1 files changed, 199 insertions, 0 deletions
diff --git a/src/libstore/pathlocks.cc b/src/libstore/pathlocks.cc
new file mode 100644
index 000000000000..b858ed238de0
--- /dev/null
+++ b/src/libstore/pathlocks.cc
@@ -0,0 +1,199 @@
+#include "pathlocks.hh"
+#include "util.hh"
+
+#include <cerrno>
+#include <cstdlib>
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+
+
+namespace nix {
+
+
+int openLockFile(const Path & path, bool create)
+{
+    AutoCloseFD fd;
+
+    fd = open(path.c_str(), O_RDWR | (create ? O_CREAT : 0), 0600);
+    if (fd == -1 && (create || errno != ENOENT))
+        throw SysError(format("opening lock file `%1%'") % path);
+
+    closeOnExec(fd);
+
+    return fd.borrow();
+}
+
+
+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, (const unsigned char *) "d", 1);
+    /* 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)
+{
+    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);
+}
+
+
+bool PathLocks::lockPaths(const PathSet & _paths,
+    const 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. */
+
+    /* 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. */
+    foreach (Paths::iterator, i, paths) {
+        checkInterrupt();
+        Path path = *i;
+        Path lockPath = path + ".lock";
+
+        debug(format("locking path `%1%'") % path);
+
+        if (lockedPaths.find(lockPath) != lockedPaths.end())
+            throw Error("deadlock: trying to re-acquire self-held lock");
+
+        AutoCloseFD fd;
+        
+        while (1) {
+
+            /* Open/create the lock file. */
+	    fd = openLockFile(lockPath, true);
+
+            /* Acquire an exclusive lock. */
+            if (!lockFile(fd, ltWrite, false)) {
+                if (wait) {
+                    if (waitMsg != "") printMsg(lvlError, waitMsg);
+                    lockFile(fd, ltWrite, true);
+                } else {
+                    /* Failed to lock this path; release all other
+                       locks. */
+                    unlock();
+                    return false;
+                }
+            }
+
+            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);
+    }
+
+    return true;
+}
+
+
+PathLocks::~PathLocks()
+{
+    unlock();
+}
+
+
+void PathLocks::unlock()
+{
+    foreach (list<FDPair>::iterator, i, fds) {
+        if (deletePaths) deleteLockFile(i->second, i->first);
+
+        lockedPaths.erase(i->second);
+        if (close(i->first) == -1)
+            printMsg(lvlError,
+                format("error (ignored): cannot close lock file on `%1%'") % i->second);
+
+        debug(format("lock released on `%1%'") % i->second);
+    }
+
+    fds.clear();
+}
+
+
+void PathLocks::setDeletion(bool deletePaths)
+{
+    this->deletePaths = deletePaths;
+}
+
+
+bool pathIsLockedByMe(const Path & path)
+{
+    Path lockPath = path + ".lock";
+    return lockedPaths.find(lockPath) != lockedPaths.end();
+}
+
+ 
+}