Trying to work out how to get a single mega-compilation-database out of Buck. This is a pretty naive approach, but I think it works?

BUCK

# compdb/BUCK

python_bootstrap_binary(
    name = "combine_compdbs",
    main = "combine_compdbs.py",
    visibility = ["PUBLIC"]
)

combine_compdbs.py

# compdb/combine_compdbs.py

#!/usr/bin/env python3

import sys
import argparse
import json

parser = argparse.ArgumentParser()

parser.add_argument("database", nargs='+')
parser.add_argument('--output', '-o', dest='output', required=True)

def main(argv):
    args = parser.parse_args(argv)

    result = {}

    for fn in args.database:
        content = json.load(open(fn, 'r'))

        for row in content:
            result[row['file']] = row

    res = list(result.values())

    json.dump(res, open(args.output, 'w'), indent = 4)

if __name__ == '__main__':
    main(sys.argv[1:])

defs.bzl

# compdb/defs.bzl

def _impl(ctx: "context"):
    output = ctx.actions.declare_output('compile_commands.json')

    cmd = [ctx.attrs._combine_compdbs[RunInfo]]
    cmd.extend(ctx.attrs.databases)
    cmd.append(cmd_args(output.as_output(), format="--output={}"))

    ctx.actions.run(
        cmd,
        category="combine_compilation_databases",
    )

    return [DefaultInfo(default_output=output)]

combine_compdbs = rule(
    impl=_impl,
    attrs = {
        "databases": attrs.list(attrs.source()),
        "_combine_compdbs": attrs.default_only(attrs.exec_dep(providers = [RunInfo], default="//compdb:combine_compdbs"))
    }
)

example_BUCK

# example BUCK file.  Can go any old place.

load("//compdb:defs.bzl", "combine_compdbs")

combine_compdbs(
    name='compile-commands',
    databases=[
        '//:foo[compilation-database]',
        '//:bar[compilation-database]',
        '//:baz[compilation-database]',
        '//:quux[compilation-database]',
    ],
)