child_process: Check stderr before accessing it

If something bad happens in spawnSync, stderr might be null. Therefore,
we have to check it before using it, so we won't mask the actual
exception.
PR-URL: #6877
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Robert Jefe Lindstädt <robert.lindstaedt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

Author: robertchiras

lib/child_process.js

@@ -489,7 +489,7 @@ function execFileSync(/*command, args, options*/) {
 
 varret=spawnSync(opts.file,opts.args.slice(1),opts.options);
 
-if(inheritStderr)
+if(inheritStderr&&ret.stderr)
 process.stderr.write(ret.stderr);
 
 varerr=checkExecSyncError(ret);
@@ -509,7 +509,7 @@ function execSync(command /*, options*/) {
 varret=spawnSync(opts.file,opts.options);
 ret.cmd=command;
 
-if(inheritStderr)
+if(inheritStderr&&ret.stderr)
 process.stderr.write(ret.stderr);
 
 varerr=checkExecSyncError(ret);

test/sequential/test-child-process-execsync.js

@@ -12,6 +12,18 @@ var start = Date.now();
 varerr;
 varcaught=false;
 
+// Verify that stderr is not accessed when a bad shell is used
+assert.throws(
+function(){execSync('exit -1',{shell: 'bad_shell'});},
+/spawnSyncbad_shellENOENT/,
+'execSync did not throw the expected exception!'
+);
+assert.throws(
+function(){execFileSync('exit -1',{shell: 'bad_shell'});},
+/spawnSyncbad_shellENOENT/,
+'execFileSync did not throw the expected exception!'
+);
+
 try{
 varcmd=`"${process.execPath}" -e "setTimeout(function(){}, ${SLEEP});"`;
 varret=execSync(cmd,{timeout: TIMER});