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#include "libstore/pathlocks.hh"
#include <cerrno>
#include <cstdlib>
#include <fcntl.h>
#include <glog/logging.h>
#include <sys/file.h>
#include <sys/stat.h>
#include <sys/types.h>
#include "libutil/sync.hh"
#include "libutil/util.hh"
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) {
int type = 0;
if (lockType == ltRead) {
type = LOCK_SH;
} else if (lockType == ltWrite) {
type = LOCK_EX;
} else if (lockType == ltNone) {
type = LOCK_UN;
} else {
abort();
}
if (wait) {
while (flock(fd, type) != 0) {
checkInterrupt();
if (errno != EINTR) {
throw SysError(format("acquiring/releasing lock"));
}
return false;
}
} else {
while (flock(fd, type | LOCK_NB) != 0) {
checkInterrupt();
if (errno == EWOULDBLOCK) {
return false;
}
if (errno != EINTR) {
throw SysError(format("acquiring/releasing lock"));
}
}
}
return true;
}
PathLocks::PathLocks() : deletePaths(false) {}
PathLocks::PathLocks(const PathSet& paths, const std::string& waitMsg)
: deletePaths(false) {
lockPaths(paths, waitMsg);
}
bool PathLocks::lockPaths(const PathSet& paths, const std::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. */
/* Acquire the lock for each path in sorted order. This ensures
that locks are always acquired in the same order, thus
preventing deadlocks. */
for (auto& path : paths) {
checkInterrupt();
Path lockPath = path + ".lock";
DLOG(INFO) << "locking path '" << path << "'";
AutoCloseFD fd;
while (true) {
/* Open/create the lock file. */
fd = openLockFile(lockPath, true);
/* Acquire an exclusive lock. */
if (!lockFile(fd.get(), ltWrite, false)) {
if (wait) {
if (!waitMsg.empty()) {
LOG(WARNING) << waitMsg;
}
lockFile(fd.get(), ltWrite, true);
} else {
/* Failed to lock this path; release all other
locks. */
unlock();
return false;
}
}
DLOG(INFO) << "lock acquired on '" << 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. */
DLOG(INFO) << "open lock file '" << lockPath << "' has become stale";
} else {
break;
}
}
/* Use borrow so that the descriptor isn't closed. */
fds.emplace_back(fd.release(), lockPath);
}
return true;
}
PathLocks::~PathLocks() {
try {
unlock();
} catch (...) {
ignoreException();
}
}
void PathLocks::unlock() {
for (auto& i : fds) {
if (deletePaths) {
deleteLockFile(i.second, i.first);
}
if (close(i.first) == -1) {
LOG(WARNING) << "cannot close lock file on '" << i.second << "'";
}
DLOG(INFO) << "lock released on '" << i.second << "'";
}
fds.clear();
}
void PathLocks::setDeletion(bool deletePaths) {
this->deletePaths = deletePaths;
}
} // namespace nix
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