zapcore: add stopped for Stop method called exactly once

[zchee]

Add stopOnce field to BufferedWriteSyncer for Stop method called exactly once.

benchmark results mostly no-op:

name                                           old time/op    new time/op    delta
BufferedWriteSyncer/write_file_with_buffer-20     137ns ± 1%     139ns ± 2%    ~     (p=0.082 n=5+6)
MultiWriteSyncer/2_discarder-20                  8.05ns ± 6%    7.93ns ± 2%    ~     (p=0.662 n=6+5)
MultiWriteSyncer/4_discarder-20                  9.15ns ± 4%    9.14ns ± 4%    ~     (p=1.000 n=5+5)
MultiWriteSyncer/4_discarder_with_buffer-20       131ns ± 2%     130ns ± 0%    ~     (p=0.370 n=6+6)
WriteSyncer/write_file_with_no_buffer-20         4.36µs ± 1%    4.24µs ± 2%  -2.96%  (p=0.004 n=5+6)
ZapConsole-20                                     787ns ± 1%     800ns ± 2%    ~     (p=0.052 n=5+6)
JSONLogMarshalerFunc-20                           738ns ± 1%     737ns ± 0%    ~     (p=0.970 n=6+6)
ZapJSON-20                                        586ns ± 4%     582ns ± 3%    ~     (p=0.818 n=6+6)
StandardJSON-20                                   991ns ± 9%     977ns ± 2%    ~     (p=0.892 n=6+5)
Sampler_Check/7_keys-20                          8.58ns ± 3%    8.49ns ± 2%    ~     (p=0.485 n=6+6)
Sampler_Check/50_keys-20                         8.18ns ± 1%    8.20ns ± 1%    ~     (p=0.690 n=5+5)
Sampler_Check/100_keys-20                        8.02ns ± 4%    8.02ns ± 2%    ~     (p=0.589 n=6+6)
Sampler_CheckWithHook/7_keys-20                  23.4ns ± 2%    23.2ns ± 2%    ~     (p=0.370 n=6+6)
Sampler_CheckWithHook/50_keys-20                 24.3ns ± 3%    24.8ns ± 7%    ~     (p=0.732 n=6+6)
Sampler_CheckWithHook/100_keys-20                24.1ns ± 2%    24.2ns ± 1%    ~     (p=0.662 n=5+6)
TeeCheck-20                                       118ns ± 1%     120ns ± 5%    ~     (p=1.000 n=4+6)
[Geo mean]                                       77.7ns         77.6ns       -0.08%

name                                           old alloc/op   new alloc/op   delta
BufferedWriteSyncer/write_file_with_buffer-20     16.0B ± 0%     16.0B ± 0%    ~     (all equal)
MultiWriteSyncer/2_discarder-20                   16.0B ± 0%     16.0B ± 0%    ~     (all equal)
MultiWriteSyncer/4_discarder-20                   16.0B ± 0%     16.0B ± 0%    ~     (all equal)
MultiWriteSyncer/4_discarder_with_buffer-20       16.0B ± 0%     16.0B ± 0%    ~     (all equal)
WriteSyncer/write_file_with_no_buffer-20          16.0B ± 0%     16.0B ± 0%    ~     (all equal)
ZapConsole-20                                    1.36kB ± 0%    1.36kB ± 0%    ~     (all equal)
JSONLogMarshalerFunc-20                          1.31kB ± 0%    1.31kB ± 0%    ~     (all equal)
ZapJSON-20                                       1.25kB ± 0%    1.25kB ± 0%    ~     (all equal)
StandardJSON-20                                  1.45kB ± 0%    1.45kB ± 0%    ~     (all equal)
Sampler_Check/7_keys-20                           0.00B          0.00B         ~     (all equal)
Sampler_Check/50_keys-20                          0.00B          0.00B         ~     (all equal)
Sampler_Check/100_keys-20                         0.00B          0.00B         ~     (all equal)
Sampler_CheckWithHook/7_keys-20                   0.00B          0.00B         ~     (all equal)
Sampler_CheckWithHook/50_keys-20                  0.00B          0.00B         ~     (all equal)
Sampler_CheckWithHook/100_keys-20                 0.00B          0.00B         ~     (all equal)
TeeCheck-20                                       64.0B ± 0%     64.0B ± 0%    ~     (all equal)
[Geo mean]                                         108B           108B       +0.00%

name                                           old allocs/op  new allocs/op  delta
BufferedWriteSyncer/write_file_with_buffer-20      1.00 ± 0%      1.00 ± 0%    ~     (all equal)
MultiWriteSyncer/2_discarder-20                    1.00 ± 0%      1.00 ± 0%    ~     (all equal)
MultiWriteSyncer/4_discarder-20                    1.00 ± 0%      1.00 ± 0%    ~     (all equal)
MultiWriteSyncer/4_discarder_with_buffer-20        1.00 ± 0%      1.00 ± 0%    ~     (all equal)
WriteSyncer/write_file_with_no_buffer-20           1.00 ± 0%      1.00 ± 0%    ~     (all equal)
ZapConsole-20                                      7.00 ± 0%      7.00 ± 0%    ~     (all equal)
JSONLogMarshalerFunc-20                            5.00 ± 0%      5.00 ± 0%    ~     (all equal)
ZapJSON-20                                         3.00 ± 0%      3.00 ± 0%    ~     (all equal)
StandardJSON-20                                    27.0 ± 0%      27.0 ± 0%    ~     (all equal)
Sampler_Check/7_keys-20                            0.00           0.00         ~     (all equal)
Sampler_Check/50_keys-20                           0.00           0.00         ~     (all equal)
Sampler_Check/100_keys-20                          0.00           0.00         ~     (all equal)
Sampler_CheckWithHook/7_keys-20                    0.00           0.00         ~     (all equal)
Sampler_CheckWithHook/50_keys-20                   0.00           0.00         ~     (all equal)
Sampler_CheckWithHook/100_keys-20                  0.00           0.00         ~     (all equal)
TeeCheck-20                                        1.00 ± 0%      1.00 ± 0%    ~     (all equal)
[Geo mean]                                         2.21           2.21       +0.00%

[codecov]

Codecov Report

Merging #966 (dedda5e) into master (aa3e73e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #966   +/-   ##
=======================================
  Coverage   98.16%   98.17%           
=======================================
  Files          45       45           
  Lines        2020     2027    +7     
=======================================
+ Hits         1983     1990    +7     
  Misses         29       29           
  Partials        8        8           

Impacted Files	Coverage Δ
zapcore/buffered_write_syncer.go	100.00% <100.00%> (ø)

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[prashantv]

I'm supportive of allowing Stop to be called twice, though I wonder if we can do it with a simpler implementation, taking advantage of the existing mutex.

This patch changes behaviour of Stop slightly. Previously, if Stop was called before initialization, it was a no-op, and calls after it was initialized would actually stop it. Now, the first Stop will result in the sync.Once being completed, and further calls won't have any impact (nothing is stopped). Checking for a stopped bool under the mutex may be a simpler alternative that avoids the behaviour change.

[zchee]

@prashantv changed to stopped bool and add local value for avoid call mutex twice.
How about it?

[abhinav]

LGTM. Thanks!

[zchee]

@prashantv Thanks!

[prashantv]

Thanks for the contribution @zchee!