1 files changed, 90 insertions, 0 deletions
diff --git a/src/libnix/pathlocks.cc b/src/libnix/pathlocks.cc
new file mode 100644
index 000000000..3ecbbbcba
--- /dev/null
+++ b/src/libnix/pathlocks.cc
@@ -0,0 +1,90 @@
+#include <cerrno>
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+
+#include "pathlocks.hh"
+
+
+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)
+            if (errno != EINTR)
+                throw SysError(format("acquiring/releasing lock"));
+    } else {
+        while (fcntl(fd, F_SETLK, &lock) != 0) {
+            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(const PathSet & _paths)
+{
+    /* 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++) {
+        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;
+        }
+        
+        /* Open/create the lock file. */
+        int fd = open(lockPath.c_str(), O_WRONLY | O_CREAT, 0666);
+        if (fd == -1)
+            throw SysError(format("opening lock file `%1%'") % lockPath);
+
+        fds.push_back(fd);
+        this->paths.push_back(lockPath);
+
+        /* Acquire an exclusive lock. */
+        lockFile(fd, ltWrite, true);
+
+        lockedPaths.insert(lockPath);
+    }
+}
+
+
+PathLocks::~PathLocks()
+{
+    for (list<int>::iterator i = fds.begin(); i != fds.end(); i++)
+        close(*i);
+
+    for (Paths::iterator i = paths.begin(); i != paths.end(); i++)
+        lockedPaths.erase(*i);
+}