Initial support for devtools inspector protocol

[jasnell]

A work-in-progress prototype enabling devtools inspector protocol support for workerd.

Adds an additional option to workerd serve to enable a local inspector port:

$ ./workerd serve -i 127.0.0.1:9229 config.capnp

(The inspector socket can also be passed as an fd instead)

When enabled, the inspector protocol wil be available on the given address and will be discoverable in devtools.

Each worker in a configuration will be listed separately in the devtools discovery view. The name of the worker itself will be displayed in the discovered list to make it possible to distinguish.

Very rudimentary for now.

This PR also adds support for using devtools to capture heap snapshots.

[jasnell]

One issue this currently has is an error stack that is printed to the terminal when an attached debugger disconnects:

workerd/server/server.c++:1638: error: Uncaught exception: kj/compat/http.c++:2398: disconnected: WebSocket disconnected between frames without sending `Close`.
stack: /home/james/.cache/bazel/_bazel_james/728d89b94395895921db6cbedc832917/execroot/workerd/bazel-out/k8-fastbuild/bin/src/workerd/server/workerd@2aac760 /home/james/.cache/bazel/_bazel_james/728d89b94395895921db6cbedc832917/execroot/workerd/bazel-out/k8-fastbuild/bin/src/workerd/server/workerd@6fd8e10 /home/james/.cache/bazel/_bazel_james/728d89b94395895921db6cbedc832917/execroot/workerd/bazel-out/k8-fastbuild/bin/src/workerd/server/workerd@6fdd2f0
    ??:0: returning here
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workerd/server/server.c++:1564: error: exception = kj/compat/http.c++:1969: failed: expected !inBody; previous HTTP message body incomplete; can't write more messages
stack: /home/james/.cache/bazel/_bazel_james/728d89b94395895921db6cbedc832917/execroot/workerd/bazel-out/k8-fastbuild/bin/src/workerd/server/workerd@6f776cc /home/james/.cache/bazel/_bazel_james/728d89b94395895921db6cbedc832917/execroot/workerd/bazel-out/k8-fastbuild/bin/src/workerd/server/workerd@70009d9 /home/james/.cache/bazel/_bazel_james/728d89b94395895921db6cbedc832917/execroot/workerd/bazel-out/k8-fastbuild/bin/src/workerd/server/workerd@6f3946f /home/james/.cache/bazel/_bazel_james/728d89b94395895921db6cbedc832917/execroot/workerd/bazel-out/k8-fastbuild/bin/src/workerd/server/workerd@6f3963d /home/james/.cache/bazel/_bazel_james/728d89b94395895921db6cbedc832917/execroot/workerd/bazel-out/k8-fastbuild/bin/src/workerd/server/workerd@241d998 /home/james/.cache/bazel/_bazel_james/728d89b94395895921db6cbedc832917/execroot/workerd/bazel-out/k8-fastbuild/bin/src/workerd/server/workerd@241d9e3 /home/james/.cache/bazel/_bazel_james/728d89b94395895921db6cbedc832917/execroot/workerd/bazel-out/k8-fastbuild/bin/src/workerd/server/workerd@6fdd52b /home/james/.cache/bazel/_bazel_james/728d89b94395895921db6cbedc832917/execroot/workerd/bazel-out/k8-fastbuild/bin/src/workerd/server/workerd@6fdd44c /home/james/.cache/bazel/_bazel_james/728d89b94395895921db6cbedc832917/execroot/workerd/bazel-out/k8-fastbuild/bin/src/workerd/server/workerd@6fddb6f /home/james/.cache/bazel/_bazel_james/728d89b94395895921db6cbedc832917/execroot/workerd/bazel-out/k8-fastbuild/bin/src/workerd/server/workerd@6fdd8b0 /home/james/.cache/bazel/_bazel_james/728d89b94395895921db6cbedc832917/execroot/workerd/bazel-out/k8-fastbuild/bin/src/workerd/server/workerd@72a9350 /home/james/.cache/bazel/_bazel_james/728d89b94395895921db6cbedc832917/execroot/workerd/bazel-out/k8-fastbuild/bin/src/workerd/server/workerd@72a9328 /home/james/.cache/bazel/_bazel_james/728d89b94395895921db6cbedc832917/execroot/workerd/bazel-out/k8-fastbuild/bin/src/workerd/server/workerd@737dcef /home/james/.cache/bazel/_bazel_james/728d89b94395895921db6cbedc832917/execroot/workerd/bazel-out/k8-fastbuild/bin/src/workerd/server/workerd@72a1a4c /home/james/.cache/bazel/_bazel_james/728d89b94395895921db6cbedc832917/execroot/workerd/bazel-out/k8-fastbuild/bin/src/workerd/server/workerd@2c29060
    ??:0: returning here
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[jasnell]

Another issue is that memory profiling does not work. The devtools client will say that it's recording the snapshot but it never completes.

[kentonv]

I would have opened this as a draft PR but it seems we don't have that feature available.

FWIW apparently this is some sort of premium feature but will become available when the repo is public.

[jasnell]

Ok, so I updated the implementation so that it is not added to the list of named/registered services but it still implements from Service. The reason is that it's just easier to implement this way since all of the mechanism for integrating Service and Socket are already in place.

I've also made it so that disconnects are handled cleanly and added some logging when --verbose is used. Fixed a couple of other bugs also. This should be ready to go for the initial rudimentary version.

[jasnell]

Doh, conflict with one of the other changes... pushing a fix in a sec

Updated

[kentonv]

Ok, so I updated the implementation so that it is not added to the list of named/registered services but it still implements from Service. The reason is that it's just easier to implement this way since all of the mechanism for integrating Service and Socket are already in place.

But you aren't actually using config::Socket to configure these sockets so there's no reason to use the socket code in the first place, and therefore no need to define the inspector in a way that integrates with that.

You should be able to use kj::HttpServer::listenHttp() directly, you don't need to use Server::listenHttp() which exists mainly to support those config options you're not using....

[jasnell]

But you aren't actually using config::Socket to configure these sockets so there's no reason to use the socket code in the first place...

Yes, I know, but the mechanism was there and very simple to extend from. Easiest thing that just worked.

I've reworked it as you've suggested.

[github-actions]

CLA Assistant Lite bot All contributors have signed the CLA ✍️ ✅

[jasnell]

Bad CLA-bot! Bad! No I will not sign the CLA. (for those reading...I don't have to since I'm Cloudflare already...)

[kentonv]

@jasnell Its gonna be easier if you just write the comment it wants you to write. You could also edit yourself into the allow list but that seems like more work.

[jasnell]

I have read the CLA Document and I hereby sign the CLA

[jasnell]

(silly CLA bot... you annoy me already)