fs: add long stacktrace debugging facility

Enable long stacktraces if NODE_DEBUG=fs is set in the environment. Only
applies to the default rethrow callback; it's to help you find places where
you forgot to pass in a callback.

lib/fs.js

@@ -52,20 +52,39 @@ var O_WRONLY = constants.O_WRONLY || 0;
 
 var isWindows = process.platform === 'win32';
 
-function rethrow(err) {
-  if (err) throw err;
+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.
+  if (DEBUG) {
+    var backtrace = new Error;
+    return function(err) {
+      if (err) {
+        backtrace.message = err.message;
+        err = backtrace;
+        throw err;
+      }
+    };
+  }
+
+  return function(err) {
+    if (err) {
+      throw err;  // Forgot a callback but don't know where? Use NODE_DEBUG=fs
+    }
+  };
 }
 
 function maybeCallback(cb) {
-  return typeof cb === 'function' ? cb : rethrow;
+  return typeof cb === 'function' ? cb : rethrow();
 }
 
 // Ensure that callbacks run in the global context. Only use this function
 // for callbacks that are passed to the binding layer, callbacks that are
 // invoked from JS already run in the proper scope.
 function makeCallback(cb) {
   if (typeof cb !== 'function') {
-    return rethrow;
+    return rethrow();
   }
 
   return function() {

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

@@ -0,0 +1,22 @@
+// Copyright Joyent, Inc. and other Node contributors.
+//
+// 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.
+
+require('fs').readFile('/');  // throws EISDIR

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

@@ -0,0 +1,55 @@
+// Copyright Joyent, Inc. and other Node contributors.
+//
+// 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.
+
+var common = require('../common');
+var assert = require('assert');
+var exec = require('child_process').exec;
+var path = require('path');
+
+var callbacks = 0;
+
+function test(env, cb) {
+  var filename = path.join(common.fixturesDir, 'test-fs-readfile-error.js');
+  var execPath = process.execPath + ' ' + filename;
+  var options = { env: env || {} };
+  exec(execPath, options, function(err, stdout, stderr) {
+    assert(err);
+    assert.equal(stdout, '');
+    assert.notEqual(stderr, '');
+    cb('' + stderr);
+  });
+}
+
+test({ NODE_DEBUG: '' }, function(data) {
+  assert(/EISDIR/.test(data));
+  assert(!/test-fs-readfile-error/.test(data));
+  callbacks++;
+});
+
+test({ NODE_DEBUG: 'fs' }, function(data) {
+  assert(/EISDIR/.test(data));
+  assert(/test-fs-readfile-error/.test(data));
+  callbacks++;
+});
+
+process.on('exit', function() {
+  assert.equal(callbacks, 2);
+});