…-state Handshake calls

The Read and Write methods of *tls.Conn call Handshake
unconditionally, every time they are called, expecting it to only
perform a new handshake if required.

However in go 1.17 handshakeContext was extended to set up a
cancelable Context, and importantly did so prior to checking if a
handshake is required. This thus causes it to allocate on every call,
even in those that are no-ops when called in a Read or Write on an
established connection, sometimes leading to very large numbers of
allocations during reads.

This change adds an early return, prior to setting up the context or
proceeding into the handshakeMutex and checking the handshake error, if
the handshake status atomic indicates handshake is already complete.

name                                     old allocs/op  new allocs/op  delta
Throughput/MaxPacket/1MB/TLSv12-10          1.07k ± 0%     0.62k ± 0%  -42.16%  (p=0.000 n=4+5)
Throughput/MaxPacket/1MB/TLSv13-10          1.70k ± 0%     1.25k ± 0%     ~     (p=0.079 n=4+5)
Throughput/MaxPacket/2MB/TLSv12-10          1.62k ± 0%     0.73k ± 0%  -55.18%  (p=0.008 n=5+5)
Throughput/MaxPacket/2MB/TLSv13-10          2.26k ± 0%     1.36k ± 0%  -39.64%  (p=0.008 n=5+5)
Throughput/MaxPacket/4MB/TLSv12-10          2.74k ± 0%     0.95k ± 0%  -65.35%  (p=0.008 n=5+5)
Throughput/MaxPacket/4MB/TLSv13-10          3.37k ± 0%     1.58k ± 0%  -53.15%  (p=0.008 n=5+5)
Throughput/MaxPacket/8MB/TLSv12-10          4.96k ± 0%     1.39k ± 0%  -72.06%  (p=0.016 n=4+5)
Throughput/MaxPacket/8MB/TLSv13-10          5.60k ± 0%     2.01k ± 0%  -64.05%  (p=0.008 n=5+5)
Throughput/MaxPacket/16MB/TLSv12-10         9.42k ± 0%     2.27k ± 1%  -75.92%  (p=0.016 n=4+5)
Throughput/MaxPacket/16MB/TLSv13-10         10.0k ± 0%      2.9k ± 0%  -71.39%  (p=0.008 n=5+5)
Throughput/MaxPacket/32MB/TLSv12-10         18.3k ± 0%      4.0k ± 0%  -77.97%  (p=0.008 n=5+5)
Throughput/MaxPacket/32MB/TLSv13-10         18.9k ± 0%      4.6k ± 0%  -75.62%  (p=0.008 n=5+5)
Throughput/MaxPacket/64MB/TLSv12-10         36.2k ± 0%      7.5k ± 0%  -79.15%  (p=0.008 n=5+5)
Throughput/MaxPacket/64MB/TLSv13-10         36.7k ± 0%      8.1k ± 0%  -78.06%  (p=0.008 n=5+5)
Throughput/DynamicPacket/1MB/TLSv12-10      1.12k ± 0%     0.63k ± 0%  -44.20%  (p=0.008 n=5+5)
Throughput/DynamicPacket/1MB/TLSv13-10      1.76k ± 0%     1.26k ± 0%  -28.22%  (p=0.016 n=5+4)
Throughput/DynamicPacket/2MB/TLSv12-10      1.68k ± 0%     0.74k ± 0%  -56.11%  (p=0.008 n=5+5)
Throughput/DynamicPacket/2MB/TLSv13-10      2.32k ± 0%     1.37k ± 0%  -40.80%  (p=0.008 n=5+5)
Throughput/DynamicPacket/4MB/TLSv12-10      2.80k ± 0%     0.96k ± 0%  -65.81%  (p=0.008 n=5+5)
Throughput/DynamicPacket/4MB/TLSv13-10      3.43k ± 0%     1.59k ± 0%  -53.57%  (p=0.008 n=5+5)
Throughput/DynamicPacket/8MB/TLSv12-10      5.03k ± 0%     1.39k ± 0%  -72.27%  (p=0.008 n=5+5)
Throughput/DynamicPacket/8MB/TLSv13-10      5.66k ± 0%     2.02k ± 0%  -64.27%  (p=0.008 n=5+5)
Throughput/DynamicPacket/16MB/TLSv12-10     9.48k ± 0%     2.28k ± 1%  -75.98%  (p=0.008 n=5+5)
Throughput/DynamicPacket/16MB/TLSv13-10     10.1k ± 0%      2.9k ± 0%  -71.34%  (p=0.008 n=5+5)
Throughput/DynamicPacket/32MB/TLSv12-10     18.4k ± 0%      4.0k ± 0%  -78.13%  (p=0.008 n=5+5)
Throughput/DynamicPacket/32MB/TLSv13-10     19.0k ± 0%      4.6k ± 0%  -75.54%  (p=0.008 n=5+5)
Throughput/DynamicPacket/64MB/TLSv12-10     36.2k ± 0%      7.6k ± 1%  -79.02%  (p=0.008 n=5+5)
Throughput/DynamicPacket/64MB/TLSv13-10     36.8k ± 0%      8.2k ± 1%  -77.76%  (p=0.008 n=5+5)

Fixes #52790

Change-Id: Iacb1f9bf7802022960d9dbce141b8c0587a614d4
Reviewed-on: https://go-review.googlesource.com/c/go/+/379034
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <valsorda@google.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
(cherry picked from commit a4af356)
Reviewed-on: https://go-review.googlesource.com/c/go/+/405544
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Alex Rakoczy <alex@golang.org>