illumos gate

During the discussion in the Austin Group I learned that our generic file locking implementation is not aligned with POSIX. We implement the write-preferring locks, but the standard requires the read-preferring locks:

When a shared lock is set on a segment of a file, other processes shall be able to set shared locks on that segment or a portion of it. A shared lock prevents any other process from setting an exclusive lock on any portion of the protected area. A request for a shared lock shall fail if the file descriptor was not opened with read access.

An exclusive lock shall prevent any other process from setting a shared lock or an exclusive lock on any portion of the protected area. A request for an exclusive lock shall fail if the file descriptor was not opened with write access.

The difference could be reproduced using the attached lock.c file:

# gcc -Wall -o lock lock.c

This is the current write-preferring behavior:

# ./lock r file &
F_SETLK: F_RDLCK
sleeping
[1] 1635
# sleep 2
# ./lock W file &
F_SETLKW: F_WRLCK
[2] 1638
# sleep 2
# ./lock r file &
F_SETLK: F_RDLCK
fcntl failed
F_GETLK l_type: F_UNLCK
sleeping
[3] 1645
#

With the read-preferring behavior, the last lock command above should not fail.