Slow mocks shutdown in Go 1.15

[fetinin]

In Go 1.15 behavior of http.Server.Shutdown changed and now server waits for all listeners to disconnect.

// go/1.15/libexec/src/net/http/server.go:2699
...
ticker := time.NewTicker(shutdownPollInterval) // 500 ms
defer ticker.Stop()
for {
	if srv.closeIdleConns() && srv.numListeners() == 0 { // for some reason this is always false first time
		return lnerr
	}
	select {
	case <-ctx.Done():
		return ctx.Err()
	case <-ticker.C:  // we always wait here half a second
	}
...

For some reason, on mock shutdown we always still have some active connections and shutdown takes about 500ms. And since we shutdown all mocks synchronously with a lot of mocks we wait for tests to finish for up to 18 seconds.

Does anyone else have this issue?

As a solution we can close each mock server in a separate goroutine, that should reduce wait time for us from 18s to 500ms.

[luza]

Suggested solution looks more like a work around. I'd better find out why it can't be shut down immediately as I don't see any reason to wait even 500 ms.

[irr123]

Look at this PR #58

[fetinin]

Fixed in #59