If I'm understanding you correctly, you could do away with the extra file (and the need for locking) by putting the info in the filenames themselves.  For example 

    CollectionID_IDX_N_imgname.jpg

[download]

where CollectionID is a unique collection identifier, N is the expected total number of images, and IDX the image number within the collection (the collection ID could also be a directory).  All you then have to do after having received a new image is a simple glob plus a check for completeness.

Update: forgot to mention that to avoid potential concurrency issues (reading yet incompletely written files), you'd rename a file to its final name only after you've finished writing it.

In reply to Re: Caching Format by Eliya
in thread Caching Format by hok_si_la

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