node: implement DEBUG_FD env variable support

Closes #146.

node.js

@@ -25,14 +25,26 @@ exports.useColors = useColors;
 
 exports.colors = [6, 2, 3, 4, 5, 1];
 
+/**
+ * The file descriptor to write the `debug()` calls to.
+ * Set the `DEBUG_FD` env variable to override with another value. i.e.:
+ *
+ *   $ DEBUG_FD=3 node script.js 3>debug.log
+ */
+
+var fd = parseInt(process.env.DEBUG_FD, 10) || 2;
+var stream = 1 === fd ? process.stdout :
+             2 === fd ? process.stderr :
+             createWritableStdioStream(fd);
+
 /**
  * Is stdout a TTY? Colored output is enabled when `true`.
  */
 
 function useColors() {
   var debugColors = (process.env.DEBUG_COLORS || '').trim().toLowerCase();
   if (0 === debugColors.length) {
-    return tty.isatty(1);
+    return tty.isatty(fd);
   } else {
     return '0' !== debugColors
         && 'no' !== debugColors
@@ -91,7 +103,7 @@ function formatArgs() {
  */
 
 function log() {
-  return console.error.apply(console, arguments);
+  return stream.write(util.format.apply(this, arguments) + '\n');
 }
 
 /**
@@ -122,6 +134,74 @@ function load() {
   return process.env.DEBUG;
 }
 
+/**
+ * Copied from `node/src/node.js`.
+ *
+ * XXX: It's lame that node doesn't expose this API out-of-the-box. It also
+ * relies on the undocumented `tty_wrap.guessHandleType()` which is also lame.
+ */
+
+function createWritableStdioStream (fd) {
+  var stream;
+  var tty_wrap = process.binding('tty_wrap');
+
+  // Note stream._type is used for test-module-load-list.js
+
+  switch (tty_wrap.guessHandleType(fd)) {
+    case 'TTY':
+      stream = new tty.WriteStream(fd);
+      stream._type = 'tty';
+
+      // Hack to have stream not keep the event loop alive.
+      // See https://github.com/joyent/node/issues/1726
+      if (stream._handle && stream._handle.unref) {
+        stream._handle.unref();
+      }
+      break;
+
+    case 'FILE':
+      var fs = require('fs');
+      stream = new fs.SyncWriteStream(fd, { autoClose: false });
+      stream._type = 'fs';
+      break;
+
+    case 'PIPE':
+    case 'TCP':
+      var net = require('net');
+      stream = new net.Socket({
+        fd: fd,
+        readable: false,
+        writable: true
+      });
+
+      // FIXME Should probably have an option in net.Socket to create a
+      // stream from an existing fd which is writable only. But for now
+      // we'll just add this hack and set the `readable` member to false.
+      // Test: ./node test/fixtures/echo.js < /etc/passwd
+      stream.readable = false;
+      stream.read = null;
+      stream._type = 'pipe';
+
+      // FIXME Hack to have stream not keep the event loop alive.
+      // See https://github.com/joyent/node/issues/1726
+      if (stream._handle && stream._handle.unref) {
+        stream._handle.unref();
+      }
+      break;
+
+    default:
+      // Probably an error on in uv_guess_handle()
+      throw new Error('Implement me. Unknown stream file type!');
+  }
+
+  // For supporting legacy API we put the FD here.
+  stream.fd = fd;
+
+  stream._isStdio = true;
+
+  return stream;
+}
+
 /**
  * Enable namespaces listed in `process.env.DEBUG` initially.
  */