| Commit message (Collapse) | Author | Age |
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The doxygen comments actually make the source code harder to read,
because they tend to document the obvious. Besides, they are not used
uniformly accross the code base anyway.
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It's easier if dss.c passes the hash of the the config file path to
the ipc functions than to pass a char pointer.
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This gets rid of existing copyright templates in favor of just the
one-liner SPDX (Software Package Data Exchange) notice. All files are
licensed under the GPL-2.0, so the same tag is added to each file. No
copyright is changed by this commit.
Several files (mostly the very short ones) did not contain a license
text so far. By default all files without license information are
under the default license of this package, which is GPL version 2.
This commit adds the missing SPDX line so that now all files except
dss.css, index.html.in, INSTALL, NEWS and README have it.
We also remove author and copyright year, since the author is the same
everywhere, and the year hasn't been updated any more since at least
six years. Accurate information is available from the git log.
The COPYING file can also be removed because the license text at
https://spdx.org/licenses/GPL-2.0.html is immutable.
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It works as follows: Whenever a semaphore operation is performed, the
PID of the process is stored in the sempid field of the semaphore.
This PID can be obtained from a different process by calling semctl
with the GETPID command.
com_kill() first tries to acquire the lock by calling the new
mutex_try_lock() function of ipc.c. In contrast to mutex_lock(),
mutex_try_lock() only operates on the first semaphore in the semaphore
set, leaving the sempid field of the second semaphore unchanged. If
mutex_try_lock() succeeds, no running dss process is holding the lock
and the kill command fails. Otherwise, some dss process is running
whose PID can be obtained by calling semctl() on the second semaphore.
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It's trickier than one might expect but it is hopefully also much
better than any pidfile-based approach.
This patch adds ipc.c and ipc.h containing the public lock_dss()
function which acquires a semaphore-based lock whose key is based
on the hash of the resolved path name of the dss config file. This
allows different instances of dss to coexist.
All semaphore operations are called with both the SEM_UNDO and the
IPC_NOWAIT flag. SEM_UNDO guarantees that no stale lock remains after
dss was killed by SIGKIlL while IPC_NOWAIT makes the call to lock_dss()
fail if another process is already holding the lock.
The prune/create/run commands simply take the lock at startup and
exit if it could not be acquired.
The underlying semaphore set contains two semaphores. This is necessary
to implement the --kill subcommand which is done in a subsequent patch.
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