So I'm using flock, but I have this noxious race condition when I try to
clean up the lock files.
I need a way of knowing if anything has a file open at the same time as
me, or I must always leave lock files lying around. Any suggestions?
Here is the sequence..
Process A: fd = open( "lockfile", 'a')
Process A: flock(fd, LOCK_EX)
Process A: do stuff in critical section...
Process B : fd = open( "lockfile", 'a')
Process B: flock(fd, LOCK_EX) // Blocks waiting for lock
Process A: Finishes, doesn't know about B, wants to clean up...
Process A: unlink fd
Process A: close(fd)
Process B: Unblocks, B grabs lock.
Process C : fd = open( "lockfile", 'a') // CREATES A NEW ONE!
Process C: flock(fd, LOCK_EX) // DOESN'T BLOCK!
Here is a chunk of ruby that demonstrates this...
fork do
fd = open( "lockfile", 'a')
puts fd.stat.ino
fd.flock(File::LOCK_EX)
puts "Sleeping"
sleep 10
File.unlink "lockfile"
fd.close
end
sleep 1
fork do
fd = open( "lockfile", 'a')
puts fd.stat.ino
puts "Waiting for lock"
fd.flock(File::LOCK_EX) # Blocks waiting for lock
puts "Got lock"
sleep 100
end
sleep 15
puts "Open file #{Time.now}"
fd = open( "lockfile", 'a') # CREATES A NEW ONE!
puts fd.stat.ino
fd.flock(File::LOCK_EX) # DOESN'T BLOCK!
puts "Did it block #{Time.now}"
···
====================================
Here is the result...
ruby -w try.rb
6157824
Sleeping
6157824
Waiting for lock
Got lock
Open file Mon Aug 14 14:10:55 +1200 2006
6158309 <-------- Note new INODE NUMBER, Did it block Mon Aug 14 14:10:55 +1200 2006 <---- No it didn't!
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