os.Pipe not working as expected with log package

Question

I have a piece of code which sets up an os.Pipe to capture Stdout/Stderr:

https://github.com/sevagh/stdcap/blob/master/stdcap.go

// Capture executes f() and returns the captured data
func (s *stdcap) Capture(f func()) string {
    s.mu.Lock()
    defer s.mu.Unlock()

    var old, r, w *os.File

    if s.out {
        old = os.Stdout
        r, w, _ = os.Pipe()
        os.Stdout = w
    } else {
        old = os.Stderr
        r, w, _ = os.Pipe()
        os.Stderr = w
    }

    f()

    outC := make(chan string)
    defer close(outC)

    go func() {
        var buf bytes.Buffer
        io.Copy(&buf, r)
        outC <- buf.String()
    }()

    w.Close()

    if s.out {
        os.Stdout = old
    } else {
        os.Stderr = old
    }

    return <-outC
}

Today I tried using this code with the log package and it doesn't work.

This works:

func TestOutCapture(t *testing.T) {
    sc := StdoutCapture()
    out := sc.Capture(func() {
        fmt.Printf("Hello world!")
    })

    if out != "Hello world!" {
        t.Errorf("Expected \"Hello world!\", got: %s\n", out)
    }
}

func TestErrCapture(t *testing.T) {
    sc := StderrCapture()
    err := sc.Capture(func() {
        fmt.Fprintf(os.Stderr, "Hello world!")
    })

    if err != "Hello world!" {
        t.Errorf("Expected \"Hello world!\", got: %s\n", err)
    }
}

These don't work:

func TestLogOutCapture(t *testing.T) {
    sc := StdoutCapture()

    log.SetOutput(os.Stdout)
    out := sc.Capture(func() {
        log.Printf("Hello world!")
    })

    if out != "Hello world!" {
        t.Errorf("Expected \"Hello world!\", got: %s\n", out)
    }
}

func TestLogErrCapture(t *testing.T) {
    sc := StderrCapture()
    log.SetOutput(os.Stderr)

    err := sc.Capture(func() {
        log.Printf("Hello world!")
    })

    if err != "Hello world!" {
        t.Errorf("Expected \"Hello world!\", got: %s\n", err)
    }
}

Any ideas on where I can debug this? Does the Golang log package not use os.Stdout/os.Stderr?

Answer 1

The logging package initializes the standard logger with the value os.Stderr at initialization time. Modifications to the variable os.Stderr do not change the value in the logger field.

Call log.SetOutput to change the output location for the standard logger from your capture function. Unfortunately, there's not a way to get standard logger's output so you can save and restore it in your capture function.