Well I tried my code on Linux and the failure case looks a little different:
$ ./locktest & sleep 4; ./locktest
[1] 1553
Using flock()...
1553 shares.
1553 owns
1553 shares
Using flock()...
1557 shares.
1557 waiting for previous instance(s) to exit...
1553 owns
1557 owns
Running...
1557 owns
1553 can't revert self lock to shared: Resource temporarily unavailabl
+e
1557 shares
^C
[1]+ Exit 11 ./locktest
Which demonstrates that Linux doesn't have the strong preference for non-blocking requests like FreeBSD appears to have.
Having lock up-/down-grading introduce a race condition where the lock is freed first is such a horrid design choice to my mind that I didn't even consider the possibility when reading "man flock" (this is not even mentioned in Linux's extremely short version of "man flock" tho my test cases show that it is the case there as well).
Thanks for the enlightenment. Now I have one more reason to hate flock. I should find a module that provides a convenient wrapper for fcntl locks... (:
- tye (but my friends call me "Tye")
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