nix/src/libstore/pathlocks.cc

#include <cerrno>

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

#include "pathlocks.hh"

#ifdef __CYGWIN__
#include <windows.h>
#include <sys/cygwin.h>
#endif


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 {
        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 StringSet lockedPaths; /* !!! not thread-safe */


PathLocks::PathLocks()
    : deletePaths(false)
{
}


PathLocks::PathLocks(const PathSet & paths, const string & waitMsg)
    : deletePaths(false)
{
    lockPaths(paths, waitMsg);
}


void PathLocks::lockPaths(const PathSet & _paths, const string & waitMsg)
{
    /* May be called only once! */
    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 (Paths::iterator i = paths.begin(); i != paths.end(); i++) {
        checkInterrupt();
        Path path = *i;
        Path lockPath = path + ".lock";

        debug(format("locking path `%1%'") % path);

        if (lockedPaths.find(lockPath) != lockedPaths.end()) {
            debug(format("already holding lock on `%1%'") % lockPath);
            continue;
        }

        AutoCloseFD fd;
        
        while (1) {

            /* Open/create the lock file. */
#ifdef __CYGWIN__
	    char win32Path[MAX_PATH];
	    cygwin_conv_to_full_win32_path(lockPath.c_str(), win32Path);

	    SECURITY_ATTRIBUTES sa;
	    sa.nLength = sizeof sa;
	    sa.lpSecurityDescriptor = 0;
	    sa.bInheritHandle = TRUE;
	    HANDLE h = CreateFile(win32Path, GENERIC_READ, 
	        FILE_SHARE_READ | FILE_SHARE_WRITE, &sa, OPEN_ALWAYS, 
                FILE_ATTRIBUTE_NORMAL, 0);
	    if (h == INVALID_HANDLE_VALUE)
		throw Error(format("opening lock file `%1%'") % lockPath);

	    fd = cygwin_attach_handle_to_fd((char *) lockPath.c_str(), -1, h, 1, O_RDWR);
#else        
            fd = open(lockPath.c_str(), O_WRONLY | O_CREAT, 0666);
            if (fd == -1)
                throw SysError(format("opening lock file `%1%'") % lockPath);
#endif

            /* Acquire an exclusive lock. */
            if (!lockFile(fd, ltWrite, false)) {
                if (waitMsg != "") printMsg(lvlError, waitMsg);
                lockFile(fd, ltWrite, true);
            }

            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, &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.borrow(), lockPath));
        lockedPaths.insert(lockPath);
    }
}


PathLocks::~PathLocks()
{
    for (list<FDPair>::iterator i = fds.begin(); i != fds.end(); i++) {
#ifndef __CYGWIN__
        if (deletePaths) {
	    /* Get rid of the lock file.  Have to be careful not to
	       introduce races. */
            /* On Unix, write a (meaningless) token to the file to
               indicate to other processes waiting on this lock that
               the lock is stale (deleted). */
            unlink(i->second.c_str());
            writeFull(i->first, (const unsigned char *) "d", 1);
            /* Note that the result of unlink() is ignored; removing
               the lock file is an optimisation, not a necessity. */
        }
#endif
        lockedPaths.erase(i->second);
        if (close(i->first) == -1)
            printMsg(lvlError,
                format("error (ignored): cannot close lock file on `%1%'") % i->second);
#ifdef __CYGWIN__
	if (deletePaths) {
	    /* On Windows, just try to delete the lock file.  This
	       will fail if anybody still has the file open.  We
	       cannot use unlink() here, because Cygwin emulates Unix
	       semantics of allowing an open file to be deleted (but
	       fakes it - the file isn't actually deleted until later,
	       so a file with the same name cannot be created in the
	       meantime). */
	    char win32Path[MAX_PATH];
	    cygwin_conv_to_full_win32_path(i->second.c_str(), win32Path);
	    if (DeleteFile(win32Path))
		debug(format("delete of `%1%' succeeded") % i->second.c_str());
	    else
		debug(format("delete of `%1%' failed: %2%") % i->second.c_str() % GetLastError());
	}
#endif
        debug(format("lock released on `%1%'") % i->second);
    }
}


void PathLocks::setDeletion(bool deletePaths)
{
    this->deletePaths = deletePaths;
}