--- Urlader/urlader.c	2011/12/30 12:21:28	1.6
+++ Urlader/urlader.c	2011/12/30 14:51:45	1.7
@@ -48,8 +48,6 @@
 
   #define u_lockfile(path) CreateFile (path, GENERIC_READ | GENERIC_WRITE, FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL)
   #define u_cloexec(handle)
-  // acquire shared in addition to excl, then unlock excl
-  #define u_lock_write_to_read(wait) u_lock (U_LOCK_READ, 1); u_lock (U_UNLOCK, 1);
 
 #else
 
@@ -59,6 +57,8 @@
 
   #include <sys/mman.h>
   #include <sys/types.h>
+  #include <sys/stat.h>
+  #include <unistd.h>
   #include <pwd.h>
 
   #if defined (MAP_ANON) && !defined (MAP_ANONYMOUS)
@@ -91,7 +91,6 @@
 
   #define u_lockfile(path) open (path, O_RDWR | O_CREAT, 0666)
   #define u_cloexec(handle) fcntl (handle, F_SETFD, FD_CLOEXEC)
-  #define u_lock_write_to_read(wait) u_lock (U_LOCK_READ, 1);
 
 #endif
 
@@ -218,41 +217,84 @@
     }
 }
 
-enum
+static u_handle
+u_lock (const char *path, int excl, int dowait)
 {
-  U_UNLOCK,
-  U_LOCK_READ,
-  U_LOCK_WRITE
-};
+  u_handle h;
 
-static int
-u_lock (int mode, int dowait)
-{
-#ifdef _WIN32
-  if (mode == U_UNLOCK)
-    return !UnlockFile (lock_handle, 0, 0, 1, 0);
-  else
+  h = u_lockfile (path);
+  if (!u_valid (h))
+    return h;
+
+  u_cloexec (h);
+
+  for (;;)
     {
+      int success;
+
+      // acquire the lock
+#ifdef _WIN32
       OVERLAPPED ov = { 0 };
 
-      return !LockFileEx (lock_handle,
-                          0
-                          | (mode == U_LOCK_WRITE ? LOCKFILE_EXCLUSIVE_LOCK : 0)
-                          | (dowait ? 0 : LOCKFILE_FAIL_IMMEDIATELY),
-                          0,
-                          1, 0,
-                          &ov);
-    }
+      success = LockFileEx (h,
+                            (excl ? LOCKFILE_EXCLUSIVE_LOCK : 0)
+                            | (dowait ? 0 : LOCKFILE_FAIL_IMMEDIATELY),
+                            0,
+                            1, 0,
+                            &ov);
+#else
+      struct flock lck = { 0 };
+
+      lck.l_type   = excl ? F_WRLCK : F_RDLCK;
+      lck.l_whence = SEEK_SET;
+      lck.l_len    = 1;
+
+      success = !fcntl (h, dowait ? F_SETLKW : F_SETLK, &lck);
+#endif
+
+      if (!success)
+        break;
+
+      // we have the lock, now verify that the lockfile still exists
+
+#ifdef _WIN32
+      // apparently, we have to open the file to get its info :(
+      BY_HANDLE_FILE_INFORMATION s1, s2;
+      u_handle h2 = u_lockfile (path);
+      if (!u_valid (h))
+        break;
+
+      success = GetFileInformationByHandle (h, &s1)
+                && GetFileInformationByHandle (h2, &s2);
+
+      u_close (h2);
+
+      if (!success)
+        break;
+
+      success = s1.dwVolumeSerialNumber == s2.dwVolumeSerialNumber
+             && s1.nFileIndexHigh       == s2.nFileIndexHigh
+             && s1.nFileIndexLow        == s2.nFileIndexLow;
 #else
-  static short mode2lck [] = { F_UNLCK, F_RDLCK, F_WRLCK };
-  struct flock lck;
-  lck.l_type   = mode2lck [mode];
-  lck.l_whence = SEEK_SET;
-  lck.l_start  = 0;
-  lck.l_len    = 1;
+      struct stat s1, s2;
 
-  return fcntl (lock_handle, dowait ? F_SETLKW : F_SETLK, &lck);
+      if (fstat (h, &s1) || stat (path, &s2))
+        break;
+
+      success = s1.st_dev == s2.st_dev
+             && s1.st_ino == s2.st_ino;
 #endif
+
+      if (success)
+        return h; // lock successfully acquired
+
+      // files differ, close and retry - should be very rare
+      u_close (h);
+    }
+
+  // failure
+  u_close (h);
+  return u_invalid_handle;
 }
 
 static void
@@ -465,16 +507,9 @@
   strcat (execdir, exe_ver);
 
   strcat (strcpy (tmppath, execdir), ".lck");
-  lock_handle = u_lockfile (tmppath);
+  lock_handle = u_lock (tmppath, 0, 1);
   if (!lock_handle)
-    fatal ("unable to create lock file");
-
-  u_cloexec (lock_handle);
-
-  // all users that uee an instance acquire a shared lock
-  // acquiring an exclusive lock is enough to delete the dir
-  // deleeting the lockfile is another topic altogether
-  u_lock (U_LOCK_READ, 1);
+    fatal ("unable to lock application instance");
 
   if (access (execdir, F_OK))
     {