Enhanced fatal calls so exit handlers can be invoked

While GO offers the ability to recover from panic there is no way to intercept an os.Exit event. To allow graceful shutdown and clean-up or programs which use Logrus to Fatal out I've borrowed ideas from the `atexit` package and enhanced Logrus.

Usage:
* When setting up the logger one call `RegisterExitHandler( func() {...} )` to add a handler that will be invoked for any `Fatal` call to the logger.

alt_exit.go

@@ -0,0 +1,60 @@
+package logrus
+
+// The following code was sourced and modified from the
+// https://bitbucket.org/tebeka/atexit package governed by the following license:
+//
+// Copyright (c) 2012 Miki Tebeka <miki.tebeka@gmail.com>.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy of
+// this software and associated documentation files (the "Software"), to deal in
+// the Software without restriction, including without limitation the rights to
+// use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+// the Software, and to permit persons to whom the Software is furnished to do so,
+// subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+// FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+// COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+// IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+// CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+import (
+"fmt"
+"os"
+)
+
+var handlers = []func(){}
+
+func runHandler(handler func()) {
+defer func() {
+if err := recover(); err != nil {
+fmt.Fprintln(os.Stderr, "Error: Logrus exit handler error:", err)
+}
+}()
+
+handler()
+}
+
+func runHandlers() {
+for _, handler := range handlers {
+runHandler(handler)
+}
+}
+
+// Exit runs all the Logrus atexit handlers and then terminates the program using os.Exit(code)
+func Exit(code int) {
+runHandlers()
+os.Exit(code)
+}
+
+// RegisterExitHandler adds a Logrus atexit handler, call logrus.Exit to invoke
+// all handlers. The handlers will also be invoked when any Fatal log entry is
+// made.
+func RegisterExitHandler(handler func()) {
+handlers = append(handlers, handler)
+}
+

alt_exit_test.go

@@ -0,0 +1,75 @@
+package logrus
+
+import (
+"os/exec"
+"testing"
+"io/ioutil"
+"time"
+)
+
+func TestRegister(t *testing.T) {
+current := len(handlers)
+RegisterExitHandler(func() {})
+if len(handlers) != current+1 {
+t.Fatalf("can't add handler")
+}
+}
+
+func TestHandler(t *testing.T) {
+gofile := "/tmp/testprog.go"
+if err := ioutil.WriteFile(gofile, testprog, 0666); err != nil {
+t.Fatalf("can't create go file")
+}
+
+outfile := "/tmp/testprog.out"
+arg := time.Now().UTC().String()
+err := exec.Command("go", "run", gofile, outfile, arg).Run()
+if err == nil {
+t.Fatalf("completed normally, should have failed")
+}
+
+data, err := ioutil.ReadFile(outfile)
+if err != nil {
+t.Fatalf("can't read output file %s", outfile)
+}
+
+if string(data) != arg {
+t.Fatalf("bad data")
+}
+}
+
+var testprog = []byte(`
+// Test program for atexit, gets output file and data as arguments and writes
+// data to output file in atexit handler.
+package main
+
+import (
+	"github.com/powerchordinc/logrus"
+	"flag"
+	"fmt"
+	"io/ioutil"
+)
+
+var outfile = ""
+var data = ""
+
+func handler() {
+	ioutil.WriteFile(outfile, []byte(data), 0666)
+}
+
+func badHandler() {
+	n := 0
+	fmt.Println(1/n)
+}
+
+func main() {
+	flag.Parse()
+	outfile = flag.Arg(0)
+	data = flag.Arg(1)
+
+	logrus.RegisterExitHandler(handler)
+	logrus.RegisterExitHandler(badHandler)
+	logrus.Fatal("Bye bye")
+}
+`)
+

entry.go

@@ -150,7 +150,7 @@ func (entry *Entry) Fatal(args ...interface{}) {
 	if entry.Logger.Level >= FatalLevel {
 		entry.log(FatalLevel, fmt.Sprint(args...))
 	}
-	os.Exit(1)
+	Exit(1)
 }
 
 func (entry *Entry) Panic(args ...interface{}) {
@@ -198,7 +198,7 @@ func (entry *Entry) Fatalf(format string, args ...interface{}) {
 	if entry.Logger.Level >= FatalLevel {
 		entry.Fatal(fmt.Sprintf(format, args...))
 	}
-	os.Exit(1)
+	Exit(1)
 }
 
 func (entry *Entry) Panicf(format string, args ...interface{}) {
@@ -245,7 +245,7 @@ func (entry *Entry) Fatalln(args ...interface{}) {
 	if entry.Logger.Level >= FatalLevel {
 		entry.Fatal(entry.sprintlnn(args...))
 	}
-	os.Exit(1)
+	Exit(1)
 }
 
 func (entry *Entry) Panicln(args ...interface{}) {

logger.go

@@ -108,7 +108,7 @@ func (logger *Logger) Fatalf(format string, args ...interface{}) {
 	if logger.Level >= FatalLevel {
 		NewEntry(logger).Fatalf(format, args...)
 	}
-	os.Exit(1)
+	Exit(1)
 }
 
 func (logger *Logger) Panicf(format string, args ...interface{}) {
@@ -155,7 +155,7 @@ func (logger *Logger) Fatal(args ...interface{}) {
 	if logger.Level >= FatalLevel {
 		NewEntry(logger).Fatal(args...)
 	}
-	os.Exit(1)
+	Exit(1)
 }
 
 func (logger *Logger) Panic(args ...interface{}) {
@@ -202,7 +202,7 @@ func (logger *Logger) Fatalln(args ...interface{}) {
 	if logger.Level >= FatalLevel {
 		NewEntry(logger).Fatalln(args...)
 	}
-	os.Exit(1)
+	Exit(1)
 }
 
 func (logger *Logger) Panicln(args ...interface{}) {