don't remove closed connections from the server's accept queue

[marten-seemann]

In preparation for #3549.

This changes the connection queuing logic on the server side. So far, if the server is falling behind on Accepting new connections, we queue up to protocol.MaxAcceptQueueSize connections. Each of these connections has a Go routine watching for the connection to close, and removes this connection from the accept queue if it is closed for any reason.

I don't really see any reason to do it that way. First of all, the server should be able to keep with accepting connections, so at best this is an optimization for a corner case. Second, the server might be interested in learning why the connection was closed. It shouldn't make a difference if the connection was closed 1ms before or after it was accepted.

I hope to eventually get rid of the handleNewConn goroutine altogether. This will require figuring out what happens to these connections when the server is closed. As of v0.40.0, closing the server doesn't close the associated connections anymore: It's now the application's responsibility to deal with these connections. However, we do need to close the connections that weren't accepted yet, since they weren't passed to the application yet. I suspect the current code has a bug here, I don't see this condition being handled anywhere.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (cb1775a) 84.08% compared to head (28c730d) 84.09%.

Files	Patch %	Lines
server.go	50.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4245   +/-   ##
=======================================
  Coverage   84.08%   84.09%           
=======================================
  Files         150      150           
  Lines       15403    15401    -2     
=======================================
- Hits        12951    12950    -1     
  Misses       1950     1950           
+ Partials      502      501    -1     

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