test: move getTTYfd() to common helpers

This utility is fairly generic and likely useful for more than one test.

PR-URL: https://github.com/nodejs/node/pull/18800
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

test/common/README.md

@@ -679,6 +679,12 @@ The realpath of the testing temporary directory.
 
 Deletes and recreates the testing temporary directory.
 
+### getTTYfd()
+
+Attempts to get a valid TTY file descriptor. Returns `-1` if it fails.
+
+The TTY file descriptor is assumed to be capable of being writable.
+
 ## WPT Module
 
 The wpt.js module is a port of parts of

test/common/index.js

@@ -775,6 +775,23 @@ exports.crashOnUnhandledRejection = function() {
              (err) => process.nextTick(() => { throw err; }));
 };
 
+exports.getTTYfd = function getTTYfd() {
+  // Do our best to grab a tty fd.
+  const tty = require('tty');
+  // Don't attempt fd 0 as it is not writable on Windows.
+  // Ref: ef2861961c3d9e9ed6972e1e84d969683b25cf95
+  const ttyFd = [1, 2, 4, 5].find(tty.isatty);
+  if (ttyFd === undefined) {
+    try {
+      return fs.openSync('/dev/tty');
+    } catch (e) {
+      // There aren't any tty fd's available to use.
+      return -1;
+    }
+  }
+  return ttyFd;
+};
+
 // Hijack stdout and stderr
 const stdWrite = {};
 function hijackStdWritable(name, listener) {

test/parallel/test-tty-get-color-depth.js

@@ -3,26 +3,9 @@
 const common = require('../common');
 const assert = require('assert').strict;
 /* eslint-disable no-restricted-properties */
-const { openSync } = require('fs');
-const tty = require('tty');
-
 const { WriteStream } = require('tty');
 
-// Do our best to grab a tty fd.
-function getTTYfd() {
-  const ttyFd = [1, 2, 4, 5].find(tty.isatty);
-  if (ttyFd === undefined) {
-    try {
-      return openSync('/dev/tty');
-    } catch (e) {
-      // There aren't any tty fd's available to use.
-      return -1;
-    }
-  }
-  return ttyFd;
-}
-
-const fd = getTTYfd();
+const fd = common.getTTYfd();
 
 // Give up if we did not find a tty
 if (fd === -1)