[cmake-developers] New command 'file(LOCK_DIRECTORY ...)'

[Ruslan Baratov]

On 13-Oct-14 18:39, Brad King wrote:
> With all the above in mind, please brainstorm and propose a more complete
> signature set.  You can use the keyword/value pairing common in other
> commands to avoid needing a positional argument for every value.
What do you think about this:

file(
     LOCK <path>
     [DIRECTORY] # if present locked file will be <path>/cmake.lock 
(instead of <path>)
     [RELEASE] # do explicit unlock
     [GUARD <FUNCTION|FILE|PROCESS>] # if not present - set to `GUARD 
PROCESS` (not used if RELEASE)
     [RESULT_VARIABLE <variable>] # 0 on success, error message 
otherwise; if not present - any error is FATAL_ERROR
     [TIMEOUT <seconds>]
         # 0 - return immediately if operation failed (try_lock), 
otherwise timeout (timed_lock);
         # if not present - lock until success (or error);
         # not used if RELEASE;
)

?

On 13-Oct-14 18:39, Brad King wrote:
> Also, please post a summary of how the implementation will work on each
> platform.
Boost implementation of file locking mechanism use 
LockFileEx/UnlockFileEx for windows and fcntl for unix-like platforms.
These functions lock file only for current process. When process crashes 
lock removed by OS automatically.
In terms of boost documentation `file(LOCK ...)` locks are exclusive and 
advisory.

boost.interprocess:
http://www.boost.org/doc/libs/1_56_0/doc/html/interprocess/synchronization_mechanisms.html#interprocess.synchronization_mechanisms.file_lock
LockFileEx:
http://msdn.microsoft.com/library/windows/desktop/aa365203%28v=vs.85%29.aspx
UnlockFileEx:
http://msdn.microsoft.com/library/windows/desktop/aa365716%28v=vs.85%29.aspx
fcntl (linux):
   http://linux.die.net/man/2/fcntl
fcntl (mac):
https://developer.apple.com/library/Mac/documentation/Darwin/Reference/ManPages/man2/fcntl.2.html

I've tried (Un)LockFileEx/fcntl on windows (including mingw and cygwin), 
linux and mac - works fine for me with one exception: cygwin's lock is 
not visible by win32's lock. I.e. you can synchronize multiple cygwin 
processes and multiple windows "normal" processes, but you can't mix them.

On 13-Oct-14 18:34, Ben Boeckel wrote:
> Maybe we need something like the 'trap' shell builtin which runs code on
> various triggers rather than something like file/directory locks...
Note that you can't set trap for SIGKILL. So if somebody will terminate 
your CMake instance by `kill -9 <pid>` probably you will have a problem.