DevTools/Features/Debugger

Feature	Status	ETA	Owner
DevTools/Features/Debugger	planning	YYYY-MM-DD	Kevin Dangoor

Summary

Initial take on an integrated JavaScript debugger for Firefox.

Team

Have some thoughts on what you want out of a debugger? Inspiration on how to do it? Join us on #devtools on irc.mozilla.org

• Feature Manager: Kevin Dangoor (irc: kdangoor)
• Lead Developer: Dave Camp (irc: dcamp)
• Product Manager: Kevin Dangoor (irc: kdangoor)
• QA: TBD
• UX: TBD
• Security: TBD

Release Requirements

TBD

Next Steps

• scope out the project

Related Bugs & Dependencies

See the status page for the bug list and current status.

Designs

TBD

Planning

What follows is some plan-related sketching, please update appropriately.

Basic Protocol Support

Basic implementation of the Remote Debugging Protocol.

None of this is implemented yet.

Protocol Transports

• Simple TCP socket protocol - Needed for remote debugging.
• Shouldn't need sockets for in-process debugging, can probably just post events directly to/from the handler thread.
• Need to figure out transport between chrome/content for e10s (See inter-process dispatch, below).
• WebSockets (not needed for an initial implementation).
• How do we represent the actor tree across multiple connections
  • Same actor tree exposed to each connection
  • Thread attachment scoped to connection, or are all connections treated equally?

Protocol Handler Thread

• Handles incoming protocol requests on a thread, needed to interrupt running script on other threads.
• Will handle transport IO as needed (for socket/websocket/etc transports).
• Handle inter-thread dispatching
  • Dispatching incoming protocol messages to actors on the proper thread.
  • An actor's parent or child might be on a different thread (for example when a browser tab actor might have a WebWorker actor as a child). Releasing a parent actor needs to release children, including on the other threads.
• Handle inter-process dispatching
  • Similar issues to inter-thread dispatch, but communicating to content processes using the debugging protocol.

Actor registration API

• Including maintenance of the actor tree.

Client API

• With appropriate client-side transport support.

Debugging compartments

• The debugger must be in a separate compartment from the debuggee, some sort of sandbox/new global-and-compartment for hosting a given thread's debugging actors.

Browser Protocol Integration

Firefox-specific integration of the remote protocol.

Content Tab Contexts

• Will need to maintain a list of compartments needing debugging for the document tree loaded in the tab.
• Manage lifetimes and notifications related to navigation.
• Tabs exposed as a thread-like actor (or maybe with an immediate child for the main thread running in that tab?)

Chrome Context

• For debugging firefox.

WebWorker/ChromeWorker

• Related to inter-thread dispatch above. Likely to require some platform work on webworkers to get them to load the debug protocol implementation on startup?

UI Shell

• For basic debugging, want a single debugger UI per tab
  • Docked
  • Separate Window

• Unconnected debugging shell for connecting to remote debugging targets
  • Always separate window.

• Shell for debugging chrome.

• Until we get some spidermonkey improvements, we're going to need a reload to debug correctly
  • XXX jimb/jorendorff: please review this claim.
  • Ability to recompile scripts with debug support.
  • Firebug currently needs to observe creation to infer information (see elsewhere about script information)
  • Debugger UI per tab, either docked or separate window

• Connection management.

Stack Traces

JSD2 support

• Debug_Object#Debug.Frame
• Mostly (?) implemented in jsdbg2 branch.

Remote Protocol Support

• Remote_Debugging_Protocol#Listing_Stack_Frames
• Not implemented.

UI

• Not implemented.

UI Notes: Other Debuggers

Firebug

• Stack frame shows function name/source/line num, args
• Expand frames to view function args
• Selecting stack frame moves source view
• Right-clicking stack frames allowed viewing properties of function objects in the DOM tab.
• Breadcrumb view of the stack in the source view.

Chrome

• Stack frame shows function name/source/line
• Scope variables/environment in a different pane (see Environments)
• Selecting stack frame moves source view

Lexical Environments

JSD2

• Debug_Object#Debug.Environment
• Not implemented.

Remote Protocol Support

• Remote_Debugging_Protocol#Lexical_Environments
• Not implemented.

UI

• Not implemented.

UI Notes: Other Debuggers

Firebug:

• XXX: I couldn't find where/if firebug exposes lexical scopes.

Chrome:

• Scope chain exposed as a list, property viewers for each scope.

Execution Control

JSD2

• Debug_Object#Resumption_Values
• Debug_Object#Debug.Script (specifically "singleStepMode")
• Not implemented?

Remote Protocol Support

• Remote_Debugging_Protocol#Interacting_with_Thread-Like_Actors
• jimb mentioned on irc being unhappy with the pause/resume protocol, may need another spec iteration.
• Not implemented.

Implementation Notes

(this is a mess, ignore for now).

Pause Reasons:

• stepped
• pre-call
• pre-return
• pre-eval?

Breaking on exception:

• pre-throw/caught/uncaught

UI

Not implemented.

UI Notes

Chrome

• Allows toggling of throw/caught/uncaught pause reasons during execution.
• Pause/Continue/Step Over/Step Into/Step Out (finish)

Firebug

• Has Rerun/Continue/Step Into/Step Over/Step Out (finish)
• Calls pause Break-on-next (makes a bit more sense when spinning the event loop, and it can't actually pause?).

Breakpoints

JSD2

• Debug_Object#The_Debug_Object
• setBreakpoint/clearBreakpoint/clearAllBreakpoints
• Not implemented.

Protocol Support

• Remote_Debugging_Protocol#Breakpoints
• Not implemented.
• Looks like the protocol implementation will need to refcount breakpoints on the debug object (which has one-breakpoint-per-line) to support multiple breakpoints on a given line (potentially with different conditions).
• The protocol will also aggregate breakpoints on multiple scripts (for example, if the same script is loaded twice, there might be two scripts covering the same source location) for a given breakpoint actor.
• Protocol should be extended with breakpoint conditions?

UI

Not implemented.

UI Notes

XXX

Watchpoints

No thoughts yet.

Source List

https://bugzilla.mozilla.org/show_bug.cgi?id=637572 (Relevant, but not likely to be available too soon).

Firebug does a lot a crazy stuff to get scripts right for all origins (inline, eval, document.write, dom-appended script tags, etc). We'd prefer to avoid this with 637572, and we might be weaker than firebug with the more exotic sources in the meantime. XXX: Need to figure out exactly how much weaker.

JSD2 support

• Debug_Object#Debug.Script
• Debug_Object#Debugging_hooks - newScript, specifically.
• Not implemented.

Protocol Support

• Debug_Object#Debugging_hooks
• Not implemented.

Source Viewer

JSD2 Support

• Would be nice to have originalSource support in spidermonkey/JSD2, but none is currently specced/planned.

Protocol Support

• In the meantime, we can do protocol-side with necko (like firebug does currently).
• For the in-process content case, might want shortcut using the protocol.
• This needs spec thought.

UI Notes

Chrome:

• Line numbers/breakpoints/current line in gutter.
• Syntax highlighting
• Left-click in gutter sets/unsets breakpoint
• Right-click in gutter brings up a context menu with:
  • Continue to here
  • Add/Remove Breakpoint
  • Add Conditional Breakpoint
  • Disable Breakpoint

Firebug:

• Line numbers/breakpoints/current line in gutter.
• Line numbers seem to be colored by executability.
• No syntax highlighting
• Left-click in gutter sets/unsets breakpoint.
• Right-click in gutter sets conditional breakpoint.