Howto launch and debug in VSCode using the debug adapter protocol (part 2)

Ok, after the basic infracstructure, the next thing to do is actually launch some program without worrying about the debugger, so, we'll just run a program without being in debug mode to completion, show its output and terminate it when requested.

To launch a program, our debug adapter must treat the 'LaunchRequest' message and actually run the program (bear in mind that we'll just launch it without doing any debugging at this point).

The first point then is how to actually launch it. We provided options for the debugger to be launched with different console arguments specifying where to launch it (either just showing output to the debug console, using the integrated terminal or using an external terminal).

So, let's start with just showing output in the debug console.

Launching it should be simple: just generate a command line while treating the 'LaunchRequest' message, but then, based on the console specified some things may be different...

Let's start handling just showing the output on the debug console.

To do that we have launch the command properly redirecting the output to pipes (for python it's something as subprocess.Popen([sys.executable, '-u', file_to_run], stdout=subprocess.PIPE, stderr=subprocess.PIPE) and then create threads which will read that output to provide it back to vscode (so, when output is obtained, an OutputEvent must be given).

Also, create another thread so that when the process finishes, a TerminatedEvent is given (in python, just do a popen.wait() in a thread and when complete send the TerminatedEvent -- you may want to synchronize to make sure other threads related to output have finished before doing that).

At this point, we can run something and should be able to see anything printed both to stdout and stderr and when the process finishes, VSCode itself acknowledges that and closes the related controls.

Great! On to launching in the integrated terminal!

So, to launch in the terminal we have to first actually check if the client does support running in the terminal... in the InitializeRequest, if it is supported, we should've received in the arguments "supportsRunInTerminalRequest": True (if it doesn't, in my case I just fall back to the debug console).

This also becomes a little bit trickier because at this point we're the ones doing the request (RunInTerminalRequest) and the client should send a response (RunInTerminalResponse). So, on to it: when the client launches, create a RunInTerminalRequest with the proper kind ('internal' or 'external') and wait for the response.

At this point, the processId may not actually be available after launching in that mode (the RunInTerminalResponse processId is optional), which means that if we didn't really create a debugger (just a simple run), we're blind... we could do another program to launch it and return the pid to be able to notify that it was stopped and to kill it when needed, but this seems a bit overkill for me and I couldn't find any info on the proper behavior here, so, I decided that when the user chooses that mode with 'noDebug' I'll simply notify that the debug session is finished for the adapter with a TerminatedEvent (and the user can see the output and Ctrl+C it in the actual terminal).

As a note the 'noDebug' option is added behind the scenes by VSCode depending on whether the user has chosen to do a debug or run for the selected launch (so, it shouldn't be a part of the declared configuration in the extension).

Now, thinking a bit more about it, there's a caveat: when launching with the redirection to the debug console, we should treat sending to stdin too (we don't want to create a process he can't do any communication with later on).

To do that in 'noDebug' should be simple... when we receive an 'EvaluateRequest', we'll send it to stdin (when actually in debug mode we probably have to check the current debugger state to determine if we want to do an evaluation or send to stdin -- i.e.: if we are stopped in a breakpoint we may want to evaluate and not send to stdin).

As a note, after playing with it more I renamed the "console" option to "terminal" with options "none", "internal", "external" as I think that's a better representation of what's expected.

So, that's it for part 2: we're launching a program and redirecting the output as requested by the user (albeit without actually debugging it for now).

The related code may be seen at: https://github.com/fabioz/python_debug_adapter_tutorial/tree/master/part2

Howto launch and debug in VSCode using the debug adapter protocol (part 1)

This is a walkthrough with the steps I'm taking to add support to launch and debug a Python script in PyDev for VSCode (note that I'm writing as I'm learning).

The debugger protocol is the protocol used in VSCode to talk to debuggers and handle launching in general (the naming may be a bit weird as the same protocol is used for regular launches and debugging, but apparently the team first did the debugging and then launching came as an afterthought just passing a separate flag during the launching of the program to specify that no debugging should be done -- and not the other way around as I think would be more common).

There is an overview of the protocol at https://code.visualstudio.com/docs/extensionAPI/api-debugging and https://code.visualstudio.com/docs/extensionAPI/extension-points provides more information on what an extension must use to provide a debugger.

There's also a json schema which specifies the format of the messages sent back and forth in the debugger at https://raw.githubusercontent.com/Microsoft/vscode-debugadapter-node/master/debugProtocol.json.

But, after reading all that, it seems that many things are still cloudy on my head on how to actually go on about it and what should be done concretely to implement a debugger in VSCode.

