Portable File Locking
Place an exclusive or shared lock on a file. It uses LockFile
on Windows
and fcntl
locks on Unix-like systems.
devtools::install_github("r-lib/filelock")
library(filelock)
This is R process 1, it gets an exclusive lock. If you want to lock file
myfile
, always create a
separate lock file instead of placing the lock on this file directly!
R1> lck <- lock("/tmp/myfile.lck")
This is R process 2, it fails to acquire a lock.
R2> lock("/tmp/myfile.lck", timeout = 0)
Specifying a timeout interval, before giving up:
R2> lock("/tmp/myfile.lck", timeout = 5000)
Wait indefinetely:
R2> lock("/tmp/myfile.lck", timeout = Inf)
Once R process 1 released the lock (or terminated), R process 2 can acquire the lock:
R1> unlock(lck)
R2> lock("/tmp/myfile.lck")
#> Lock on ‘/tmp/myfile.lck’
Always use special files for locking. I.e. if you want to restict access
to a certain file, do not place the lock on this file. Create a special
file, e.g. by appending .lock
to the original file name and place the
lock on that. (The lock()
function creates the file for you, actually,
if it does not exist.) Reading from or writing to a locked file has
undefined behavior! (See more about this below at the Internals Section.)
It is hard to determine whether and when it is safe to remove these special files, so our current recommendation is just to leave them around.
It is best to leave the special lock file empty, simply because on some OSes you cannot write to it (or read from it), once the lock is in place.
All locks set by this package might be advisory. A process that does not respect this locking machanism may be able to read and write the locked file, or even remove it (assuming it has capabilities to do so).
If a process terminates (with a normal exit, a crash or on a signal), the lock(s) it is holding are automatically released.
If the R object that represents the lock (the return value of lock
)
goes out of scope, then the lock will be released automatically as
soon as the object is garbage collected. This is more of a safety
mechanism, and the user should still unlock()
locks manually, maybe
using base::on.exit()
, so that the lock is released in case of errors
as well, as soon as possible.
File locking needs support from the file system, and some non-standard
file systems do not support it. For example on network file systems
like NFS or CIFS, user mode file systems like sshfs
or ftpfs
, etc.,
support might vary. Recent Linux versions and recent NFS versions (from
version 3) do support file locking, if enabled.
In theory it is possible to simply test for lock support, using two
child processes and a timeout, but filelock
does not do this
currently.
While this is possible in general, filelock
does not suport it
currently. The main purpose of filelock
is to lock using special
lock files, and locking part of these is not really useful.
On Unix (i.e. Linux, macOS, etc.), we use fcntl
to acquire and
release the locks. You can read more about it here:
https://www.gnu.org/software/libc/manual/html_node/File-Locks.html
Some important points:
SIGALRM
signals are
delivered via a single call to the signal handler, then alarms might
get lost. Currently base R does not use the SIGALRM
signal for
anything, but other packages might.)On Windows, LockFileEx
is used to create the lock on the file.
If a finite timeout is specified for the lock request, asynchronous
(overlapped) I/O is used to wait for the locking event with a timeout.
See more about LockFileEx
here:
https://msdn.microsoft.com/en-us/library/aa365203.aspx
Some important points:
LockFileEx
locks are mandatory (as opposed to advisory), so indeed
no other processes have access to the locked file. Actually, even the
locking process has no access to it through a different file handle,
than the one used for locking. In general, R cannot read from the
locked file, and cannot write to it. (Although, the current R version
does not fail, it just does nothing, which is quite puzzling.)
Remember, always use a special lock file, instead of putting the lock
on the main file, so that you are not affected by these problems.MIT © RStudio
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