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#include <fcntl.h>
#include "pathlocks.hh"
/* This enables us to check whether are not already holding a lock on
a file ourselves. POSIX locks (fcntl) suck in this respect: if we
close a descriptor, the previous lock will be closed as well. And
there is no way to query whether we already have a lock (F_GETLK
only works on locks held by other processes). */
static StringSet lockedPaths; /* !!! not thread-safe */
PathLocks::PathLocks(const Strings & _paths)
{
/* Note that `fds' is built incrementally so that the destructor
will only release those locks that we have already acquired. */
/* Sort the paths. This assures that locks are always acquired in
the same order, thus preventing deadlocks. */
Strings paths(_paths);
paths.sort();
/* Acquire the lock for each path. */
for (Strings::iterator i = paths.begin(); i != paths.end(); i++) {
string path = *i;
string lockPath = path + ".lock";
debug(format("locking path `%1%'") % path);
if (lockedPaths.find(lockPath) != lockedPaths.end())
throw Error(format("already holding lock on `%1%'") % lockPath);
/* Open/create the lock file. */
int fd = open(lockPath.c_str(), O_WRONLY | O_CREAT, 0666);
if (fd == -1)
throw SysError(format("opening lock file `%1%'") % lockPath);
fds.push_back(fd);
this->paths.push_back(lockPath);
/* Lock it. */
struct flock lock;
lock.l_type = F_WRLCK; /* exclusive lock */
lock.l_whence = SEEK_SET;
lock.l_start = 0;
lock.l_len = 0; /* entire file */
while (fcntl(fd, F_SETLKW, &lock) == -1)
if (errno != EINTR)
throw SysError(format("acquiring lock on `%1%'") % lockPath);
lockedPaths.insert(lockPath);
}
}
PathLocks::~PathLocks()
{
for (list<int>::iterator i = fds.begin(); i != fds.end(); i++)
close(*i);
for (Strings::iterator i = paths.begin(); i != paths.end(); i++)
lockedPaths.erase(*i);
}
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