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rustle / SRVResolver

Run loop based NSOperation that resolves DNS SRV records. Based on SRVResolver Apple sample code.

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SRVResolver

Based on Apple Sample Code.

Purpose

NSOperation subclass to resolve DNS SRV records based on host name.

Usage

class ThatWhichRequiresResolve: NSObject, SRVResolverDelegate {
    let queue = OperationQueue()
    func resolve(host: String) {
        let operation = SRVResolverOperation(srvName: "_jmap._tcp.\(host)")
        operation.delegate = self
        operation.delegateQueue = queue
        queue.addOperation(operation)
    }

    func srvResolverOperation(_ resolver: SRVResolverOperation,
                              didReceiveResult result: [SRVResolverResultsKey: Any]) {
        guard
            let priority = result[.priority] as? Int,
            let weight = result[.weight] as? Int,
            let port = result[.port] as? Int,
            let target = result[.target] as? String
        else {
            return
        }
        ...
    }

    func srvResolverOperation(_ resolver: SRVResolverOperation,
                              didFinishWithError error: Error?) {
        if let error = error {
            ...
        } else {
            ...
        }
    }
}

SRVRunLoopOperation Theory of Operation

Assumptions

  1. By the time we're running on the run loop thread, all further state transitions must happen on the run loop thread. We can ensure this by defining three states (inited, executing, and finished), only allowing run loop thread code to run in the last two states, and only allowing the executing to finished transition on the run loop thread.
  2. start must only be called once. Run loop thread code doesn't have to worry about racing with start because, by the time the run loop thread code runs, start has already been called.
  3. cancel can be called multiple times from any thread. Run loop thread code must take care to do the right thing with cancellation.

Discussion of state transitions

  1. It's valid to allocate an operation and never run it.
    • `init (any thread)
    • `dealloc (any thread)
  2. It's valid to allocate an operation, cancel it, and never run it.
    • init (any thread)
    • cancel (any thread)
    • dealloc (any thread)
  3. This case can never happen because while it's valid to cancel an operation before it starting it, this case doesn't happen because -start always bounces to the run loop thread to maintain the invariant that the executing to finished transition always happens on the run loop thread.
    • init (any thread)
    • cancel (any thread)
    • start (any thread)
    • finishWithError: (required to be run loop thread, but in this case would be undefined)
    • dealloc (any thread)
  4. An operation is allocated, cancel and start are both called, either on the same thread, with cancel going first, or on different threads, with cancel winning the race to set state. Both cancel and start bounce to the their run loop thread counterparts (cancelOnRunLoopThread and startOnRunLoopThread) to maintain "the executing to finished transition always happens on the run loop thread" as an invariant. When startOnRunLoopThread checks state and finds it isn't executing, it returns early.
    • init (any thread)
      • cancel (thread A)
      • start (thread A)
    • OR
      • cancel (thread A, cancelOnRunLoopThread wins race with startOnRunLoopThread)
      • start (thread B, startOnRunLoopThread loses race with cancelOnRunLoopThread)
    • cancelOnRunLoopThread (run loop thread, calls finishWithError: with an appropriate error)
    • startOnRunLoopThread (run loop thread, returns early after consulting state)
    • finishWithError: (run loop thread, receiving error indicating cancellation)
    • dealloc (any thread)
  5. An operation is allocated, cancel and start are both called, either on the same thread, with start going first, or on different threads, with cancel winning the race to set isCancelled before startOnRunLoopThread runs. Both cancel and start bounce to the their run loop thread counterparts (cancelOnRunLoopThread and startOnRunLoopThread) to maintain "the executing to finished transition always happens on the run loop thread" as an invariant. When startOnRunLoopThread checks isCancelled and finds it is true, it returns early. When cancelOnRunLoopThread
    • init (any thread)
      • start (thread A)
      • cancel (thread A)
    • OR
      • start (thread A, startOnRunLoopThread wins race with cancelOnRunLoopThread)
      • cancel (thread B, cancelOnRunLoopThread loses race with startOnRunLoopThread, but cancel wins race to set isCancelled to true before startOnRunLoopThread runs)
    • cancelOnRunLoopThread (run loop thread, sees state is finished and returns early)
    • startOnRunLoopThread (run loop thread, sees isCancelled is true and calls finishWithError: with an appropriate error)
    • finishWithError: (run loop thread, receiving error indicating cancellation)
    • dealloc (any thread)
  6. This case can never happen because work scheduled with performSelector:onThread:withObject:waitUntilDone:modes: happens in the order they're submitted in and start and cancel both take the state lock, and cancel only schedules if start has run.
    • init (any thread)
    • start (any thread)
    • cancel (any thread)
    • cancelOnRunLoopThread (run loop thread)
    • startOnRunLoopThread (run loop thread)
    • finishWithError: (run loop thread)
    • dealloc (any thread)
  7. This case can never happen because startOnRunLoopThread will finish immediately if it detects isCancelled (see case 5).
    • init (any thread)
    • start (any thread)
    • cancel (any thread)
    • startOnRunLoopThread (run loop thread)
    • cancelOnRunLoopThread (run loop thread)
    • finishWithError: (run loop thread)
    • dealloc (any thread)
  8. This is the standard run-to-completion case.
    • init (any thread)
    • start (any thread)
    • startOnRunLoopThread (run loop thread)
    • finishWithError: (run loop thread)
    • dealloc (any thread)
  9. This is the standard cancellation case. cancelOnRunLoopThread wins the race with finishWithError:, and it detects that the operation is executing and actually cancels.
    • init (any thread)
    • start (any thread)
    • startOnRunLoopThread (run loop thread)
    • cancel (any thread)
    • cancelOnRunLoopThread (run loop thread)
    • finishWithError: (run loop thread, receiving error indicating cancellation)
    • dealloc (any thread)
  10. In this case the cancelOnRunLoopThread loses the race with finishWithError:, but that's OK because cancelOnRunLoopThread does nothing if the operation is already finished.
    • init (any thread)
    • start (any thread)
    • startOnRunLoopThread (run loop thread)
    • cancel (any thread)
    • finishWithError: (run loop thread)
    • cancelOnRunLoopThread (run loop thread)
    • dealloc (any thread)
  11. Cancellating after finishing still sets isCancelled to true but has no impact on the run loop thread code.
    • init (any thread)
    • start (any thread)
    • startOnRunLoopThread (run loop thread)
    • finishWithError: (run loop thread)
    • cancel (any thread)
    • dealloc (any thread) */

