156 lines
4 KiB
C++
156 lines
4 KiB
C++
#include "pathlocks.hh"
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#include <fcntl.h>
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#include <sys/file.h>
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#include <sys/stat.h>
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#include <sys/types.h>
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#include <cerrno>
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#include <cstdlib>
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#include "sync.hh"
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#include "util.hh"
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namespace nix {
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AutoCloseFD openLockFile(const Path& path, bool create) {
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AutoCloseFD fd;
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fd = open(path.c_str(), O_CLOEXEC | O_RDWR | (create ? O_CREAT : 0), 0600);
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if (!fd && (create || errno != ENOENT))
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throw SysError(format("opening lock file '%1%'") % path);
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return fd;
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}
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void deleteLockFile(const Path& path, int fd) {
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/* Get rid of the lock file. Have to be careful not to introduce
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races. Write a (meaningless) token to the file to indicate to
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other processes waiting on this lock that the lock is stale
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(deleted). */
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unlink(path.c_str());
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writeFull(fd, "d");
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/* Note that the result of unlink() is ignored; removing the lock
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file is an optimisation, not a necessity. */
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}
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bool lockFile(int fd, LockType lockType, bool wait) {
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int type;
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if (lockType == ltRead)
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type = LOCK_SH;
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else if (lockType == ltWrite)
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type = LOCK_EX;
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else if (lockType == ltNone)
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type = LOCK_UN;
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else
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abort();
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if (wait) {
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while (flock(fd, type) != 0) {
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checkInterrupt();
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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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return false;
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}
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} else {
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while (flock(fd, type | LOCK_NB) != 0) {
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checkInterrupt();
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if (errno == EWOULDBLOCK) return false;
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if (errno != EINTR) 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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PathLocks::PathLocks() : deletePaths(false) {}
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PathLocks::PathLocks(const PathSet& paths, const string& waitMsg)
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: deletePaths(false) {
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lockPaths(paths, waitMsg);
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}
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bool PathLocks::lockPaths(const PathSet& paths, const string& waitMsg,
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bool wait) {
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assert(fds.empty());
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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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/* Acquire the lock for each path in sorted order. This ensures
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that locks are always acquired in the same order, thus
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preventing deadlocks. */
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for (auto& path : paths) {
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checkInterrupt();
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Path lockPath = path + ".lock";
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debug(format("locking path '%1%'") % path);
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AutoCloseFD fd;
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while (1) {
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/* Open/create the lock file. */
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fd = openLockFile(lockPath, true);
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/* Acquire an exclusive lock. */
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if (!lockFile(fd.get(), ltWrite, false)) {
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if (wait) {
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if (waitMsg != "") printError(waitMsg);
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lockFile(fd.get(), ltWrite, true);
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} else {
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/* Failed to lock this path; release all other
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locks. */
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unlock();
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return false;
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}
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}
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debug(format("lock acquired on '%1%'") % lockPath);
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/* Check that the lock file hasn't become stale (i.e.,
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hasn't been unlinked). */
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struct stat st;
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if (fstat(fd.get(), &st) == -1)
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throw SysError(format("statting lock file '%1%'") % lockPath);
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if (st.st_size != 0)
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/* This lock file has been unlinked, so we're holding
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a lock on a deleted file. This means that other
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processes may create and acquire a lock on
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`lockPath', and proceed. So we must retry. */
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debug(format("open lock file '%1%' has become stale") % lockPath);
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else
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break;
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}
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/* Use borrow so that the descriptor isn't closed. */
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fds.push_back(FDPair(fd.release(), lockPath));
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}
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return true;
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}
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PathLocks::~PathLocks() {
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try {
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unlock();
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} catch (...) {
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ignoreException();
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}
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}
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void PathLocks::unlock() {
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for (auto& i : fds) {
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if (deletePaths) deleteLockFile(i.second, i.first);
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if (close(i.first) == -1)
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printError(format("error (ignored): cannot close lock file on '%1%'") %
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i.second);
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debug(format("lock released on '%1%'") % i.second);
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}
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fds.clear();
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}
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void PathLocks::setDeletion(bool deletePaths) {
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this->deletePaths = deletePaths;
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}
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} // namespace nix
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