test: fix test-fs-access when uid is 0

Superusers can open files with W_OK permission even though their
mode is set to 0444. This commit makes the test attempt to change
its uid to nobody on non-Windows platforms. Patch originally from
nodejs/node-v0.x-archive@28d0cbbd.

Fixes: #1031
PR-URL: #1037
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/test-fs-access.js

@@ -4,6 +4,7 @@ var fs = require('fs');
 var path = require('path');
 var doesNotExist = __filename + '__this_should_not_exist';
 var readOnlyFile = path.join(common.tmpDir, 'read_only_file');
+var readWriteFile = path.join(common.tmpDir, 'read_write_file');
 
 var removeFile = function(file) {
   try {
@@ -13,13 +14,47 @@ var removeFile = function(file) {
   }
 };
 
-var createReadOnlyFile = function(file) {
+var createFileWithPerms = function(file, mode) {
   removeFile(file);
   fs.writeFileSync(file, '');
-  fs.chmodSync(file, 0444);
+  fs.chmodSync(file, mode);
 };
 
-createReadOnlyFile(readOnlyFile);
+createFileWithPerms(readOnlyFile, 0444);
+createFileWithPerms(readWriteFile, 0666);
+
+/*
+ * On non-Windows supported platforms, fs.access(readOnlyFile, W_OK, ...)
+ * always succeeds if node runs as the super user, which is sometimes the
+ * case for tests running on our continuous testing platform agents.
+ *
+ * In this case, this test tries to change its process user id to a
+ * non-superuser user so that the test that checks for write access to a
+ * read-only file can be more meaningful.
+ *
+ * The change of user id is done after creating the fixtures files for the same
+ * reason: the test may be run as the superuser within a directory in which
+ * only the superuser can create files, and thus it may need superuser
+ * priviledges to create them.
+ *
+ * There's not really any point in resetting the process' user id to 0 after
+ * changing it to 'nobody', since in the case that the test runs without
+ * superuser priviledge, it is not possible to change its process user id to
+ * superuser.
+ *
+ * It can prevent the test from removing files created before the change of user
+ * id, but that's fine. In this case, it is the responsability of the
+ * continuous integration platform to take care of that.
+ */
+var hasWriteAccessForReadonlyFile = false;
+if (process.platform !== 'win32' && process.getuid() === 0) {
+  hasWriteAccessForReadonlyFile = true;
+  try {
+    process.setuid('nobody');
+    hasWriteAccessForReadonlyFile = false;
+  } catch (err) {
+  }
+}
 
 assert(typeof fs.F_OK === 'number');
 assert(typeof fs.R_OK === 'number');
@@ -45,8 +80,12 @@ fs.access(readOnlyFile, fs.F_OK | fs.R_OK, function(err) {
 });
 
 fs.access(readOnlyFile, fs.W_OK, function(err) {
-  assert.notEqual(err, null, 'error should exist');
-  assert.strictEqual(err.path, readOnlyFile);
+  if (hasWriteAccessForReadonlyFile) {
+    assert.equal(err, null, 'error should not exist');
+  } else {
+    assert.notEqual(err, null, 'error should exist');
+    assert.strictEqual(err.path, readOnlyFile);
+  }
 });
 
 assert.throws(function() {
@@ -68,7 +107,7 @@ assert.doesNotThrow(function() {
 assert.doesNotThrow(function() {
   var mode = fs.F_OK | fs.R_OK | fs.W_OK;
 
-  fs.accessSync(__filename, mode);
+  fs.accessSync(readWriteFile, mode);
 });
 
 assert.throws(function() {
@@ -79,4 +118,5 @@ assert.throws(function() {
 
 process.on('exit', function() {
   removeFile(readOnlyFile);
+  removeFile(readWriteFile);
 });