Write to temp outfile until success, fixes #899

Write browserify command output to a temporary file, overwriting the target file atomically upon success (and simply cleaning up the tempfile on error).

This has two benefits:

* user won't be left with an empty file if compilation fails (which can break toolchains that rely on destination files/timestamps to indicate build status, Make or django-pipeline for just two examples)
* output file can never end up in a half-written state during even a successful compilation

bin/cmd.js

@@ -54,16 +54,23 @@ if (b.argv.list) {
 
 var bundle = b.bundle();
 bundle.on('error', errorExit);
+bundle.on('end', successExit);
 
+var tmpfile;
 var outfile = b.argv.o || b.argv.outfile;
 if (outfile) {
-    bundle.pipe(fs.createWriteStream(outfile));
+    // we'll output to a temp file within same filesystem, then atomically overwrite outfile once successful
+    tmpfile = outfile + ".tmp-browserify-" + Math.random().toFixed(20).slice(2)
+    bundle.pipe(fs.createWriteStream(tmpfile));
 }
 else {
     bundle.pipe(process.stdout);
 }
 
 function errorExit(err) {
+    if (tmpfile) fs.unlink(tmpfile, function (err) {
+      if (err) /* no-op, we're already exiting unhappily… */;
+    });
     if (err.stack) {
         console.error(err.stack);
     }
@@ -72,3 +79,9 @@ function errorExit(err) {
     }
     process.exit(1);
 }
+
+function successExit() {
+  if (tmpfile && outfile) fs.rename(tmpfile, outfile, function (err) {
+    if (err) errorExit(err);
+  });
+}