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:

https://github.com/trailofbits/blight/blob/92b3c190e848055239cb0d6cacfd0edab1f2608e/src/blight/util.py#L201-L218