Exclusive file open
source link: https://lispblog.xach.com/post/189664324263/exclusive-file-open
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The Planet Lisp twitter fix involved tracking post status with a file. Although it’s not 100% bulletproof, there’s a trick when using files to reduce races.
Here’s a naive approach:
- Assign a Planet Lisp post a unique pathname
- If that pathname exists, do nothing
- Otherwise, send a tweet for the post and save its details to the pathname
The problem is between 2 and 3 - another process could post the tweet first.
There’s another option:
- Assign a Planet Lisp post a unique pathname
- Attempt to open the file with
O_EXCL|O_CREATmode - If the attempt succeeds, send a tweet for the post and save its details to the pathname
The key bit is O_EXCL|O_CREAT mode, a Unix option that means open
will fail if the file exists, and succeed atomically
otherwise. Specifically:
The check for the existence of the file and the creation of the file if it does not exist shall be atomic with respect to other threads executing open() naming the same filename in the same directory with O_EXCL and O_CREAT set.
In SBCL, you can
getO_EXCL|O_CREAT semantics by specifing :direction :output and
NIL, :error, or :new-version as the :if-exists argument to
CL:OPEN. For NIL, you can check to see if
the returned value is NIL, instead of a stream, to know if you have
successfully exclusively opened the file and can proceed. I used the
NIL
approach
in the Planet Lisp twitter gateway.
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