[KPhotoAlbum] More on thumbnails

[Tobias Leupold]

Nice work :-) Don't forget to create a branch and push your changes from time 
to time!

Am Freitag, 18. Mai 2018, 03:20:31 CEST schrieb Robert Krawitz:
> There are some really serious problems in the thumbnail cache code:
> 
> 1) It misuses timers.  Timers can only be started and stopped in the
>    same thread.  Even putting them all in the same function is not
>    good enough, if that function can be called from different threads;
>    with the multi-threaded nature of thumbnail building, calling
>    ThumbnailCache::save() directly means it can be called from
>    multiple threads.
> 
>    The practical impact is that the 1-second timer for thumbnail
>    saving accomplished nothing, because the timer was seldom getting
>    started (or stopped).
> 
>    The solution is to never call save() directly; always call it
>    indirectly through emitting a signal.
> 
> 2) The thumbnail cache itself is saved very inefficiently; it's
>    written in full to a temporary file, which is then copied (yes,
>    copied, not renamed) to the real file.  Even with a big database,
>    such as my own (270,303 images) that's probably not killing me with
>    I/O, but it's a lot of work to be doing every 100 images.
> 
>    I'm changing it to do an incremental save where possible.  That
>    technically violates the intent of the file format, which stores
>    the most recently written file and offset as well as the image
>    count, but those can be reconstituted on cache reload easily
>    enough.
> 
>    Also, in order to do that, I had to add an additional list of
>    unsaved files (auto-save list).  And there's no good reason for the
>    data structure to be a QMap; a QHash is faster, and ordering
>    doesn't matter in the cache.
> 
> 3) The save() routine wasn't protected by a mutex; with multiple
>    threads processing thumbnails, I'm not sure how we didn't get a lot
>    of data corruption in the index cache.
> 
> I'm working on all of these problems.