Clean up some of the log path detection code

/cc @markpeek I think this is much cleaner. Also, I made it so it
doesn't append by default... I like the idea of appending but it scares
me that for a bug report someone might put like a multi-megabyte gist of
logs at me. HAHA. We'll see.

CHANGELOG.md

@@ -10,6 +10,8 @@ FEATURES:
   floppy disks when booting. This allows for unattended Windows installs.
 * `packer build` has a new `-force` flag that forces the removal of
   existing artifacts if they exist. [GH-173]
+* You can now log to a file (instead of just stderr) by setting the
+  `PACKER_LOG_FILE` environmental variable. [GH-168]
 
 IMPROVEMENTS:
 

packer.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"github.com/mitchellh/packer/packer"
 	"github.com/mitchellh/packer/packer/plugin"
+	"io"
 	"io/ioutil"
 	"log"
 	"os"
@@ -16,24 +17,25 @@ import (
 func main() {
 	// Setup logging if PACKER_LOG is set.
 	// Log to PACKER_LOG_PATH if it is set, otherwise default to stderr.
-	if os.Getenv("PACKER_LOG") == "" {
-		// If we don't have logging explicitly enabled, then disable it
-		log.SetOutput(ioutil.Discard)
-	} else {
-		if log_path := os.Getenv("PACKER_LOG_PATH"); log_path == "" {
-			log.SetOutput(os.Stderr)
-		} else {
-			file, err := os.OpenFile(log_path, os.O_WRONLY|os.O_CREATE|os.O_APPEND, 0600)
-			if err == nil {
-				log.SetOutput(file)
-			} else {
-				// Problem opening the file, fail back to Stderr
-				log.SetOutput(os.Stderr)
-				log.Printf("Could not open %s for logging (%s). Using stderr instead.", log_path, err.Error())
+	var logOutput io.Writer = ioutil.Discard
+	if os.Getenv("PACKER_LOG") != "" {
+		logOutput = os.Stderr
+
+		if logPath := os.Getenv("PACKER_LOG_PATH"); logPath != "" {
+			var err error
+			logOutput, err = os.Create(logPath)
+			if err != nil {
+				fmt.Fprintf(
+					os.Stderr,
+					"Couldn't open '%s' for logging: %s",
+					logPath, err)
+				os.Exit(1)
 			}
 		}
 	}
 
+	log.SetOutput(logOutput)
+
 	// If there is no explicit number of Go threads to use, then set it
 	if os.Getenv("GOMAXPROCS") == "" {
 		runtime.GOMAXPROCS(runtime.NumCPU())