* Make lock removal safe by signalling to blocked processes that the
lock they are waiting on has become stale (we do this by writing a meaningless token to the unlinked file).
This commit is contained in:
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a24b78e9f1
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59682e6188
4 changed files with 52 additions and 21 deletions
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@ -61,7 +61,7 @@ PathLocks::PathLocks(const PathSet & paths)
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void PathLocks::lockPaths(const PathSet & _paths)
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void PathLocks::lockPaths(const PathSet & _paths)
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{
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{
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/* May be called only once! */
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/* May be called only once! */
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assert(this->paths.empty());
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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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/* 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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will only release those locks that we have already acquired. */
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@ -84,19 +84,37 @@ void PathLocks::lockPaths(const PathSet & _paths)
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continue;
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continue;
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}
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}
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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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/* Open/create the lock file. */
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int fd = open(lockPath.c_str(), O_WRONLY | O_CREAT, 0666);
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fd = open(lockPath.c_str(), O_WRONLY | O_CREAT, 0666);
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if (fd == -1)
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if (fd == -1)
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throw SysError(format("opening lock file `%1%'") % lockPath);
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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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/* Acquire an exclusive lock. */
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lockFile(fd, ltWrite, true);
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lockFile(fd, ltWrite, true);
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debug(format("lock acquired on `%1%'") % lockPath);
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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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lockedPaths.insert(lockPath);
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}
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}
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}
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}
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@ -104,19 +122,22 @@ void PathLocks::lockPaths(const PathSet & _paths)
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PathLocks::~PathLocks()
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PathLocks::~PathLocks()
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{
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{
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for (list<int>::iterator i = fds.begin(); i != fds.end(); i++)
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for (list<FDPair>::iterator i = fds.begin(); i != fds.end(); i++) {
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if (close(*i) != 0) throw SysError("closing fd");
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for (Paths::iterator i = paths.begin(); i != paths.end(); i++) {
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checkInterrupt();
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if (deletePaths) {
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if (deletePaths) {
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/* This is not safe in general! */
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/* Write a (meaningless) token to the file to indicate to
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unlink(i->c_str());
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other processes waiting on this lock that the lock is
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stale (deleted). */
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if (write(i->first, "d", 1) == 1) {
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unlink(i->second.c_str());
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}
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/* Note that the result of unlink() is ignored; removing
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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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the lock file is an optimisation, not a necessity. */
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}
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}
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lockedPaths.erase(*i);
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lockedPaths.erase(i->second);
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debug(format("lock released on `%1%'") % *i);
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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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debug(format("lock released on `%1%'") % i->second);
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}
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}
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}
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}
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@ -12,8 +12,8 @@ bool lockFile(int fd, LockType lockType, bool wait);
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class PathLocks
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class PathLocks
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{
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{
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private:
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private:
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list<int> fds;
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typedef pair<int, Path> FDPair;
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Paths paths;
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list<FDPair> fds;
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bool deletePaths;
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bool deletePaths;
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public:
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public:
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@ -415,6 +415,15 @@ bool AutoCloseFD::isOpen()
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}
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}
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/* Pass responsibility for closing this fd to the caller. */
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int AutoCloseFD::borrow()
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{
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int oldFD = fd;
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fd = -1;
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return oldFD;
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}
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void Pipe::create()
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void Pipe::create()
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{
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{
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int fds[2];
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int fds[2];
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@ -184,6 +184,7 @@ public:
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operator int();
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operator int();
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void close();
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void close();
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bool isOpen();
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bool isOpen();
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int borrow();
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};
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};
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