Fixes #7782
Fixes #9554
Updates #7237 (original metabug, before we switched to specific bugs)
Updates #11932 (plan9 still doesn't have net I/O deadline support)

Change-Id: I96f311b88b1501d884ebc008fd31ad2cf1e16d75
Reviewed-on: https://go-review.googlesource.com/15941
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David du Colombier <0intro@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Since CL 22101, network tests are failing on Plan 9
due to the lack of deadline support.

Instead of panicking, we just ignore the deadline
when set.

Update #11932.
Fixes #15328.

Change-Id: I1399303b0b3d6d81e0b8b8d327980d978b411a46
Reviewed-on: https://go-review.googlesource.com/22127
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

My previous https://golang.org/cl/22101 to add context throughout the
net package broke Plan 9, which isn't currently tested (#15251).

It also broke some old unsupported version of Windows (Windows 2000?)
which doesn't have the ConnectEx function, but that was only found
visually, since our minimum supported Windows version has ConnectEx.
This change simplifies the Windows and deletes the non-ConnectEx code
path.  Windows 2000 will work even less now, if it even worked
before. Windows XP remains our minimum supported version.

Specifically, the previous CL stopped using the "dial" function, which
0intro noted:
#15333 (comment)

This CL removes the dial function instead and makes plan9's net
implementation respect contexts, which likely fixes a number of
t.Skipped tests. I'm leaving that to 0intro to investigate.

In the process of propagating and respecting contexts for plan9, I had
to change some signatures to add contexts to more places and ended up
pushing contexts down into the Go-based DNS resolution as well,
replacing the pure-Go DNS implementation's use of "timeout
time.Duration" with a context instead.

Updates #11932
Updates #15328

Fixes #15333

Change-Id: I6ad1e62f38271cdd86b3f40921f2d0f23374936a
Reviewed-on: https://go-review.googlesource.com/22144
Reviewed-by: David du Colombier <0intro@gmail.com>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

…cel on Plan 9

TestDialParallel, TestDialerFallbackDelay and TestDialCancel
require dialTCP to support cancellation, which has been
implemented for Plan 9 in CL 22144.

Updates #11225.
Updates #11932.

Change-Id: I3b30a645ef79227dfa519cde8d46c67b72f2485c
Reviewed-on: https://go-review.googlesource.com/22203
Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

DualStack mode requires dialTCP to support cancellation,
which has been implemented for Plan 9 in CL 22144.

Updates #11225.
Updates #11932.

Change-Id: I6e468363dc147326b097b604c122d5af80362787
Reviewed-on: https://go-review.googlesource.com/22204
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

All implementations have always implemented this behavior, it's
tested, and it's depended on by other packages. (notably, by net/http)

The one exception is Plan 9 which doesn't support I/O deadlines at all
(tracked in #11932). As a result, a bunch of tests fail on plan9
(#7237). But once Plan 9 adds I/O deadline support, it'll also need
this behavior.

Change-Id: Idb71767f0c99279c66dce29f7bdc78ef467e47aa
Reviewed-on: https://go-review.googlesource.com/30164
Reviewed-by: Sam Whited <sam@samwhited.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>