Exception `-[NSConcreteFileHandle readDataOfLength:]: Bad file descriptor` when running a NSUserUnixTask - objective-c

Question

I'm using NSUserUnixTask to run an unsandboxed NSTask on my sandboxed app. However, my code hangs on calls to [NSFileHandle readDataToEndOfFile]. If I remove those calls it works perfectly. If I replace [NSFileHandle readDataToEndOfFile] with [NSFileHandle availableData] it hangs as well.
Here is the code:
NSUserUnixTask* unixTask = [[NSUserUnixTask alloc] initWithURL: [NSURL fileURLWithPath: path] error: nil];
// Create error file handle
NSFileHandle* errorFH = [NSFileHandle fileHandleWithStandardError];
[unixTask setStandardError: errorFH];
// Create output file handle
NSFileHandle* outputFH = [NSFileHandle fileHandleWithStandardOutput];
[unixTask setStandardOutput: outputFH];
// Run task with termination handler
[unixTask executeWithArguments: nil completionHandler: ^(NSError* error2) {
// Save output
NSString* output = [[NSString alloc] initWithData: [outputFH readDataToEndOfFile] encoding: NSUTF8StringEncoding];
if ([output length]) { // <-- Execution never reaches this line when the block is called
NSLog(#"%#", output);
}
// Read error 1
NSString* error1 = [[NSString alloc] initWithData: [errorFH readDataToEndOfFile] encoding: NSUTF8StringEncoding];
if ([error1 length]) {
NSLog(#"%#", error1);
}
// Read error 2
if (error2) {
NSLog(#"%#", error2);
}
}];
Nothing is logged on Xcode's console, but if I open Console.app I see this line:
Warning: Exception caught during decoding of received reply to message 'executeScript:interpreter:arguments:standardInput:standardOutput:standardError::', dropping incoming message and calling failure block.
Exception: *** -[NSConcreteFileHandle readDataOfLength:]: Bad file descriptor
Hmmm, how can I solve this? Before I had my app sandboxed I used NSPipe in conjunction with NSTask and everything worked, but now NSUserUnixTask requires the use of NSFileHandle instead and I reckon my problem is simply because I'm not using it correctly. But still I have no idea what I'm doing wrong.

Answer 1

It seems to me that you have to use NSPipe to read stderr and stdout from NSUserUnixTask in the same way it is done with NSTask.
I have tested it with a "/bin/ls" task and it produced the expected output:
NSPipe *outPipe = [NSPipe pipe];
NSPipe *errPipe = [NSPipe pipe];
NSString *path = #"/bin/ls";
NSUserUnixTask *unixTask = [[NSUserUnixTask alloc] initWithURL: [NSURL fileURLWithPath: path] error: nil];
[unixTask setStandardOutput:[outPipe fileHandleForWriting]];
[unixTask setStandardError:[errPipe fileHandleForWriting]];
[unixTask executeWithArguments:nil completionHandler:^(NSError *error) {
NSString *output = [[NSString alloc] initWithData: [[outPipe fileHandleForReading] readDataToEndOfFile] encoding: NSUTF8StringEncoding];
NSLog(#"stdout: %#", output);
NSString *error1 = [[NSString alloc] initWithData: [[errPipe fileHandleForReading] readDataToEndOfFile] encoding: NSUTF8StringEncoding];
NSLog(#"stderr: %#", error1);
}];