testing: print log output immediately when it can

[rogpeppe]

Diagnosing a test that hangs up is made harder because it's
T.Logf doesn't print any output until the test completes (which might
never happen).

Before a test has called t.Parallel, it would be nice if it could print
any log output immediately.

This would also make it easier to see test Logf output in sequential context with
print statements made by the code being called by the test.

This issue is made worse when logging infrastructure is hooked up
to T.Log so that it doesn't pollute normal test output by printing to
stdout.

[cznic]

But only with -v.

[rogpeppe]

@cznic Absolutely - I'm not proposing any change in whether the log output is shown, just when it appears.

[ALTree]

As noted in #23567 (a duplicate of this), the issue also affects Errorf calls (since Errorf calls Logf).

[nerdralph]

This is also a problem if the test crashes. I noticed this when testing go code with a buggy OpenCL driver. Having the log output flush after each call would make it much easier to find the exact point in the test that failed.

[mvdan]

Sounds like there is no investigation required here - the proposed change is clear. Labelling as NeedsDecision.

/cc @mpvl

[jeanbza]

I'll add my 2c here quickly:

The fact that -v does not show logs means I get no sense of timing in failed test runs. Nor do I see failed tests, as @nerdralph mentioned.

It's a real bummer to mass replace t.Log with fmt.Println, get my tests fixed, and then mass replace fmt.Println with t.Log.

[agnivade]

@ianlancetaylor - Can we get this in for 1.11 ? What are the factors that are yet to be "decided" (because of the needsdecision label) ?

I have got bitten by this several times and would be great not to resort to printing through fmt package.

Also curious regarding why it was initially designed not to print any output until the test has finished. Printing output immediately after a call to t.Log is what most people would think will happen. This is just a very weird counter-intuitive behavior.

[rsc]

The current output is:

r$ go test -run Reader
--- FAIL: TestReader (0.00s)
	reader_test.go:18: bye
FAIL
exit status 1
FAIL	strings	0.105s
r$ 

We can't start printing log output without knowing whether to start with PASS or FAIL. Various things read this output, we can't just reorder it.