JSON workers should delimit work requests with newlines

[wolfd]

Description of the problem / feature request:

JSON workers are difficult to write because work requests are not newline delimited. This issue was brought up in #13599, and the discussion generally indicated that this is a usability bug.

The proposal is to add a newline between requests according to ndjson.

Feature requests: what underlying problem are you trying to solve with this feature?

Right now an implementation of a JSON worker work request loop using python looks like this:

# This is a pretty cumbersome way of getting work requests.
# It waits for something to be available on stdin, and then tries parsing
# whatever text is available as JSON. It keeps accumulating more text until a
# full work request is available.
def wait_for_stdin_json():
    stdin = open("/dev/stdin", "rb")
    os.set_blocking(stdin.fileno(), False)
    stream_bytes = b""
    while True:
        _, _, _ = select.select([stdin.fileno()], [], [])
        maybe_bytes = stdin.read()
        if maybe_bytes is None:
            continue
        stream_bytes += maybe_bytes
        try:
            work_request = json.loads(stream_bytes)
        except json.decoder.JSONDecodeError:
            # stream couldn't decode, wait until next select
            sys.stderr.write(f"failed to decode: {stream_bytes.decode('utf-8')}\n")
            sys.stderr.flush()
            continue

        yield work_request
        stream_bytes = b""  # reset

With newline-delimited JSON it would look like this:

# Simplified json loop with newline-delimited json.
def wait_for_stdin_json():
    while True:
        yield json.loads(sys.stdin.readline())

Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

Current version

Proposed version

[meisterT]

I agree, see #13599 (comment)

[meisterT]

cc @larsrc-google

[brentleyjones]

@wolfd Do you want to make a PR against the 5.0 rc? Seems like a good thing to try to get in.

[wolfd]

@wolfd Do you want to make a PR against the 5.0 rc? Seems like a good thing to try to get in.

Sure: #14314