init: Fix incorrect warning "Reducing -maxconnections from N to N-1, because of system limitations" #20024

Fix incorrect warning Reducing -maxconnections from N to N-1, because of system limitations.

Before this patch (only the first warning is correct):

$ src/bitcoind -maxconnections=10000000 | grep Warning
2020-09-26T01:23:45Z Warning: Reducing -maxconnections from 10000000 to 1048417, because of system limitations.

$ src/bitcoind -maxconnections=1000000 | grep Warning
2020-09-26T01:23:45Z Warning: Reducing -maxconnections from 1000000 to 999999, because of system limitations.

$ src/bitcoind -maxconnections=100000 | grep Warning
2020-09-26T01:23:45Z Warning: Reducing -maxconnections from 100000 to 99999, because of system limitations.

$ src/bitcoind -maxconnections=10000 | grep Warning
2020-09-26T01:23:45Z Warning: Reducing -maxconnections from 10000 to 9999, because of system limitations.

$ src/bitcoind -maxconnections=1000 | grep Warning
2020-09-26T01:23:45Z Warning: Reducing -maxconnections from 1000 to 999, because of system limitations.

$ src/bitcoind -maxconnections=100 | grep Warning
[no warning]

After this patch (no incorrect warnings):

$ src/bitcoind -maxconnections=10000000 | grep Warning
2020-09-26T01:23:45Z Warning: Reducing -maxconnections from 10000000 to 1048417, because of system limitations.

$ src/bitcoind -maxconnections=1000000 | grep Warning
[no warning]

$ src/bitcoind -maxconnections=100000 | grep Warning
[no warning]

$ src/bitcoind -maxconnections=10000 | grep Warning
[no warning]

$ src/bitcoind -maxconnections=1000 | grep Warning
[no warning]

$ src/bitcoind -maxconnections=100 | grep Warning
[no warning]

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #19509 (Per-Peer Message Capture by troygiorshev)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

Just ran your change on macOS and Debian and wasn´t able to reproduce the error or saw any difference with and without your change applied, so this error may only be affecting some systems 🤔 macOS 10.15.6 at ea93bbeb2 & 23d3ae7ac

$ ./src/bitcoind --maxconnections=10000000 | grep Warning
Warning: Reducing -maxconnections from 10000000 to 865, because of system limitations.
2020-09-27T21:41:03Z Warning: Reducing -maxconnections from 10000000 to 865, because of system limitations.

$ ./src/bitcoind --maxconnections=1000000 | grep Warning 
Warning: Reducing -maxconnections from 1000000 to 865, because of system limitations.
2020-09-27T21:41:17Z Warning: Reducing -maxconnections from 1000000 to 865, because of system limitations.

$ ./src/bitcoind --maxconnections=100000 | grep Warning 
Warning: Reducing -maxconnections from 100000 to 865, because of system limitations.
2020-09-27T21:41:23Z Warning: Reducing -maxconnections from 100000 to 865, because of system limitations.

$ ./src/bitcoind --maxconnections=10000 | grep Warning 
Warning: Reducing -maxconnections from 10000 to 865, because of system limitations.
2020-09-27T21:41:29Z Warning: Reducing -maxconnections from 10000 to 865, because of system limitations.

$ ./src/bitcoind --maxconnections=1000 | grep Warning 
Warning: Reducing -maxconnections from 1000 to 865, because of system limitations.
2020-09-27T21:41:35Z Warning: Reducing -maxconnections from 1000 to 865, because of system limitations.

$ ./src/bitcoind --maxconnections=866 | grep Warning 
Warning: Reducing -maxconnections from 866 to 865, because of system limitations.
2020-09-27T21:41:44Z Warning: Reducing -maxconnections from 866 to 865, because of system limitations.

$ ./src/bitcoind --maxconnections=865 | grep Warning
[no warning]

Debian Buster (10.5, 4.19.76-linuxkit) in Docker on macOS at ea93bbeb2 & 23d3ae7ac

$ ./src/bitcoind --maxconnections=10000000 | grep Warning
Warning: Reducing -maxconnections from 10000000 to 1048417, because of system limitations.
2020-09-27T21:52:54Z Warning: Reducing -maxconnections from 10000000 to 1048417, because of system limitations.

$ ./src/bitcoind --maxconnections=1000000 | grep Warning
[no warning]
$ ./src/bitcoind --maxconnections=100000 | grep Warning
[no warning]

Related to #18911.

@n-thumann Thanks for testing! Good to know that you're not affected. FWIW I've seen this on multiple machines :)

Hopefully others will chime in to add their observations: that might allow us to pinpoint exactly which systems that are affected.

tACK https://github.com/bitcoin/bitcoin/pull/20024/commits/ea93bbeb26948c0ddba39b589bd166eaecf446c8, Ran on other systems running Debian 10.5 (4.19.0-8-amd64) and Debian bullseye/sid (5.3.0-1-amd64) and was able to reproduce the issue exactly as you described above on both of them. After applying your patch the issue is fixed :v:

tACK ea93bbeb26948c0ddba39b589bd166eaecf446c8 I can confirm the behaviour in the PR description (for both in master branch and PR branch where it's fixed) with the same warning output numbers, running Ubuntu 16.04.6 LTS in x86_64 with Kernel 4.4.0 here.

Code review ACK ea93bbeb26948c0ddba39b589bd166eaecf446c8