CouchQuery stores error of the last request

[kaalita]

Readonly error property stores the error of the last request.

[snej]

Thanks! Just a few minor issues...

[kaalita]

Updated the code as you suggested.
This also includes a fix for another mistake in my original code (after an error occured, the error property was never set to nil again, when subsequent requests were successful).

[kaalita]

Hm, this method works fine with all sorts of failed requests that return at least some kind of response.
But I just simulated a request timeout and the error property wasn't changed to the timeout error.
It looks like the -operation:willCompleteWithError: implementation in the CouchQuery class isn't called at all, it only calls the -operation:willCompleteWithError: implemention of the CouchResource class.

To catch also the timeout error I could of course move the error property to the CouchResource class, but not sure whether this is the best way to do it.

[snej]

It looks like the -operation:willCompleteWithError: implementation in the CouchQuery class isn't called at all, it only calls the -operation:willCompleteWithError: implemention of the CouchResource class.

Can you explain in more detail? CouchQuery overrides that method, so I don't see a way for the CouchResource implementation to be called without going through the CouchQuery override.

RESTOperation treats HTTP error statuses and socket-level errors the same way; they both call -completedWithError:, which in turn calls the owning resource's -operation:willCompleteWithError:.

[kaalita]

For my test I made all requests to xyz.iriscouch.com time out.
I started my app which contains a UITableView with a CouchUITableSource.
When I create the CouchLiveQuery with the asLiveQuery method, this happens:

• the method seems to create the CouchLiveQuery synchronously, because it doesn't return until the timeout occurs
• the operation:willCompleteWithError: is called, but not on CouchQuery, but on CouchResource (*)

(*) I added comments to all the operation:willCompleteWithError: methods to see which where called and to find out why the error isn't stored in the error property.

[snej]

the method seems to create the CouchLiveQuery synchronously, because it doesn't return until the timeout occurs

I don't think it does — I can't see any place in that code path that will block. And I ran the TouchDB iOS demo and looked at all the calls to -[RESTOperation wait], and none were invoked on a view. Can you pause the app while it's blocked in the synchronous call and show me the backtrace (use the 'bt' debugger command)?

the operation:willCompleteWithError: is called, but not on CouchQuery, but on CouchResource (*)

Please set a breakpoint in the CouchResource method and send me the backtrace when it was hit. I think it must be invoked from the 'super' call in the CouchQuery method.

[kaalita]

To the first point:
Oh, you were right, it's not the asLiveQuery method that doesn't return immediately, it's the setQuery method of the CouchUITableSource in the same code line: self.query = [query asLiveQuery];

