IssueTracker

Library used: support-v4
Version used: 23.1.1
Devices/Android versions reproduced on: Genymotion with 5.0 image, Nexus 6P with Android 6.0

Source code: https://github.com/chalup/android-loaders-bug

Steps to reproduce:
1. Start the application
2. You should see "Loading, Slow, Fast" on the screen
3. Rotate the screen

Expected results:
4. You should see "Loading, Slow, Slow, Fast" on the screen (although I'm not 100% sure the duplicated "Slow" caused by onLoadFinished called twice on screen rotation is not another bug).

Actual results:
4. You see "Loading, Slow, Slow" sometimes (reproduction rate probably depends on the device, on my devices it happens once every ~5 times).

More info:
The issue happens only if the "fast" loader finished its background job before it's cancelled inside LoaderManagerImpl.restartLoader call. Here's the sample logcat output when the issue happens:

V/LoaderManager( 1133): Retaining in LoaderManager{38b61f33 in HostCallbacks{1f86268}}
V/LoaderManager( 1133):   Retaining: LoaderInfo{1f882bb2 #1 : FastLoader{27f70703}}
V/LoaderManager( 1133):   Retaining: LoaderInfo{9999069 #0 : SlowLoader{3a98aaee}}
V/LoaderManager( 1133): Destroying Inactive in LoaderManager{38b61f33 in HostCallbacks{1f86268}}
V/LoaderManager( 1133): initLoader in LoaderManager{38b61f33 in HostCallbacks{9f451f1}}: args=null
V/LoaderManager( 1133):   Re-using existing loader LoaderInfo{9999069 #0 : SlowLoader{3a98aaee}}
V/LoaderManager( 1133): Starting in LoaderManager{38b61f33 in HostCallbacks{9f451f1}}
V/LoaderManager( 1133):   onLoadFinished in SlowLoader{3a98aaee id=0}: String{2dcc41ab}
V/LoaderManager( 1133): restartLoader in LoaderManager{38b61f33 in HostCallbacks{9f451f1}}: args=null
V/LoaderManager( 1133):   Making last loader inactive: LoaderInfo{1f882bb2 #1 : FastLoader{27f70703}}
V/LoaderManager( 1133):   Starting: LoaderInfo{3f6481d6 #1 : FastLoader{1edeac57}}
V/LoaderManager( 1133): Finished Retaining in LoaderManager{38b61f33 in HostCallbacks{9f451f1}}
D/LoaderManager( 1133): Start loadInBackground FastLoader{1edeac57 id=1}
D/LoaderManager( 1133): Finished loadInBackground FastLoader{1edeac57 id=1}
V/LoaderManager( 1133):   Finished Retaining: LoaderInfo{9999069 #0 : SlowLoader{3a98aaee}}
V/LoaderManager( 1133):   onLoadFinished in SlowLoader{3a98aaee id=0}: String{2dcc41ab}
V/LoaderManager( 1133): restartLoader in LoaderManager{38b61f33 in HostCallbacks{9f451f1}}: args=null
V/LoaderManager( 1133):   Current loader is running; attempting to cancel
V/LoaderManager( 1133):   Canceling: LoaderInfo{3f6481d6 #1 : FastLoader{1edeac57}}
V/LoaderManager( 1133): onLoadCanceled: LoaderInfo{3f6481d6 #1 : FastLoader{1edeac57}}
V/LoaderManager( 1133):   Enqueuing as new pending loader

I have implemented the fix/workaround here: https://github.com/futuresimple/android-support-v4/commit/5c96b3562d59baf9468bd0f2d183d9529a7af145, but as I wrote in the commit message, I'm not sure if this is the right way to fix this issue.