grpclog: change logger to avoid Printf when the logger is io.Discard

[rob05c]

Changes the logger to not call Printf when the logger is io.Discard.

Also changes the logger constructor to use io.Discard instead of io.MultiWriter(io.Discard, io.Discard), which is necessary to detect all levels being discarded.

This is a significant performance improvement for disabled logs, see #7463 for details.

Notes:

• This is similar to how the std does it, see log.go
• I was hoping to use the std log.Printf itself, but I don't think that's possible, because the JSON option needs to Printf and then escape the result after interpolating, which I don't think is possible with what's exposed by the std (possibly sans some overly complex io.Writer interceptors).
• It could use log.Printf for the non-JSON, but there's not much point if we can't use it for the JSON, and it would incur an unnecessary atomic call.
• The constructor io.Discard logic could be made faster (e.g. with more nested if's), and could do smarter things around the discard/multiwriter. I wrote it this way for readability, and because I don't think the performance is significant. But I'm happy to change it.

Fixes #7463

RELEASE NOTES:

• grpclog: Avoid expensive printf calls for io.Discard loggers.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: rob05c / name: Robert O Butts (2bf7f34)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.83%. Comparing base (31ffeee) to head (2bf7f34).
Report is 71 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7471      +/-   ##
==========================================
+ Coverage   81.73%   81.83%   +0.10%     
==========================================
  Files         361      361              
  Lines       27817    27833      +16     
==========================================
+ Hits        22735    22776      +41     
+ Misses       3871     3850      -21     
+ Partials     1211     1207       -4     

Files with missing lines	Coverage Δ
grpclog/internal/loggerv2.go	100.00% <100.00%> (+48.00%)	⬆️

... and 14 files with indirect coverage changes

[purnesh42H]

Adding @dfawley as 2nd reviewer

[rob05c]

Ok, I think I addressed all the comments again. Let me know if I missed anything.

Also, the PR started failing a new Github check for RELEASE NOTES in the description, so I attempted to add it like I see in other PRs. I'm happy to change it if it isn't right, just let me know

[purnesh42H]

lgtm

[rob05c]

Is there anything else I need to do for this to be merged?

[purnesh42H]

Doug is reviewing it. Nothing is needed from you right now.

[dfawley]

Is there anything else I need to do for this to be merged?

Apologies; I have a lot on my plate right now, but will try to get to this in the next week or so.

[dfawley]

Thanks for the PR and sorry again for the delay.

[dfawley]

The closure here seems unfortunate, and I think this approach has performance impacts for non-Discard use cases (creating the closure and the call to concatenate the strings inside if !g.jsonFormat).

Can we keep output and add an outputf that duplicates the io.Discard check, and avoids the closure and different formatting?

[dfawley]

I think we could optimize this a bit, and make it easier to read as well?

// Returns a combined logger for both higher & lower severity logs, or higher only if lower is io.Discard.
func combineLoggers(lower, higher io.Writer) io.Writer {
	if lower == io.Discard || higher == io.Discard {
		return higher
	}
	return io.MultiWriter(lower, higher)		
}

func NewLoggerV2(..) {
	// ...

	// Just overwrite instead of creating new locals with different names:
	warningW = combineLoggers(infoW, warningW)
	errorW = combineLoggers(warningW, errorW)
}

This ends up with 2 nested multiwriters if you have 3 different loggers, but that's probably fairly uncommon, and would be no worse than the behavior this PR adds of creating a multiwriter for (warningW, io.Discard) if infoW is io.Discard.

[github-actions]

This PR is labeled as requiring an update from the reporter, and no update has been received after 6 days. If no update is provided in the next 7 days, this issue will be automatically closed.

[rob05c]

This PR is labeled as requiring an update from the reporter, and no update has been received after 6 days. If no update is provided in the next 7 days, this issue will be automatically closed.

Sorry, I was on vacation last week. I'll try to address the comments sometime this week