Add debug logger (#604)

Signed-off-by: David Jimenez <dvejmz@sgfault.com>

logging/logger.go

@@ -0,0 +1,44 @@
+package logging
+
+import (
+	"io"
+	"io/ioutil"
+	"log"
+	"os"
+)
+
+var logger *log.Logger
+
+// GetLogger retrieves an instance of an application logger which outputs
+// to a file if the debug flag is enabled
+func GetLogger() (*log.Logger, error) {
+	if logger != nil {
+		return logger, nil
+	}
+
+	var writer io.Writer
+	var err error
+
+	debugModeEnabled := os.Getenv("DEBUG") == "1"
+	if debugModeEnabled {
+		writer, err = os.Create("revive.log")
+		if err != nil {
+			return nil, err
+		}
+	} else {
+		// Suppress all logging output if debug mode is disabled
+		writer = ioutil.Discard
+	}
+
+	logger = log.New(writer, "", log.LstdFlags)
+
+	if !debugModeEnabled {
+		// Clear all flags to skip log output formatting step to increase
+		// performance somewhat if we're not logging anything
+		logger.SetFlags(0)
+	}
+
+	logger.Println("Logger initialised")
+
+	return logger, nil
+}