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#include "pathlocks.hh"
#include "util.hh"
#include "sync.hh"
#include <cerrno>
#include <cstdlib>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
namespace nix {
AutoCloseFD openLockFile(const Path & path, bool create)
{
AutoCloseFD fd;
fd = open(path.c_str(), O_CLOEXEC | O_RDWR | (create ? O_CREAT : 0), 0600);
if (!fd && (create || errno != ENOENT))
throw SysError(format("opening lock file '%1%'") % path);
return fd;
}
void deleteLockFile(const Path & path, int fd)
{
/* Get rid of the lock file. Have to be careful not to introduce
races. Write a (meaningless) token to the file to indicate to
other processes waiting on this lock that the lock is stale
(deleted). */
unlink(path.c_str());
writeFull(fd, "d");
/* Note that the result of unlink() is ignored; removing the lock
file is an optimisation, not a necessity. */
}
bool lockFile(int fd, LockType lockType, bool wait)
{
struct flock lock;
if (lockType == ltRead) lock.l_type = F_RDLCK;
else if (lockType == ltWrite) lock.l_type = F_WRLCK;
else if (lockType == ltNone) lock.l_type = F_UNLCK;
else abort();
lock.l_whence = SEEK_SET;
lock.l_start = 0;
lock.l_len = 0; /* entire file */
if (wait) {
while (fcntl(fd, F_SETLKW, &lock) != 0) {
checkInterrupt();
if (errno != EINTR)
throw SysError(format("acquiring/releasing lock"));
else
return false;
}
} else {
while (fcntl(fd, F_SETLK, &lock) != 0) {
checkInterrupt();
if (errno == EACCES || errno == EAGAIN) return false;
if (errno != EINTR)
throw SysError(format("acquiring/releasing lock"));
}
}
return true;
}
/* 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 Sync<StringSet> lockedPaths_;
PathLocks::PathLocks()
: deletePaths(false)
{
}
PathLocks::PathLocks(const PathSet & paths, const string & waitMsg)
: deletePaths(false)
{
lockPaths(paths, waitMsg);
}
bool PathLocks::lockPaths(const PathSet & _paths,
const string & waitMsg, bool wait)
{
assert(fds.empty());
/* 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. */
Paths paths(_paths.begin(), _paths.end());
paths.sort();
/* Acquire the lock for each path. */
for (auto & path : paths) {
checkInterrupt();
Path lockPath = path + ".lock";
debug(format("locking path '%1%'") % path);
{
auto lockedPaths(lockedPaths_.lock());
if (lockedPaths->count(lockPath)) {
if (!wait) return false;
throw AlreadyLocked("deadlock: trying to re-acquire self-held lock '%s'", lockPath);
}
lockedPaths->insert(lockPath);
}
try {
AutoCloseFD fd;
while (1) {
/* Open/create the lock file. */
fd = openLockFile(lockPath, true);
/* Acquire an exclusive lock. */
if (!lockFile(fd.get(), ltWrite, false)) {
if (wait) {
if (waitMsg != "") printError(waitMsg);
lockFile(fd.get(), ltWrite, true);
} else {
/* Failed to lock this path; release all other
locks. */
unlock();
lockedPaths_.lock()->erase(lockPath);
return false;
}
}
debug(format("lock acquired on '%1%'") % lockPath);
/* Check that the lock file hasn't become stale (i.e.,
hasn't been unlinked). */
struct stat st;
if (fstat(fd.get(), &st) == -1)
throw SysError(format("statting lock file '%1%'") % lockPath);
if (st.st_size != 0)
/* This lock file has been unlinked, so we're holding
a lock on a deleted file. This means that other
processes may create and acquire a lock on
`lockPath', and proceed. So we must retry. */
debug(format("open lock file '%1%' has become stale") % lockPath);
else
break;
}
/* Use borrow so that the descriptor isn't closed. */
fds.push_back(FDPair(fd.release(), lockPath));
} catch (...) {
lockedPaths_.lock()->erase(lockPath);
throw;
}
}
return true;
}
PathLocks::~PathLocks()
{
try {
unlock();
} catch (...) {
ignoreException();
}
}
void PathLocks::unlock()
{
for (auto & i : fds) {
if (deletePaths) deleteLockFile(i.second, i.first);
lockedPaths_.lock()->erase(i.second);
if (close(i.first) == -1)
printError(
format("error (ignored): cannot close lock file on '%1%'") % i.second);
debug(format("lock released on '%1%'") % i.second);
}
fds.clear();
}
void PathLocks::setDeletion(bool deletePaths)
{
this->deletePaths = deletePaths;
}
bool pathIsLockedByMe(const Path & path)
{
Path lockPath = path + ".lock";
return lockedPaths_.lock()->count(lockPath);
}
}
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