debug.go

package main

import (
	"fmt"
	"os"
	"sync"
)

// Diagnostic logging. Off by default so end users get a clean console. Enable
// with the -debug flag; output then goes to debug.log (gitignored via *.log),
// never to the console.

var (
	debugEnabled bool
	debugFile    *os.File
	debugOnce    sync.Once
	debugMu      sync.Mutex
)

// debugf writes a diagnostic line to debug.log when -debug is active; otherwise
// it does nothing.
func debugf(format string, a ...interface{}) {
	if !debugEnabled {
		return
	}
	debugOnce.Do(func() {
		f, err := os.OpenFile("debug.log", os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
		if err == nil {
			debugFile = f
		}
	})
	if debugFile == nil {
		return
	}
	debugMu.Lock()
	fmt.Fprintf(debugFile, format, a...)
	debugMu.Unlock()
}