proposal: testing: add log.Logger adapter to testing.T

[SamWhited]

When working on applications I often find myself copy pasting some boilerplate similar to the following into my tests:

func testLogger(t *testing.T) *log.Logger {
	return log.New(testWriter{t}, "test", log.LstdFlags)
}

type testWriter struct {
	t *testing.T
}

func (tw testWriter) Write(p []byte) (n int, err error) {
	tw.t.Log(string(p))
	return len(p), nil
}

then when I test functions in my application I can pass in the testing logger and not end up with lots of logging unless the tests failed (in which case the logs show up in a more-or-less sane way and I only see logs generated by the specific tests that failed).

I would like to propose adding this to the testing package as a method on T:

// Logger returns a new logger that logs output using c.Print.
func (c *T) Logger() *log.Logger {}

Alternatively, making *T a writer would require no new dependencies and be more generic but would require a bit more boiler plate for the test writer:

// Write satisfies the io.Writer interface for T that "writes" to t.Print.
// Write always returns len(p), nil.
func (c *T) Write(p []byte) (n int, err error) {}

If accepted, this adds one method to type T which would need to be covered under the compatibility promise.

[rsc]

Does someone have a list of libraries etc that take a *log.Logger? It would be more compelling if there were many such packages that this would help.

As an aside, it would be nice if testing.T and log.Logger both implemented some common interface that code could be written to use, so that you could pass t instead of a logger and not need this adapter. But testing.T calls the logging Logf and log.Logger calls it Printf. (They both agree on a Fatalf.)

[SamWhited]

Does someone have a list of libraries etc that take a *log.Logger? It would be more compelling if there were many such packages that this would help.

I am not very skilled at BigTable but maybe someone with more monthly minutes than I could run something over the GitHub dataset to find out (or come up with a query that I can run once). I know @campoy isn't on the team anymore, but I remember him being good at this, maybe he'd be willing to advise on the best way to do this and I can try to run the query?

In the standard library there are at least:

• "net/http".Server.ErrorLogger
• "net/http/httputil".ReverseProxy.ErrorLogger
• "net/http/cgi".Handler.Logger

As an aside, it would be nice if testing.T and log.Logger both implemented some common interface that code could be written to use, so that you could pass t instead of a logger and not need this adapter. But testing.T calls the logging Logf and log.Logger calls it Printf. (They both agree on a Fatalf.)

Agreed. Having testing.T also have a Printf method might not be the end of the world, although it does feel redundant even if we were to say that the old one is deprecated. This wouldn't work for the cases in the standard library that already take a log.Logger though.

[rsc]

Adding a new dependency into testing is a big step, one that we're not ready for. In particular there are a few things about log.Logger that we'd change if we could, and maybe one would be arranging that it had a useful method in common with testing.T, so that an interface would be usable here. Maybe there should even be an io.Logfer or something like that, but there is not today (and as noted above testing.T and log.Logger have no useful methods in common).

[SamWhited]

What about making testing.T an io.Writer which would allow it to be passed in when constructing a logger?

[ags799]

Hey @SamWhited 😊. I like your idea about an io.Writer for testing.T. For example, when I test command-line applications, I would like to be able to set os.exec.Cmd.Stdout to a test logger.

[phinicota]

@SamWhited @ags799 just came in looking exactly for that!

edit: this might help: https://golang.org/pkg/testing/iotest/#NewWriteLogger

[gopherbot]

Change https://golang.org/cl/149379 mentions this issue: testing: add Write method to T and B