One issue which is sometimes ignored in these file-sharing schemes is a race condition which can lead to half-written files being processed.

You can have a problem where something like this happens:

  1. Writing process gets current epoch time A, makes up filename
  2. Writing process creates file, starts writing, doesn't finish
  3. Reading process comes along at time A+delta, notes an 'old' file with timestamp A, opens and reads it.
In this case, the reading process has seen a half-completed file.

One might argue that the writing process couldn't stall for long enough for this to happen, but that depends on the size of the file being written, whether it now (or in the future) will be writing over a network, whether the writing process has to wait to get more data, etc.

The safe way to do this (which you might already be doing) is for the reader and writer to agree on a pattern match of files to ignore (e.g. *.tmp). The writer can then create and write to a x-y-z.tmp file, flush and sync it to disk and then do a rename() on it once it's finished.

In reply to Re: Extract the middle part of a list by jbert
in thread Extract the middle part of a list by chrism01

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