Debugging builders?

[simolus3]

When working on a builder, it is convenient to be able to run a mostly regular build and attach breakpoints in builders or other parts of the stack like the analyzer to figure out what's going on. Of course, I could also try to reproduce every issue I run into with a build_test testcase and then debug that, but the ergonomics of just pausing a "real" build and inspecting that state are much better.

Historically, the trick has been to start .dart_tool/build/entrypoint/build.dart as a standalone program with the right arguments to have full debugging control over the process. But since that script now waits for the BuildProcessState on stdin, that doesn't seem to work anymore.

So I wonder if the build and watch commands could perhaps come with a "debug" mode that spawns the underlying process with --observe --pause-isolates-on-start? Then I could start a build with say dart run build_runner build --debug-builders, wait for that to print the VM service URI of the inner process and attach a debugger to that?

I'm not sure if I'm overlooking something that would complicate this, but if that flag / general approach sounds good to the maintainers I'd be happy to contribute.

[davidmorgan]

Sounds good to me, I see you already got started in #4243 :)