105 lines
3.0 KiB
C++
105 lines
3.0 KiB
C++
#include <cerrno>
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#include <sys/types.h>
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#include <sys/stat.h>
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#include <fcntl.h>
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#include "pathlocks.hh"
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bool lockFile(int fd, LockType lockType, bool wait)
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{
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struct flock lock;
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if (lockType == ltRead) lock.l_type = F_RDLCK;
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else if (lockType == ltWrite) lock.l_type = F_WRLCK;
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else if (lockType == ltNone) lock.l_type = F_UNLCK;
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else abort();
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lock.l_whence = SEEK_SET;
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lock.l_start = 0;
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lock.l_len = 0; /* entire file */
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if (wait) {
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while (fcntl(fd, F_SETLKW, &lock) != 0)
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if (errno != EINTR)
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throw SysError(format("acquiring/releasing lock"));
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} else {
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while (fcntl(fd, F_SETLK, &lock) != 0) {
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if (errno == EACCES || errno == EAGAIN) return false;
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if (errno != EINTR)
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throw SysError(format("acquiring/releasing lock"));
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}
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}
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return true;
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}
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/* This enables us to check whether are not already holding a lock on
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a file ourselves. POSIX locks (fcntl) suck in this respect: if we
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close a descriptor, the previous lock will be closed as well. And
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there is no way to query whether we already have a lock (F_GETLK
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only works on locks held by other processes). */
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static StringSet lockedPaths; /* !!! not thread-safe */
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PathLocks::PathLocks(const PathSet & _paths)
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: deletePaths(false)
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{
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/* Note that `fds' is built incrementally so that the destructor
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will only release those locks that we have already acquired. */
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/* Sort the paths. This assures that locks are always acquired in
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the same order, thus preventing deadlocks. */
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Paths paths(_paths.begin(), _paths.end());
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paths.sort();
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/* Acquire the lock for each path. */
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for (Paths::iterator i = paths.begin(); i != paths.end(); i++) {
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Path path = *i;
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Path lockPath = path + ".lock";
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debug(format("locking path `%1%'") % path);
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if (lockedPaths.find(lockPath) != lockedPaths.end()) {
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debug(format("already holding lock on `%1%'") % lockPath);
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continue;
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}
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/* Open/create the lock file. */
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int fd = open(lockPath.c_str(), O_WRONLY | O_CREAT, 0666);
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if (fd == -1)
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throw SysError(format("opening lock file `%1%'") % lockPath);
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fds.push_back(fd);
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this->paths.push_back(lockPath);
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/* Acquire an exclusive lock. */
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lockFile(fd, ltWrite, true);
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lockedPaths.insert(lockPath);
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}
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}
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PathLocks::~PathLocks()
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{
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for (list<int>::iterator i = fds.begin(); i != fds.end(); i++)
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close(*i);
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for (Paths::iterator i = paths.begin(); i != paths.end(); i++) {
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if (deletePaths) {
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/* This is not safe in general! */
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unlink(i->c_str());
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/* Note that the result of unlink() is ignored; removing
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the lock file is an optimisation, not a necessity. */
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}
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lockedPaths.erase(*i);
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}
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}
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void PathLocks::setDeletion(bool deletePaths)
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{
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this->deletePaths = deletePaths;
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}
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