So, my approach is getting the debugProtocol.json, converting it to a structure with Python classes (so that each message that can be sent has a Python representation) and playing a bit doing a debugger stub, just to exercise a dummy debugger talking to VSCode (but without actually doing anything).

It's interesting to note that the first thing to do is actually making the debugger available in the extension. For that, I've used the json below in package.json (as a note, my package.json is actually generated from Python code, so, the structure below is actually a Python dict which is later converted to json, not the actual json -- if you're doing a VSCode extension, I highly recommend generating your package.json and parts of the code that are related and not doing it all by hand... this way it's possible to see it in small pieces and auto generate command ids and the related code, etc... initially I haven't done so in PyDev, but as the declarative files grow, it becomes harder to follow and make changes while keeping the code and declaration in sync):

  
{
    'type': 'PyDev',
    'label': 'PyDev (Python)',
    'languages': ['python'],
    'adapterExecutableCommand': 'pydev.start.debugger', 
    
    # Note: adapterExecutableCommand will be replaced by a different API (right now still in proposal mode). 
    # See: https://code.visualstudio.com/updates/v1_20#_debug-api
    # See: https://github.com/Microsoft/vscode/blob/7636a7d6f7d2749833f783e94fd3d48d6a1791cb/src/vs/vscode.proposed.d.ts#L388-L395
    
    'enableBreakpointsFor': {
        'languageIds': ['python', 'html'],
    },
    'configurationAttributes': {
        'launch': {
            'required': [
                'mainModule'
            ],
            'properties': {

                'mainModule': {
                    'type': 'string',
                    'description': 'The .py file that should be debugged.',
                },

                'args': {
                    'type': 'string',
                    'description': 'The command line arguments passed to the program.'
                },

                "cwd": {
                    "type": "string",
                    "description": "The working directory of the program.",
                    "default": "${workspaceFolder}"
                },

                "console": {
                    "type": "string",
                    "enum": [
                        "integratedTerminal",
                        "externalTerminal"
                    ],
                    "enumDescriptions": [
                        "VS Code integrated terminal.",
                        "External terminal that can be configured in user settings."
                    ],
                    "description": "The specified console to launch the program.",
                    "default": "integratedTerminal"
                },
            }
        }
    },

    "configurationSnippets": [
        {
            "label": "PyDev: Launch Python Program",
            "description": "Add a new configuration for launching a python program with the PyDev debugger.",
            "body": {
                "type": "PyDev",
                "name": "PyDev Debug (Launch)",
                "request": "launch",
                "cwd": "^\"\\${workspaceFolder}\"",
                "console": "integratedTerminal",
                "mainModule": "",
                "args": ""
            }
        },
    ]
}

So, although there are many things there, initially we just need to make adapterExecutableCommand return the command to be executed (you could also create a standalone executable or something to run along with a supported vm -- such as mono, but there's nothing for python there, so, the adapterExecutableCommand is probably the best approach for a python debugger).

In my case it's something as:

  
commands.registerCommand('pydev.start.debugger', () => {
    return {
        command: "C:/bin/python27/python.exe",  // paths initially hardcoded for simplicity
        args: ["X:/vscode-pydev/vscode-pydev/src/debug_adapter/debugger_protocol.py"]
    }
});
  

The configurationSnippets section provides the snippets which allow VSCode to autogenerate the configuration for the user and the configurationAttributes are actually custom for each implementation (so, those will probably need more tweaking going forward).

Another interesting point is that when VSCode launches the debug adapter it'll use stdin and stdout to communicate with the adapter (this makes some things a bit quirky to develop the debugger because you have to (initially) resort to printing debug information to a file to be able to check what's happening, although on the bright side, you won't have to worry about having a firewall at that point).

Also, don't forget to flush after writing messages to stdout.

Now, on to the protocol itself... I created something which would read from stdin and then redirect that to a file to see what's coming (after digging up things a bit more I found an issue in the VSCode tracker referencing: https://github.com/buggerjs/bugger-v8-client/blob/master/PROTOCOL.md which details that a bit more -- although not all that's there is actually applicable to the VSCode debugger). 

The first message that arrives from stdin is:

  
Content-Length: 312\r\n
\r\n
{
    "arguments": {
        "adapterID": "PyDev", 
        "clientID": "vscode", 
        "clientName": "Visual Studio Code", 
        "columnsStartAt1": true, 
        "linesStartAt1": true, 
        "locale": "en-us", 
        "pathFormat": "path", 
        "supportsRunInTerminalRequest": true, 
        "supportsVariablePaging": true, 
        "supportsVariableType": true
    }, 
    "command": "initialize", 
    "seq": 1, 
    "type": "request"
}

-- this is the InitializeRequest in the json schema.

