child_process: allow buffer encoding in spawnSync

When the 'buffer' encoding is passed to spawnSync(), an exception
is thrown in Buffer's toString() method because 'buffer' is not
a valid encoding there. This commit special cases the 'buffer'
encoding.

Fixes: https://github.com/nodejs/node/issues/6930
PR-URL: https://github.com/nodejs/node/pull/6939
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

lib/child_process.js

@@ -438,7 +438,7 @@ function spawnSync(/*file, args, options*/) {
 
   var result = spawn_sync.spawn(options);
 
-  if (result.output && options.encoding) {
+  if (result.output && options.encoding && options.encoding !== 'buffer') {
     for (i = 0; i < result.output.length; i++) {
       if (!result.output[i])
         continue;

test/common.js

@@ -241,6 +241,17 @@ exports.spawnPwd = function(options) {
   }
 };
 
+
+exports.spawnSyncPwd = function(options) {
+  const spawnSync = require('child_process').spawnSync;
+
+  if (exports.isWindows) {
+    return spawnSync('cmd.exe', ['/c', 'cd'], options);
+  } else {
+    return spawnSync('pwd', [], options);
+  }
+};
+
 exports.platformTimeout = function(ms) {
   if (process.config.target_defaults.default_configuration === 'Debug')
     ms = 2 * ms;

test/parallel/test-child-process-spawnsync.js

@@ -33,3 +33,17 @@ assert.deepStrictEqual(ret_err.spawnargs, ['bar']);
 
   assert.strictEqual(response.stdout.toString().trim(), cwd);
 })();
+
+{
+  // Test the encoding option
+  const noEncoding = common.spawnSyncPwd();
+  const bufferEncoding = common.spawnSyncPwd({encoding: 'buffer'});
+  const utf8Encoding = common.spawnSyncPwd({encoding: 'utf8'});
+
+  assert.deepStrictEqual(noEncoding.output, bufferEncoding.output);
+  assert.deepStrictEqual([
+    null,
+    noEncoding.stdout.toString(),
+    noEncoding.stderr.toString()
+  ], utf8Encoding.output);
+}