module: use 'repl' as the filename arg if missing

Fix #3859

lib/module.js

@@ -437,7 +437,12 @@ Module.prototype._compile = function(content, filename) {
   var compiledWrapper = runInThisContext(wrapper, filename, true);
   if (global.v8debug) {
     if (!resolvedArgv) {
-      resolvedArgv = Module._resolveFilename(process.argv[1], null);
+      // we enter the repl if we're not given a filename argument.
+      if (process.argv[1]) {
+        resolvedArg = Module._resolveFilename(process.argv[1], null);
+      } else {
+        resolvedArg = 'repl';
+      }
     }
 
     // Set breakpoint on module start

test/simple/test-debug-brk-no-arg.js

@@ -0,0 +1,35 @@
+// 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 spawn = require('child_process').spawn;
+
+var child = spawn(process.execPath, ['--debug-brk=' + common.PORT]);
+child.stderr.once('data', function(c) {
+  console.error('%j', c.toString());
+  child.stdin.end();
+});
+
+child.on('exit', function(c) {
+  assert(c === 0);
+  console.log('ok');
+});