Support "break"/suspend command

Maybe we should expose "break" command via DebuggerAgent.

This command ("break") is intentionally non-JSON, as it corresponds to the fact that JavaScript VM is being busy right now so we can't parse JSON.
This command has higher QoS that other commands, because it evades normal command queue.

[Mikhail Naganov, 2009-08-05 07:37:00]

http://codereview.chromium.org/160605/diff/1001/5
File src/debug.cc (right):

http://codereview.chromium.org/160605/diff/1001/5#newcode2290
Line 2290: if (CompareAsciiWithUtf16("break", command)) {
FYI, an alternative to rolling out your own comparison function would be using
V8 strings:

Handle<String> request = Factory::NewStringFromTwoByte(command);
if (request->IsEqualTo(CStrVector("break")) {
...

[Søren Thygesen Gjesse, 2009-08-05 07:45:21]

On 2009/08/04 21:51:20, Peter Rybin wrote:
> 

Adding a break command the the debugger agent protocol is definitely something
which is needed. However I think this should be handled as part of the remote
debugger protocol and not through a special command. The v8:Debug::SendCommand()
should stay as a JSON interface. With your suggestion calling
v8:Debug::SendCommand() with the command "break" will be equivalent to calling
v8::Debug::DebugBreak().

I suggest adding a header to the protocol for signalling "out-of-band" stuff
like break, e.g. this message would cause a break.

Debug-Break: true
Content-Length: 0

This would require adding handling of other headers than Content-Length in the
protocol (currently they are just printed).

We also need to figure out whether there should be a "forced" break, where the
embedding application is requested to execute some JavaScript (e.g. 'void 0') to
ensure that the break is "immediate" as opposed to "when JavaScript runs". This
will required some extra facilities for calling back to the embedder as the
debugger agent cannot safely initiate execution of JavaScript due to threading
issues.

[Peter Rybin, 2009-08-30 19:21:01]

Hi Soren

Thank you for explanation. Let me withdraw my CL now.

On the other hand I'm interested in developing support of V8 debugger because of
chromedevtools project I'm working on.

As I understand our main issue is multi-thread aspects V8<->some_debug_agent
communication.

Couple of Qs:

1. Can we have all debug commands "forced", which means that if V8 is running at
the moment, it interrupts and process command.

2. Provided #1 == true, "break" seems to be a regular JSON command. Do we need
an out-of-order hi-pri "break" command in this case?  We probably only need it
to interrupt other requests, notably "evaluate". I don't know it we really want
to support this, but it looks nice.

3. Can we start adding API for embedder to run for us a spare cycle of V8 to
process pending debug command? I'll try to submit a corresponding CL soon.

Peter