75a3345734f480b08dc52d1d5184d7b63f377bd1
https://hg.mozilla.org/projects/ash/rev/f59baa8c5902b6622890bf6c6d61c34ca0b1d2f1
https://hg.mozilla.org/projects/ash/rev/2f19ece0f08acb75c77a9cca3160589cb6cf739c

Comment 5

[Brian Hackett [Laid off!]]

Attachment: Part 4 - Always trace replay debugger roots.

Fix a crash where nursery objects referred to by the middleman process weren't being traced.

https://hg.mozilla.org/projects/ash/rev/2c915dae95e8a53121ab278fa9cd662871d124ba

Comment 6

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

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

Comment 7

[Andrew McCreight [:mccr8]]

WebReplay has been removed.