Improve handling of newlines in log messages

[5tefan]

(Apologies for not starting a discussion as requested in contributing guidelines... let's have the discussion here and reject this PR if it doesn't align with your vision.)

Summary

Improves the newline handling in log forwarding from workers so that it does not always introduce newlines.

Problem

Due to some unknown change in our setup, we started seeing log messages from workers split into multiple lines:

Perhaps we are flushing log message more frequently than expected by dramatiq... and the flushes are not aligned with newlines.

Solution

After some digging, we found that the newlines are inserted by dramatiq here:

dramatiq/dramatiq/cli.py

Lines 322 to 338 in a30b787

	while event.poll():
	# StreamHandler writes newlines into the pipe separately
	# from the actual log entry; to avoid back-to-back newlines
	# in the events pipe (causing multiple entries on a single
	# line), discard newline-only data from the pipe
	try:
	data = event.recv_bytes()
	except EOFError:
	event.close()
	raise
	data = data.decode("utf-8", errors="replace").rstrip("\n")
	if not data:
	continue
	log_file.write(data + "\n")
	log_file.flush()

This code always adds a newline, even if the rstrip did not strip one out.

Given the comment above this code section explaining the goal:

discard newline-only data from the pipe

I believe this revision accomplishes this better by only using rstrip for the continue condition (to not pass blank lines),but otherwise passing the log without modification.

[Bogdanp]

Thanks! I'll try to review this PR this weeked or the next. The code in question does look like it assumes line buffering, so your setup probably changed to some other buffer mode for whatever reason.