Question: This comment seems unwarranted
plivesey opened this issue · comments
"@warning Access is protected for the duration of the block, but to maintain safe disk access do not access this fileURL after the block has ended. Do all work on the ."
I don't understand this warning. I presume this is because you want to enforce that it's done on the same queue as everything else, but why is this necessary? Isn't the file manager thread safe? Shouldn't I always be able to read files from different threads?
This is related to the next issue I'm about to post about the synchronous methods. I'll link it after I post.
Thanks for your help.
This comment also highlights the same issue:
line 270, TMDiskCache.h
Retrieves the file URL for the specified key. This method blocks the calling thread until the
url is available. Do not use this URL anywhere but on the . This method probably
shouldn't even exist, just use the asynchronous one.
The first comment is from line 183 in TMDishCache.h
The file manager itself can probably deal with reads just fine, but the intention is to maintain cache consistency. We don't want another thread moving or changing a file when there's already a block queued up that will need it. There's also the issue of file sizes and dates being changed on disk without the cache having a chance to update its own metadata.
It's also a performance consideration. 10 different caches shouldn't read 10 files in parallel off the disk, serial is a better approach here (hence the shared queue across caches.) A nice side effect of this is that two instances of TMDiskCache can use the same disk area without conflict.
I didn't notice that the disk queue was serial, but that makes a lot of sense and answers some of my other questions on how reliable GCD is for not overloading the disk. Having a shared queue definitely makes sense for this, and it also makes sense that other threads should not use the fileUrl for any reason.
Thanks for your help. Since my questions were answered, I'm going to close the issue.