(lldb) bt

• thread Block executed after CouchModel saved #1: tid = 0x1f03, 0x94271c22 libsystem_kernel.dylibmach_msg_trap + 10, stop reason = signal SIGSTOP frame #0: 0x94271c22 libsystem_kernel.dylibmach_msg_trap + 10
  frame Block executed after CouchModel saved #1: 0x942711f6 libsystem_kernel.dylibmach_msg + 70 frame #2: 0x0237f10a CoreFoundationCFRunLoopServiceMachPort + 186
  frame Some fixes and KVO niceness #3: 0x022e25d5 CoreFoundation__CFRunLoopRun + 1445 frame #4: 0x022e1d84 CoreFoundationCFRunLoopRunSpecific + 212
  frame Support for rewrites + CouchModel unit test for arrays #5: 0x022e1c9b CoreFoundationCFRunLoopRunInMode + 123 frame #6: 0x01c1f40f Foundation-[NSRunLoop(NSRunLoop) runMode:beforeDate:] + 300
  frame UITableViewCell customisation #7: 0x001b36c8 Lingya-[RESTOperation wait] + 296 at RESTOperation.m:186 frame #8: 0x001b4ce3 Lingya-[RESTOperation responseBody] + 51 at RESTOperation.m:392
  frame Support for managing cookies & ability to replace existing replication requests #9: 0x001bcbf2 Lingya-[CouchDatabase lastSequenceNumber] + 114 at CouchDatabase.m:370 frame #10: 0x001bd585 Lingya-[CouchDatabase setTracksChanges:] + 165 at CouchDatabase.m:471
  frame Fix for memory leak in CouchChangeTracker #11: 0x001acd0d Lingya-[CouchLiveQuery start] + 157 at CouchQuery.m:247 frame #12: 0x001acbb6 Lingya-[CouchLiveQuery rows] + 70 at CouchQuery.m:231
  frame Removed a memory leak in the CouchQueryRow class. #13: 0x001bdddc Lingya-[CouchUITableSource reloadFromQuery] + 60 at CouchUITableSource.m:105 frame #14: 0x001bdd95 Lingya-[CouchUITableSource setQuery:] + 277 at CouchUITableSource.m:99
  frame Fixed memory leak relating to cachedURL in RESTResource #15: 0x0009523c Lingya-[NewsfeedDatasource init] + 620 at NewsfeedDatasource.m:40 frame #16: 0x0001aaa5 Lingya-[NewsfeedViewController viewDidLoad] + 421 at NewsfeedViewController.m:63
  frame New table view update delegate: couchTableSource:updateFromQuery:previousRows: #17: 0x013a0a1e UIKit-[UIViewController view] + 184 frame #18: 0x0139ffec UIKit-[UIViewController nextResponder] + 34
  frame More efficient CouchModel attachment handling #19: 0x013c6f1d UIKit-[UIResponder _containsResponder:] + 40 frame #20: 0x013b11cb UIKit-[UINavigationController defaultFirstResponder] + 83
  frame Https support for _changes stream #21: 0x013c7e59 UIKit-[UIResponder(Internal) _deepestDefaultFirstResponderMatching:] + 59 frame #22: 0x013c7e76 UIKit-[UIResponder(Internal) _deepestDefaultFirstResponderMatching:] + 88
  frame Retained parent links lead to cycles #23: 0x013c7e76 UIKit-[UIResponder(Internal) _deepestDefaultFirstResponderMatching:] + 88 frame #24: 0x013c7f3a UIKit-[UIResponder(Internal) _promoteDeepestDefaultFirstResponder] + 181
  frame Clean up replication .state vs .mode properties #25: 0x013066f5 UIKit-[UIWindow makeKeyWindow] + 401 frame #26: 0x0130673c UIKit-[UIWindow makeKeyAndVisible] + 53
  frame MAPBLOCK macro is difficult to debug #27: 0x00019cf3 Lingya-[EmphrasesAppDelegate application:didFinishLaunchingWithOptions:] + 979 at EmphrasesAppDelegate.m:86 frame #28: 0x012d7386 UIKit-[UIApplication _callInitializationDelegatesForURL:payload:suspended:] + 1292
  frame Implementation of CouchUser and CouchSecurity #29: 0x012d8274 UIKit-[UIApplication _runWithURL:payload:launchOrientation:statusBarStyle:statusBarHidden:] + 524 frame #30: 0x012e7183 UIKit-[UIApplication handleEvent:withNewEvent:] + 1027
  frame Documentation language #31: 0x012e7c38 UIKit-[UIApplication sendEvent:] + 68 frame #32: 0x012db634 UIKit_UIApplicationHandleEvent + 8196
  frame [performSelector: withObject: afterDelay: ] is not guarantee to be performed #33: 0x02b1fef5 GraphicsServicesPurpleEventCallback + 1274 frame #34: 0x0237f195 CoreFoundation__CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION + 53
  frame document should always choose the highest revision as current revision #35: 0x022e3ff2 CoreFoundation__CFRunLoopDoSource1 + 146 frame #36: 0x022e28da CoreFoundation__CFRunLoopRun + 2218
  frame RESTOperation crashing in connection:didReceiveData: #37: 0x022e1d84 CoreFoundationCFRunLoopRunSpecific + 212 frame #38: 0x022e1c9b CoreFoundationCFRunLoopRunInMode + 123
  frame Always remove an observer before adding it #39: 0x012d7c65 UIKit-[UIApplication _run] + 576 frame #40: 0x012d9626 UIKitUIApplicationMain + 1163
  frame Updated the error message for other linker flag #41: 0x000197ed Lingyamain + 125 at main.m:14 frame #42: 0x000027e5 Lingyastart + 53

[kaalita]

Please set a breakpoint in the CouchResource method and send me the backtrace when it was hit. I think it must be
invoked from the 'super' call in the CouchQuery method.

Sorry, I would, but I can't get that to work :-(
I added a symbolic breakpoint for the symbol:
[CouchResource operation:willCompleteWithError:]
but the breakpoint is never invoked.

I'm already googling for an answer, but until now nothing worked.

[kaalita]

But this hack http://stackoverflow.com/a/9603733 might help out a little bit.
I don't know how reliable this is, but at least it gave me now 3 times in a row the same result:

2012-07-03 19:38:12.695 Lingya[24106:11303] Stack = 1
2012-07-03 19:38:12.695 Lingya[24106:11303] Framework = Lingya
2012-07-03 19:38:12.695 Lingya[24106:11303] Memory address = 0x001b4678
2012-07-03 19:38:12.695 Lingya[24106:11303] Class caller = RESTOperation
2012-07-03 19:38:12.696 Lingya[24106:11303] Function caller = completedWithError:
2012-07-03 19:38:12.696 Lingya[24106:11303] Line caller = 920

When a timeout occurs, the method is called from RESTOperation, not from CouchQuery.

[snej]

Re the timeout: your backtrace shows it's coming from setting the tracksChanges property of the database, which CouchUITableSource does so it can listen for changes. (Setting -tracksChanges gets the db's latest sequence number via a synchronous call — this could perhaps be fixed to be async.)

I'll review your pull request again — sorry for the delay, was stuck with a bunch of Couchbase Server tasks this week.