On 08-Oct-14 16:45, Brad King wrote:
On 10/08/2014 07:52 AM, Ruslan Baratov wrote:
Okay :) I just not sure that I understand "to pass to some other
process" goal correctly.
That was just an example of why one might want to drop management
of the lock by CMake without actually unlocking it. Perhaps the
code is starting a daemon and passes off responsibility for the
unlock side to the daemon process.
Okay, got it.
* we just need to `unlock` file so the other instance will use it:
file(UNLOCK_FILE "/path/to/shared/file")
# now anybody can do the lock
Yes. We also need the locking API to return information about
whether we really acquired the lock.
So it can be TRY_LOCK and LOCK. But does the TRY_LOCK really needed?
With LOCK we can try to lock and spin until acquire, but what to do with
TRY_LOCK? Just give up?
* we need other process to "inherit" the lock (what for?), i.e. move
ownership (?):
file(LOCK_FILE "/path/to/shared/file")
execute_process(${CMAKE_COMMAND} --take-ownership "/path/to/shared/file"
...)
# unlocked by execute_process
I think all we need there is a way to ask CMake to take over
responsibility for a lock that it did not originally create.
It can also be in the file() command.
Thanks,
-Brad
Okay, so here is the commands inspired by C++:
file(TRY_LOCK "/path/to/file" result) # try to lock and return TRUE
on success (needed?)
file(LOCK ...) # lock until unlock (unlock by further code (what if
exit/crashed?) or by deamon)
file(LOCK_GUARD ...) # lock for scope of current function or until exit
file(UNLOCK ...) # explicit unlock of LOCK/LOCK_GUARD
Locking directory is equivalent to locking file `cmake.lock` in
directory "/path/to/shared/directory/":
file(DIRECTORY_TRY_LOCK ...)
file(DIRECTORY_LOCK "/path/to/shared/directory/") # == file(LOCK
"/path/to/shared/directory/cmake.lock")
file(DIRECTORY_LOCK_GUARD ...)
file(DIRECTORY_UNLOCK ...)
If any of this commands failed (e.g. have no permissions) - exit with
unsuccessful code (i.e. FATAL_ERROR) ?
* http://en.cppreference.com/w/cpp/thread/try_lock
* http://en.cppreference.com/w/cpp/thread/lock
* http://en.cppreference.com/w/cpp/thread/lock_guard
* http://en.cppreference.com/w/cpp/thread/mutex/unlock
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