* Defensive programming against POSIX locking idiocy.

* Simplified realiseSlice().

src/pathlocks.cc

@@ -3,6 +3,14 @@
 #include "pathlocks.hh"
 
 
+/* 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 Strings & _paths)
 {
     /* Note that `fds' is built incrementally so that the destructor
@@ -19,6 +27,9 @@ PathLocks::PathLocks(const Strings & _paths)
         string lockPath = path + ".lock";
 
         debug(format("locking path `%1%'") % path);
+
+        if (lockedPaths.find(lockPath) != lockedPaths.end())
+            throw Error(format("already holding lock on `%1%'") % lockPath);
         
         /* Open/create the lock file. */
         int fd = open(lockPath.c_str(), O_WRONLY | O_CREAT, 0666);
@@ -26,6 +37,7 @@ PathLocks::PathLocks(const Strings & _paths)
             throw SysError(format("opening lock file `%1%'") % lockPath);
 
         fds.push_back(fd);
+        this->paths.push_back(lockPath);
 
         /* Lock it. */
         struct flock lock;
@@ -37,6 +49,8 @@ PathLocks::PathLocks(const Strings & _paths)
         while (fcntl(fd, F_SETLKW, &lock) == -1)
             if (errno != EINTR)
                 throw SysError(format("acquiring lock on `%1%'") % lockPath);
+
+        lockedPaths.insert(lockPath);
     }
 }
 
@@ -45,4 +59,7 @@ PathLocks::~PathLocks()
 {
     for (list<int>::iterator i = fds.begin(); i != fds.end(); i++)
         close(*i);
+
+    for (Strings::iterator i = paths.begin(); i != paths.end(); i++)
+        lockedPaths.erase(*i);
 }