Add debug logger (#604)

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

.gitignore

@@ -3,3 +3,4 @@ revive
 vendor
 *.swp
 dist/
+*.log

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
+}

main.go

@@ -13,6 +13,7 @@ import (
 	"github.com/mgechev/dots"
 	"github.com/mgechev/revive/config"
 	"github.com/mgechev/revive/lint"
+	"github.com/mgechev/revive/logging"
 	"github.com/mitchellh/go-homedir"
 )
 
@@ -29,6 +30,11 @@ func fail(err string) {
 }
 
 func main() {
+	log, err := logging.GetLogger()
+	if err != nil {
+		fail(err.Error())
+	}
+
 	conf, err := config.GetConfig(configPath)
 	if err != nil {
 		fail(err.Error())
@@ -60,6 +66,8 @@ func main() {
 		fail(err.Error())
 	}
 
+	log.Println("Config loaded")
+
 	failures, err := revive.Lint(packages, lintingRules, *conf)
 	if err != nil {
 		fail(err.Error())