core: flb_scheduler fix for macOS

[bluebike]

fluent-bit/src/flb_scheduler.c

Lines 46 to 50 in d1bcfa8

	ret = flb_pipe_r(fd, &val, sizeof(val));
	if (ret <= 0) {
	flb_errno();
	return -1;
	}

src/flb_scheduler.c: cosumer_bytes() gives errors in macOS (10.13,... ?).
function flb_pipe_r can return value 0 which is consider as error.
This patch allows return value zero.

Probably we should test if ret == 0 is ok in other operating systems.

Fixes #2460

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Testing
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• [N/A] Example configuration file for the change
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---

Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[bluebike]

Test run after patch

$ bin/fluent-bit -i dummy -o stdout
Fluent Bit v1.6.0
* Copyright (C) 2019-2020 The Fluent Bit Authors
* Copyright (C) 2015-2018 Treasure Data
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

[2020/08/12 11:48:37] [ info] [engine] started (pid=67982)
[2020/08/12 11:48:37] [ info] [storage] version=1.0.5, initializing...
[2020/08/12 11:48:37] [ info] [storage] in-memory
[2020/08/12 11:48:37] [ info] [storage] normal synchronization mode, checksum disabled, max_chunks_up=128
[2020/08/12 11:48:37] [ info] [sp] stream processor started
[0] dummy.0: [1597222118.028957000, {"message"=>"dummy"}]
[1] dummy.0: [1597222119.027146000, {"message"=>"dummy"}]
[2] dummy.0: [1597222120.028602000, {"message"=>"dummy"}]
[3] dummy.0: [1597222121.027044000, {"message"=>"dummy"}]
[0] dummy.0: [1597222122.028639000, {"message"=>"dummy"}]
[1] dummy.0: [1597222123.026792000, {"message"=>"dummy"}]
[2] dummy.0: [1597222124.028137000, {"message"=>"dummy"}]

[edsiper]

thanks