p2p: refactor MaxPendingPeers handling

[battlmonstr]

• use semaphore instead of a chan struct{}
• move MaxPendingPeers default value to DefaultConfig.P2P
• log Error if Accept fails
• replace quit channel with context

[AskAlexSharov]

better do release in defer, otherwise can get deadlock on panic

[battlmonstr]

I'm not sure how to do this without a bigger refactoring of the structure of this function.

defer only runs on return, and can't be cancelled once it is set.
Here the logic wants to release either before goroutine if checks fail, or after the setup goroutine ends.

One way to do would be like so (pseudocode):

slots.Acquire()
preSetupBarrier = WaitGroup(1)
go {
    defer slots.Release()
    conn, err = preSetup(preSetupBarrier)
    if err != nil { return }
    setup(conn)
}
preSetupBarrier.Wait()

//

func preSetup(preSetupBarrier) (conn, err) {
    defer preSetupBarrier.Done()
    conn, err = accept()
    if err != nil { return nil, err }
    err = checks(conn)
    return conn, err
}

it is definitely neat and future-proof with a single Release, but it adds a barrier, and a new func such that .Done() call is also performed by defer to avoid it in multiple return paths.
besides log.Trace and fd.Close are primitives and are unlikely to panic.

Do you think it is worth this refactoring?

Also I wonder if we'd like to keep the ability to apply patches from geth upstream?

[battlmonstr]

I'm also considering using a bounded channel, but I feel that there has to be 2 channels for this. It wants to allow for parallel setup while queueing up Accept'ed conns.

[AskAlexSharov]

TestUDPv5_lookupE2E v5_udp_test.go:94: (a065529b1d5276dc) [TRACE] [04-27|18:07:53.448] UDP read error err="read udp4 127.0.0.1:55675: use of closed network connection"
probably can take a look later