Actions: write a record-style action that emits `compile_commands.json`

[woodruffw]

The JSON Compilation Database format is about as close as there is to a standard for recording each step in a compilation.

Blight currently has Record, which generates a blight-specific JSON(L) output format. We should also support compile_commands.json, probably with a separate action.

cc @kumarak and @pgoodman as potential end users.

[woodruffw]

The only major "gotcha" here is that blight's architecture is stateless, meaning that each action runs once per wrapped tool invocation. That's why all of the current actions emit JSONL -- it's much faster and easier to append newline-separated records than it is to to re-parse and update a JSON file on each step (particularly when steps happen in parallel).

That leaves two options:

• Swallow the performance hit, and have the action generate the JSON with each step;
• Write a post-processing script that turns the JSONL into a JSON document.

[pgoodman]

Do you have any atomicity guarantees with JSONL? What prevents concurrent writes from being interleaved if a particular command JSON struct is sufficiently long?

[woodruffw]

What prevents concurrent writes from being interleaved if a particular command JSON struct is sufficiently long?

Right now I have a little flock_append decorator that uses exclusive locking to get around any underlying concurrency:

blight/src/blight/util.py

Lines 201 to 218 in 92b3c19

	@contextlib.contextmanager
	def flock_append(filename: os.PathLike):
	"""
	Open the given file for appending, acquiring an exclusive lock on it in
	the process.
	Args:
	filename (str): The file to open for appending
	Yields:
	An open fileobject for `filename`
	"""
	with open(filename, "a") as io:
	try:
	fcntl.flock(io, fcntl.LOCK_EX)
	yield io
	finally:
	fcntl.flock(io, fcntl.LOCK_UN)