So, it seems a regular http-protocol, sending json contents as the actual content... so, in response to that, the debug adapter should do its initialization and return the capabilities it has -- something as:

  
{
    "seq": 1,
    "request_seq": 1, 
    "command": "initialize", 
    "body": {"supportsConfigurationDoneRequest": true, 
             "supportsConditionalBreakpoints": true}, 
    "type": "response", 
    "success": true
}

-- this is the InitializeResponse in the json schema.

and then send and event saying that it has initialized properly:

  
{"type": "event", "event": "initialized", "seq": 2}

-- this is the InitializedEvent in the json schema.

Note that those are all http responses, so, the Content-Length: $size\r\n\r\n needs to be passed on each request (note that each message sent or received has a seq, which is a number that should be raised whenever a new message is sent -- the seq is raised independently on the server and on the client and responses should reference the seq from the request in request_seq). 

Afterwards, the client (VSCode) sends the actual launch request (which should be based on the configurationAttributes previously configured). In this case:

  
{
    "arguments": {
        "__sessionId": "474aa497-0a90-4b30-8cc6-edf3bebbe703", 
        "args": "", 
        "console": "integratedTerminal", 
        "cwd": "X:\\vscode_example", 
        "name": "PyDev Debug (Launch)", 
        "program": "X:/vscode_example/robots.py", 
        "request": "launch", 
        "type": "PyDev"
    }, 
    "command": "launch", 
    "seq": 2, 
    "type": "request"
}

-- this is the launch request in the json schema (it comes with additional attributes the user specified in the launch... each extension needs to tweak the actual parameters to its use case).

At this point, it becomes clear that this is really just an adapter: we're expected to actually launch the process and provide the communication layer to the actual debugger (so, the debugger doesn't really have to be changed -- although on some cases that may be benefical if possible... for instance, the debugger could already give output on the variable frames as json so that the message doesn't need to be decoded and recoded in a new format). 

Also, the stdin and stdout may be in use (because VSCode uses it to communicate to the debug adapter), so, it may be hard to reuse this process to be the actual debugger process (for instance, launch could then make main proceed to launch the program in this process if the debugger could directly handle the debug protocol, but then if clients managed to write to the 'real' stdin/stdout handles, the debugger would stop working). 

The launch request just requires a notification that the program was launched, so, the response would be a launch response with an empty body (or if there was some error -- say, the file to be launched no longer exists -- a "message" could be set and "success" could be False). 

  
{
    "request_seq": 2, 
    "command": "launch", 
    "body": {}, 
    "type": "response", 
    "success": true
}
  

-- this is the LaunchResponse in the json schema.

Ok, now, at this point I already have a structure which parses the json and creates python instances for each protocol message (and vice-versa), so, instead of specifying each message in its full format, I'll just reference it from the identifier on the schema instead of the actual json. 

After the launch request, I get a ConfigurationDoneRequest and return the proper ConfigurationDoneResponse and for the ThreadsRequest a ThreadsResponse.

At this point, the debugger will sit idle, waiting for actions from the user or events from our debug adapter (if more than one thread was returned in the ThreadsResponse, the threads will appear in the CallStack).

Now, the only thing different at this point is that the debug controls will appear, so, a pause or stop can be activated from the UI.

Pressing stop will send us a DisconnectRequest (for which a DisconnectResponse should be sent as an acknowledgement) and the pause will send a PauseRequest (which requires us to send back a PauseResponse -- and after a thread is actually paused, a StoppedEvent should be sent). 

Ok, this is the end of part 1 (we have something which can be started and later stopped -- without actually doing anything, so, pretty much a mock debugger)... This actually took me 2 full days to implement (most of the work trying to wrap my head around how things worked and generating python code from the json schema -- I tried some libraries and none of them worked as I needed, so, I rolled my own here). 

My main gripe was the lack of a better documentation on how to approach doing a debug protocol from scratch and how it should work. For instance, it took me quite a while to find a reference to launching from the adapterExecutableCommand where I could construct a command line -- initial references I found pointed only to using an executable or a supported runtime such as mono -- some things I still don't know how to handle such as how to actually provide output based on the console type the user expects: (i.e.: integratedTerminal, externalTerminal) -- anyways, hope to get to that in the upcoming parts... 

The final code I have at this point (which also contains the code generator I did) may be seen at:

https://github.com/fabioz/python_debug_adapter_tutorial/tree/master/part1

Part 2 should get us to the point of actually launching a process...