fix: include escape codes in byte counts from Fprint, Fprintf

[qualidafial]

Fix the n byte count returned from Color.Fprint and Color.Fprintf to include the bytes in ANSI escape sequences.

Both methods delegate to SetWriter and UnsetWriter methods to start and stop the excape sequence. Since both these methods are public and do not return the byte counts needed, I extracted private versions of these methods setWriter and unsetWriter, and redirected the Fprint and Fprintf to get accurate byte counts without breaking API compatibility.

Finally, unsetWriter() is no longer deferred. If we encounter an error at any point, stop writing and return the error.

[qualidafial]

Ping. Is there any interest in this PR? I came across this bug while trying to migrate some code to FPrint so we can use io.Writer in tests instead of stdout.

[fatih]

@qualidafial thanks for the test and verification. This looks something we can merge. Did you encountered any issue before? If yes what was the use case you saw ?

[qualidafial]

We use color to format test results. We were trying to implement our own result.Fprint(w io.Writer) (n int, err error) methods, but were finding in tests that the returned byte count n didn't match the number of bytes actually written tow. The culprit turned out to be Color.Fprint(io.Writer) and Color.Fprintf(io.Writer) neglecting ANSI escape codes in the byte count totals.

As a workaround, I wrapped the passed-in writer on our side in a counting writer implementation, rather than relying on the byte counts from color:

type countingWriter struct {
	writer io.Writer
	N      int
}

func (w *countingWriter) Write(p []byte) (int, error) {
	n, err := w.writer.Write(p)
	w.N += n
	return n, err
}

[fatih]

Thank you for the fix @qualidafial.

[qualidafial]

My pleasure! Thanks @fatih for sharing and maintaining this library!