net,os: arrange zero-copy of os.File and net.TCPConn to net.UnixConn

Fixes #58808

goos: linux
goarch: amd64
pkg: net
cpu: DO-Premium-Intel
                             │      old      │                 new                  │
                             │    sec/op     │    sec/op     vs base                │
Splice/tcp-to-unix/1024-4       3.783µ ± 10%   3.201µ ±  7%  -15.40% (p=0.001 n=10)
Splice/tcp-to-unix/2048-4       3.967µ ± 13%   3.818µ ± 16%        ~ (p=0.971 n=10)
Splice/tcp-to-unix/4096-4       4.988µ ± 16%   4.590µ ± 11%        ~ (p=0.089 n=10)
Splice/tcp-to-unix/8192-4       6.981µ ± 13%   5.236µ ±  9%  -25.00% (p=0.000 n=10)
Splice/tcp-to-unix/16384-4     10.192µ ±  9%   7.350µ ±  7%  -27.89% (p=0.000 n=10)
Splice/tcp-to-unix/32768-4      19.65µ ± 13%   10.28µ ± 16%  -47.69% (p=0.000 n=10)
Splice/tcp-to-unix/65536-4      41.89µ ± 18%   15.70µ ± 13%  -62.52% (p=0.000 n=10)
Splice/tcp-to-unix/131072-4     90.05µ ± 11%   29.55µ ± 10%  -67.18% (p=0.000 n=10)
Splice/tcp-to-unix/262144-4    170.24µ ± 15%   52.66µ ±  4%  -69.06% (p=0.000 n=10)
Splice/tcp-to-unix/524288-4     326.4µ ± 13%   109.3µ ± 11%  -66.52% (p=0.000 n=10)
Splice/tcp-to-unix/1048576-4    651.4µ ±  9%   228.3µ ± 14%  -64.95% (p=0.000 n=10)
geomean                         29.42µ         15.62µ        -46.90%

                             │      old      │                  new                   │
                             │      B/s      │      B/s       vs base                 │
Splice/tcp-to-unix/1024-4      258.2Mi ± 11%   305.2Mi ±  8%   +18.21% (p=0.001 n=10)
Splice/tcp-to-unix/2048-4      492.5Mi ± 15%   511.7Mi ± 13%         ~ (p=0.971 n=10)
Splice/tcp-to-unix/4096-4      783.5Mi ± 14%   851.2Mi ± 12%         ~ (p=0.089 n=10)
Splice/tcp-to-unix/8192-4      1.093Gi ± 11%   1.458Gi ±  8%   +33.36% (p=0.000 n=10)
Splice/tcp-to-unix/16384-4     1.497Gi ±  9%   2.076Gi ±  7%   +38.67% (p=0.000 n=10)
Splice/tcp-to-unix/32768-4     1.553Gi ± 11%   2.969Gi ± 14%   +91.17% (p=0.000 n=10)
Splice/tcp-to-unix/65536-4     1.458Gi ± 23%   3.888Gi ± 11%  +166.69% (p=0.000 n=10)
Splice/tcp-to-unix/131072-4    1.356Gi ± 10%   4.131Gi ±  9%  +204.72% (p=0.000 n=10)
Splice/tcp-to-unix/262144-4    1.434Gi ± 13%   4.637Gi ±  4%  +223.32% (p=0.000 n=10)
Splice/tcp-to-unix/524288-4    1.497Gi ± 15%   4.468Gi ± 10%  +198.47% (p=0.000 n=10)
Splice/tcp-to-unix/1048576-4   1.501Gi ± 10%   4.277Gi ± 16%  +184.88% (p=0.000 n=10)
geomean                        1.038Gi         1.954Gi         +88.28%

                             │      old      │                   new                   │
                             │     B/op      │    B/op     vs base                     │
Splice/tcp-to-unix/1024-4      0.000 ±  0%     0.000 ± 0%         ~ (p=1.000 n=10) ¹
Splice/tcp-to-unix/2048-4      0.000 ±  0%     0.000 ± 0%         ~ (p=1.000 n=10) ¹
Splice/tcp-to-unix/4096-4      0.000 ±  0%     0.000 ± 0%         ~ (p=1.000 n=10) ¹
Splice/tcp-to-unix/8192-4      0.000 ±  0%     0.000 ± 0%         ~ (p=1.000 n=10) ¹
Splice/tcp-to-unix/16384-4     0.000 ±  0%     0.000 ± 0%         ~ (p=1.000 n=10) ¹
Splice/tcp-to-unix/32768-4     0.000 ±  0%     0.000 ± 0%         ~ (p=1.000 n=10) ¹
Splice/tcp-to-unix/65536-4     1.000 ±   ?     0.000 ± 0%  -100.00% (p=0.001 n=10)
Splice/tcp-to-unix/131072-4    2.000 ±  0%     0.000 ± 0%  -100.00% (p=0.000 n=10)
Splice/tcp-to-unix/262144-4    4.000 ± 25%     0.000 ± 0%  -100.00% (p=0.000 n=10)
Splice/tcp-to-unix/524288-4    7.500 ± 33%     0.000 ± 0%  -100.00% (p=0.000 n=10)
Splice/tcp-to-unix/1048576-4   17.00 ± 12%      0.00 ± 0%  -100.00% (p=0.000 n=10)
geomean                                    ²               ?                       ² ³
¹ all samples are equal
² summaries must be >0 to compute geomean
³ ratios must be >0 to compute geomean

                             │     old      │                 new                 │
                             │  allocs/op   │ allocs/op   vs base                 │
Splice/tcp-to-unix/1024-4      0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
Splice/tcp-to-unix/2048-4      0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
Splice/tcp-to-unix/4096-4      0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
Splice/tcp-to-unix/8192-4      0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
Splice/tcp-to-unix/16384-4     0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
Splice/tcp-to-unix/32768-4     0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
Splice/tcp-to-unix/65536-4     0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
Splice/tcp-to-unix/131072-4    0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
Splice/tcp-to-unix/262144-4    0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
Splice/tcp-to-unix/524288-4    0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
Splice/tcp-to-unix/1048576-4   0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
geomean                                   ²               +0.00%                ²
¹ all samples are equal
² summaries must be >0 to compute geomean

Change-Id: I829061b009a0929a8ef1a15c183793c0b9104dde
Reviewed-on: https://go-review.googlesource.com/c/go/+/472475


Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>