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= Remote Debugging =
#REDIRECT [[Remote Debugging Protocol]]
 
Mozilla will support remote debugging with a two-level protocol, roughly modeled after the [http://code.google.com/p/v8/wiki/DebuggerProtocol|V8 Debugger Protocol] and the [http://code.google.com/p/chromedevtools/wiki/ChromeDevToolsProtocol Chrome Developer Tools Protocol].
 
The remote protocol operates at the JavaScript level, not at the C++ or
machine level. It assumes that the JavaScript implementation itself is
healthy and responsive: the JavaScript program being executed may have gone
wrong, but the JavaScript implementation's internal state must not be
corrupt. Bugs in the implementation may cause the debugger to fail; bugs in
the interpreted program must not.
 
= Debugging States =
 
When a debugger client first makes a connection to a Mozilla process, no
debugging sphere has yet been selected, and the server uses the js::dbg2
sphere discovery facilities to answer client requests about what is
available to be debugged in that process:
 
[[File:Initial-connection.png]]
 
Selecting a debugging sphere establishes event handlers which invoke the
debug server to communicate the news to the debugger. Assuming the selected
sphere runs on the main thread, the stack looks like this while the event
is being reported:
 
[[File:main-thread-event.png]]
 
When the debugger asks the debuggee to continue, the frames for the
js::dbg2 dispatch facilities simply return, and the code that generated the
event resumes execution.
 
== Evaluating User Expressions ==
 
If the user asks the debugger to evaluate an expression that requires
evaluating JavaScript code (like <tt>e.x()</tt>), then the C++ stack looks
like this:
 
{| border="1"
| interpreter/JITted frames for expression given to debugger
|-
| debugger machinery frames
|-
| interpreter/JITted frames for debuggee
|-
| top-level event loop
|}
 
If evaluation of the expression throws an exception or hits a breakpoint,
then the result is a matter of user interface. Either we abandon evaluation
of the expression, and C++ control returns to the original machinery frames:
 
{| border="1"
| debugger machinery frames
|-
| interpreter/JITted frames for debuggee
|-
| top-level event loop
|}
 
Or we treat the event as something to be investigated, just as if it had
occurred in the debuggee's normal course of execution:
 
{| border="1"
| nested debugger machinery frames
|-
| interpreter/JITted frames for expression given to debugger
|-
| debugger machinery frames
|-
| interpreter/JITted frames for debuggee
|-
| top-level event loop
|}
 
Again, the debugger machinery is <em>not</em> written to tolerate corrupt
interpreter data structures or incomplete execution states; it relies on
the interpreter's debugging API working correctly.
 
== Same-Stack Debugging ==
 
In the current model for debugging Firefox, the debugger runs in the same
process as the debuggee. Since the XUL user interface only allows one
thread to interact with it, the debugger's user interface must share a
thread, and thus a stack, with the debuggee. Thus, when the debuggee is
paused and the user is interacting with the debugger's user interface, the
C++ stack looks like this:
 
{| border="1"
| debugger UI frames<br>(that is, more interpreted/JITted JS frames)
|-
| nested event loop invocation
|-
| debugger machinery frames
|-
| interpreter/JITted frames for debuggee
|-
| top-level event loop
|}
 
There are a number of complications that arise from this model:
 
* The debugger's UI and the debuggee share a DOM, and may interact with each other in unexpected ways through that DOM.
 
* The debugger should never refer to the debuggee's objects directly --- it is too easy to introduce bugs and security holes by doing so. However, avoiding this is similar to the problem of ensuring that references between Firefox chrome and content go through the proper wrapper objects. This seems to be challenging in practice.
 
== Remote Debugging ==
 
One way to avoid the issues mentioned above is to move the debugger UI into
its own process, and have it communicate with the debuggee using a wire
protocol.  (See Remote Debugging, above.)
 
This ability is also helpful when the debuggee is running on a device with
a limited user interface (say, a mobile phone or tablet computer): it can
be valuable to have the debugger's user interface running on a workstation
or laptop. In this case, the C++ call stack looks like this:
 
{| border="1"
| debug protocol server
|-
| nested event loop invocation
|-
| debugger machinery frames
|-
| interpreter/JITted frames for debuggee
|-
| top-level event loop
|}
 
The stack of the debugger's user interface can be whatever is convenient,
as long as it communicates appropriately with the debug server. But one
possible arrangement would be to treat the protocol as simply another back
end for the js::dbg2 interface; the debugger UI would behave identically
regardless of whether the debuggee was local or remote. Thus, the C++ stack
in the process running the debugger UI would look like this:
 
{| border="1"
| debugger UI frames
|-
| nested event loop invocation
|-
| debugger machinery frames
|-
| debugger back end: debug protocol client
|-
| top-level event loop
|}
 
Remote debugging also enables debugging worker threads: if the worker's
top-level event loop responds to messages registering the debugger's
interest in the sphere
 
Remote debugging also prepares us to support debugging content in an
architecture which places content JavaScript in separate processes from
chrome JavaScript.
 
== Separate Windows Cannot Be Debugged Independently ==
 
One interesting consequence of the fact that Firefox uses a single thread
for all chrome and content JavaScript is that independent windows (in the
sense of an HTML5 "Window" object; tabs are windows) cannot be debugged
independently. Suppose we hit a breakpoint in one window:
 
{| border="1"
| debugger UI frames
|-
| nested event loop invocation
|-
| debugger machinery frames
|-
| interpreter/JITted frames for first window
|-
| top-level event loop
|}
 
Then we switch to a different window and hit a breakpoint there, as well:
 
{| border="1"
| debugger UI frames
|-
| nested event loop invocation
|-
| debugger machinery frames
|-
| interpreter/JITted frames for second window
|-
| nested event loop invocation
|-
| debugger machinery frames
|-
| interpreter/JITted frames for first window
|-
| top-level event loop
|}
 
(I believe Firebug currently forbids this situation from arising, either by
refusing to allow debugging to occur in the second window, or by throwing
away the first window's JavaScript stack. But the goal here is to point out
intrinsic limitations in Firefox's execution model, regardless of how
Firebug behaves.)
 
In this case, we cannot simply switch back to the first window and resume
execution there: we must first finish (or abandon) execution in the second
window, because its stack frames are on top of the ones we wish to resume.
 
There are two general solutions. The first would be to change SpiderMonkey
to represent the JavaScript stack entirely in the heap, such that no C++
frames accumulate in the above scenario, and then use a separate JavaScript
stack for each window. However, aside from the engineering work needed,
accomodating native frames mixed with JavaScript frames in this arrangement
would be a challenge.
 
The second is to change Firefox to use a separate C++ stack for each
window, by creating a separate thread for each window. These threads would
not run concurrently (if properly designed, the functions for passing
control from one stack to another can guarantee this), avoiding the sorts
of unreproducible behavior that make most multi-threaded, shared memory
programming so difficult.
 
If Firefox evolves towards a process-per-window model, then it will have a
separate stack per window, and the debugging restrictions described above
can be lifted. However, if the user creates a large number of windows,
Firefox may need to have windows share processes; in this case, the
multiple, non-mutually-preemptive thread model described above could
provide consistency between the process-per-window and
several-windows-per-process arrangements.
Jimb
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