URL: https://developer.apple.com/library/archive/samplecode/SRVResolver/Listings/SRVResolver_m.html#//apple_ref/doc/uid/DTS40009625-SRVResolver_m-DontLinkElementID_5

Contains: Uses <dns_sd.h> APIs to resolve SRV records.

Written by: DTS

Copyright: Copyright (c) 2010-2012 Apple Inc. All Rights Reserved.

Disclaimer: IMPORTANT: This Apple software is supplied to you by Apple Inc. ("Apple") in consideration of your agreement to the following terms, and your use, installation, modification or redistribution of this Apple software constitutes acceptance of these terms. If you do not agree with these terms, please do not use, install, modify or redistribute this Apple software.

In consideration of your agreement to abide by the following terms, and subject to these terms, Apple grants you a personal, non-exclusive license, under Apple's copyrights in this original Apple software (the "Apple Software"), to use, reproduce, modify and redistribute the Apple Software, with or without modifications, in source and/or binary forms; provided that if you redistribute the Apple Software in its entirety and without modifications, you must retain this notice and the following text and disclaimers in all such redistributions of the Apple Software. Neither the name, trademarks, service marks or logos of Apple Inc. may be used to endorse or promote products derived from the Apple Software without specific prior written permission from Apple. Except as expressly stated in this notice, no other rights or licenses, express or implied, are granted by Apple herein, including but not limited to any patent rights that may be infringed by your derivative works or by other works in which the Apple Software may be incorporated.

The Apple Software is provided by Apple on an "AS IS" basis. APPLE MAKES NO WARRANTIES, EXPRESS OR IMPLIED, INCLUDING WITHOUT LIMITATION THE IMPLIED WARRANTIES OF NON-INFRINGEMENT, MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, REGARDING THE APPLE SOFTWARE OR ITS USE AND OPERATION ALONE OR IN COMBINATION WITH YOUR PRODUCTS.

IN NO EVENT SHALL APPLE BE LIABLE FOR ANY SPECIAL, INDIRECT, INCIDENTAL OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) ARISING IN ANY WAY OUT OF THE USE, REPRODUCTION, MODIFICATION AND/OR DISTRIBUTION OF THE APPLE SOFTWARE, HOWEVER CAUSED AND WHETHER UNDER THEORY OF CONTRACT, TORT (INCLUDING NEGLIGENCE), STRICT LIABILITY OR OTHERWISE, EVEN IF APPLE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

URL: http://developer.apple.com/library/ios/#samplecode/MVCNetworking/Listings/Networking_QRunLoopOperation_m.html

Contains: An abstract subclass of NSOperation for async run loop based operations.

Written by: DTS

Copyright: Copyright (c) 2010 Apple Inc. All Rights Reserved.

Disclaimer: IMPORTANT: This Apple software is supplied to you by Apple Inc. ("Apple") in consideration of your agreement to the following terms, and your use, installation, modification or redistribution of this Apple software constitutes acceptance of these terms. If you do not agree with these terms, please do not use, install, modify or redistribute this Apple software.

In consideration of your agreement to abide by the following terms, and subject to these terms, Apple grants you a personal, non-exclusive license, under Apple's copyrights in this original Apple software (the "Apple Software"), to use, reproduce, modify and redistribute the Apple Software, with or without modifications, in source and/or binary forms; provided that if you redistribute the Apple Software in its entirety and without modifications, you must retain this notice and the following text and disclaimers in all such redistributions of the Apple Software. Neither the name, trademarks, service marks or logos of Apple Inc. may be used to endorse or promote products derived from the Apple Software without specific prior written permission from Apple. Except as expressly stated in this notice, no other rights or licenses, express or implied, are granted by Apple herein, including but not limited to any patent rights that may be infringed by your derivative works or by other works in which the Apple Software may be incorporated.

The Apple Software is provided by Apple on an "AS IS" basis. APPLE MAKES NO WARRANTIES, EXPRESS OR IMPLIED, INCLUDING WITHOUT LIMITATION THE IMPLIED WARRANTIES OF NON-INFRINGEMENT, MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, REGARDING THE APPLE SOFTWARE OR ITS USE AND OPERATION ALONE OR IN COMBINATION WITH YOUR PRODUCTS.

IN NO EVENT SHALL APPLE BE LIABLE FOR ANY SPECIAL, INDIRECT, INCIDENTAL OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) ARISING IN ANY WAY OUT OF THE USE, REPRODUCTION, MODIFICATION AND/OR DISTRIBUTION OF THE APPLE SOFTWARE, HOWEVER CAUSED AND WHETHER UNDER THEORY OF CONTRACT, TORT (INCLUDING NEGLIGENCE), STRICT LIABILITY OR OTHERWISE, EVEN IF APPLE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

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Run loop based NSOperation that resolves DNS SRV records. Based on SRVResolver Apple sample code.

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