TestVariousDeadlines starts a client and server.
The client dials the server, sets a timeout on the connection,
reads from it, gets a timeout error, closes the connection.
The server writes an infinite stream of a's to each connection
it accepts.

The test was trying to run these in lockstep:
run a client dial+read+timeout+close,
wait for server to accept+write+error out on write to closed connection,
repeat.

On FreeBSD 11.2 and less frequently on macOS we see
the test timeout waiting for the server to do its half of
the lockstep dance.

I believe the problem is that the client can do its step
of the dance with such a short timeout that the read,
timeout, and close happens before the server ever returns
from the accept(2) system call. For the purposes of testing
the client-side read timeout, this is fine. But I suspect
that under some circumstances, the "TCP-accepted"
connection does not translate into a "socket-layer-accepted"
connection that triggers a return from accept(2).
That is, the Go server never sees the connection at all.
And the test sits there waiting for it to acknowledge
being done with a connection it never started with.

Fix the problem by not trying to lockstep with the server.

This definitely fixes the flake, since the specific line that
was calling t.Fatal is now deleted.

This exposes a different flake, seen on a trybot run for an
early version of this CL, in which the client's io.Copy does
not stop within the time allotted. The problem now is that
there is no guarantee that a read beyond the deadline with
available data returns an error instead of the available data,
yet the test assumes this guarantee, and in fact the opposite
is usually true - we don't bother checking the deadline unless
the read needs to block. That is, deadlines don't cut off a
flood of available data, yet this test thinks they do.

This CL therefore also changes the server not to send an
infinite flood of data - don't send any data at all - so that
the read deadline is guaranteed to be exercised.

Fixes #19519.

Change-Id: I58057c3ed94ac2aebab140ea597f317abae6e65e
Reviewed-on: https://go-review.googlesource.com/c/go/+/184137
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>