fs: Missing cb errors are deprecated, not a throw

Commit a804347 makes fs function rethrow errors when the callback is
omitted. While the right thing to do, it's a change from the old v0.8
behavior where such errors were silently ignored.

To give users time to upgrade, temporarily disable that and replace it
with a function that warns once about the deprecated behavior.

Close #5005

lib/fs.js

@@ -59,22 +59,36 @@ var DEBUG = process.env.NODE_DEBUG && /fs/.test(process.env.NODE_DEBUG);
 function rethrow() {
   // Only enable in debug mode. A backtrace uses ~1000 bytes of heap space and
   // is fairly slow to generate.
+  var callback;
   if (DEBUG) {
     var backtrace = new Error;
-    return function(err) {
+    callback = debugCallback;
+  } else
+    callback = missingCallback;
+
+  return callback;
+
+  function debugCallback(err) {
     if (err) {
       backtrace.message = err.message;
       err = backtrace;
-        throw err;
+      missingCallback(err);
     }
-    };
   }
 
-  return function(err) {
+  function missingCallback(err) {
     if (err) {
+      if (process.throwDeprecation)
         throw err;  // Forgot a callback but don't know where? Use NODE_DEBUG=fs
+      else if (!process.noDeprecation) {
+        var msg = 'fs: missing callback ' + (err.stack || err.message);
+        if (process.traceDeprecation)
+          console.trace(msg);
+        else
+          console.error(msg);
+      }
+    }
   }
-  };
 }
 
 function maybeCallback(cb) {

test/simple/test-fs-readfile-error.js

@@ -28,7 +28,7 @@ var callbacks = 0;
 
 function test(env, cb) {
   var filename = path.join(common.fixturesDir, 'test-fs-readfile-error.js');
-  var execPath = process.execPath + ' ' + filename;
+  var execPath = process.execPath + ' --throw-deprecation ' + filename;
   var options = { env: env || {} };
   exec(execPath, options, function(err, stdout, stderr) {
     assert(err);
@@ -53,3 +53,12 @@ test({ NODE_DEBUG: 'fs' }, function(data) {
 process.on('exit', function() {
   assert.equal(callbacks, 2);
 });
+
+(function() {
+  console.error('the warnings are normal here.');
+  // just make sure that this doesn't crash the process.
+  var fs = require('fs');
+  fs.readFile(__dirname);
+  fs.readdir(__filename);
+  fs.unlink('gee-i-sure-hope-this-file-isnt-important-or-existing');
+})();