CFMessagePort weirdness

Question

I am facing a weird delay issue regarding sending/receiving messages via mach ports. The basic concept of my implementation is the following:

Plugin creates local port → Launches remote process which sends a message to the said port → returns the received data.

Here is the plugin part:

static NSArray *returned=nil;
static CFDataRef handle_port (
   CFMessagePortRef local,
   SInt32 msgid,
   CFDataRef d,
   void *info
) {
    NSPropertyListFormat format;
    NSDictionary* ret = [NSPropertyListSerialization propertyListWithData:(NSData*)d
                                            options: NSPropertyListImmutable
                                            format: &format
                                            error: nil];
    returned=[NSArray arrayWithArray:[ret objectForKey:@"aKey"]]; //this is what I want returned from the portRet()
    NSLog(@"returned array %@",returned); 
    return NULL;     
}

NSArray* portRet(){
    CFMessagePortRef port = CFMessagePortCreateLocal(kCFAllocatorDefault, CFSTR("com.someport"), handle_port, NULL, NULL);
    CFRunLoopSourceRef source =  CFMessagePortCreateRunLoopSource(kCFAllocatorDefault, port, 0);
    CFRunLoopAddSource(CFRunLoopGetCurrent(), source, kCFRunLoopCommonModes);
    CFRelease(source);
    int r=system("/path/someExecutable"); 
    if(r !=0) NSLog(@"Program error");
    //CFMessagePortInvalidate(port);    
    //CFRelease(port);
    return returned; // always returns nil
}

And the important part of someExecutable's code is the following:

int main(){
...
CFMessagePortRef port = CFMessagePortCreateRemote(NULL, CFSTR("com.someport"));
    if(port == NULL) exit(1);
    CFDataRef d=CFPropertyListCreateData(kCFAllocatorDefault,[NSDictionary dictionaryWithObject:anArray forKey:@"aKey"], kCFPropertyListXMLFormat_v1_0, 0, NULL);
    CFMessagePortSendRequest (port, 0, d, 0, 0, NULL, NULL);
    NSLog(@"Program is about to exit");
    CFRelease(d);
...
    exit(0);
}

The message from the remote process is sent gracefully, but the the callback is called after the process has ended and the portRet() has returned a null value. If I invalidate the port inside the portRet() function, the message is never received.

I can't figure out the reason why this delay is happening. What I want to achieve is to get the port callback called before portRet() returns. I have also tried to use the main dispatch queue instead of a CFRunLoopSource for the port's callback scheduling:

 CFMessagePortSetDispatchQueue(port, dispatch_get_main_queue());

But the result is pretty much the same. Am not sure what I'm doing wrong. Your help is greatly appreciated.

Answer 1

You need to run your run loop in portRet until the second process returns a value. For example:

SInt32 runLoopRunReturnValue = CFRunLoopRunInMode(kCFRunLoopDefaultMode, CFDateGetTimeIntervalSinceDate((__bridge CFDateRef)[NSDate distantFuture], (__bridge CFDateRef)[NSDate date]), true);

if (runLoopRunReturnValue == kCFRunLoopRunHandledSource)
    return returned;
else {
    // Throw exception or whatever
    // (although this will never be called using the above implementation
    // since [NSDate distantFuture] is wayy into the future...)
}