CaptureLogs.go

var logLines []string

type logMock struct{}

func (l *logMock) Write(p []byte) (int, error) {
	logLines = append(logLines, string(p))
	return 0, nil
}

//CaptureLogOutput A utility to capture the log output of a function and pass it along
func CaptureLogOutput(toBeCaptured func(), handleResults func([]string)) {
	logLines = []string{}
	currentFlags := log.Flags()
	log.SetFlags(0)
	log.SetOutput(&logMock{})
	toBeCaptured()
	log.SetOutput(os.Stderr)
	log.SetFlags(currentFlags)
	handleResults(logLines)
}

It would also be handy if the return value of CaptureLogOutput were the return value of the func that it encloses. That would allow you to do things like:

got := CaptureLogOutput(func() rune {
    log.Print("about to return something")
    return "☃"
  },
  func(logs []string) {
    // assert something about what got logged
  })

// assert something about the return value "☃"

This may be a can of worms since the return value of CaptureLogOutput then becomes subject to the whims of the code that's being wrapped. So maybe another callback?

CaptureLogOutput(func() rune {
    log.Print("about to return something")
    return "☃"
  },
  func(logs []string) {
    // assert something about what got logged
  },
  func(frosty rune) {
    // assert frosty is "☃"
  })

Making the callback to test the return value of the wrapped function be the third argument to CaptureLogOutput feels sensible since not all code run in CaptureLogOutput will actually have a return value, so that avoids needing to pass in an empty function as a second argument.

Great idea, all ideas implemented: https://github.com/MediaMath/auth_go_common_libs

See for examples: https://github.com/MediaMath/auth_go_common_libs/blob/master/testhelpers/testhelpers_test.go