244 lines
7.1 KiB
C++
244 lines
7.1 KiB
C++
#include "pathlocks.hh"
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#include "util.hh"
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#include <cerrno>
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#include <cstdlib>
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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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#ifdef __CYGWIN__
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#include <windows.h>
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#include <sys/cygwin.h>
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#endif
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namespace nix {
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int openLockFile(const Path & path, bool create)
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{
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AutoCloseFD fd;
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#ifdef __CYGWIN__
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/* On Cygwin we have to open the lock file without "DELETE"
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sharing mode; otherwise Windows will allow open lock files to
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be deleted (which is almost but not quite what Unix does). */
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char win32Path[MAX_PATH + 1];
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cygwin_conv_to_full_win32_path(path.c_str(), win32Path);
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SECURITY_ATTRIBUTES sa; /* required, otherwise inexplicably bad shit happens */
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sa.nLength = sizeof sa;
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sa.lpSecurityDescriptor = 0;
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sa.bInheritHandle = TRUE;
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HANDLE h = CreateFile(win32Path, GENERIC_READ | GENERIC_WRITE,
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FILE_SHARE_READ | FILE_SHARE_WRITE, &sa,
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(create ? OPEN_ALWAYS : OPEN_EXISTING),
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FILE_ATTRIBUTE_NORMAL, 0);
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if (h == INVALID_HANDLE_VALUE) {
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if (create || GetLastError() != ERROR_FILE_NOT_FOUND)
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throw Error(format("opening lock file `%1%'") % path);
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fd = -1;
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}
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else
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fd = cygwin_attach_handle_to_fd((char *) path.c_str(), -1, h, 1, O_RDWR);
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#else
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fd = open(path.c_str(), O_RDWR | (create ? O_CREAT : 0), 0666);
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if (fd == -1 && (create || errno != ENOENT))
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throw SysError(format("opening lock file `%1%'") % path);
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#endif
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return fd.borrow();
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}
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void deleteLockFilePreClose(const Path & path, int fd)
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{
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#ifndef __CYGWIN__
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/* Get rid of the lock file. Have to be careful not to introduce
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races. */
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/* On Unix, 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, (const unsigned char *) "d", 1);
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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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#endif
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}
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void deleteLockFilePostClose(const Path & path)
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{
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#ifdef __CYGWIN__
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/* On Windows, just try to delete the lock file. This will fail
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if anybody still has the file open. We cannot use unlink()
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here, because Cygwin emulates Unix semantics of allowing an
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open file to be deleted (but fakes it - the file isn't actually
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deleted until later, so a file with the same name cannot be
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created in the meantime). */
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char win32Path[MAX_PATH + 1];
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cygwin_conv_to_full_win32_path(path.c_str(), win32Path);
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if (DeleteFile(win32Path))
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debug(format("delete of `%1%' succeeded") % path.c_str());
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else
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/* Not an error: probably means that the lock is still opened
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by someone else. */
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debug(format("delete of `%1%' failed: %2%") % path.c_str() % GetLastError());
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#endif
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}
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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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checkInterrupt();
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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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} else {
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while (fcntl(fd, F_SETLK, &lock) != 0) {
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checkInterrupt();
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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()
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: deletePaths(false)
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{
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}
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PathLocks::PathLocks(const PathSet & paths, const string & waitMsg)
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: deletePaths(false)
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{
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lockPaths(paths, waitMsg);
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}
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void PathLocks::lockPaths(const PathSet & _paths, const string & waitMsg)
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{
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/* May be called only once! */
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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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/* 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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checkInterrupt();
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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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throw Error("deadlock: trying to re-acquire self-held lock");
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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, ltWrite, false)) {
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if (waitMsg != "") printMsg(lvlError, waitMsg);
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lockFile(fd, ltWrite, true);
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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, &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.borrow(), lockPath));
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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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unlock();
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}
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void PathLocks::unlock()
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{
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for (list<FDPair>::iterator i = fds.begin(); i != fds.end(); i++) {
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if (deletePaths) deleteLockFilePreClose(i->second, i->first);
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lockedPaths.erase(i->second);
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if (close(i->first) == -1)
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printMsg(lvlError,
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format("error (ignored): cannot close lock file on `%1%'") % i->second);
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if (deletePaths) deleteLockFilePostClose(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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{
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this->deletePaths = deletePaths;
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}
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bool pathIsLockedByMe(const Path & path)
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{
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Path lockPath = path + ".lock";
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return lockedPaths.find(lockPath) != lockedPaths.end();
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}
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}
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