Try harder to make file locking succeed in case of network errors

We noticed that during nightly builds, the LockFileEx() call would
occasionally(!) fail with ERROR_NETNAME_DELETED. The lockfile was
specified via an UNC path to a network drive, but the host was
available all the time. We couldn't find a reason why this would
happen and there was no obvious pattern to this either.

Let's just try to call LockFileEx() again in case this error occurs
(at least a couple of times), it might be that it's just a temporary
failure.

withlockfile.cpp

@@ -61,8 +61,28 @@ int main( int argc, char **argv )
         OVERLAPPED ol;
         ::ZeroMemory( &ol, sizeof( ol ) );
 
-        if ( !::LockFileEx( f, LOCKFILE_EXCLUSIVE_LOCK, 0, 1, 0, &ol ) ) {
-            throw Win32Error( "LockFileEx", ::GetLastError() );
+        /* For some unknown reason, LockFileEx fails with ERROR_NETNAME_DELETED
+         * every now and then. We couldn't determine the reason, let's just
+         * ignore this error for a while if it occurs - maybe it's some
+         * network instability?
+         */
+        {
+            bool lockingSucceeded = false;
+            for ( int i = 0; i < 3; ++i ) {
+                if ( ::LockFileEx( f, LOCKFILE_EXCLUSIVE_LOCK, 0, 1, 0, &ol ) ) {
+                    lockingSucceeded = true;
+                    break;
+                }
+
+                const DWORD errorCode = ::GetLastError();
+                if ( errorCode != ERROR_NETNAME_DELETED ) {
+                    throw Win32Error( "LockFileEx", errorCode );
+                }
+            }
+
+            if ( !lockingSucceeded ) {
+                throw Win32Error( "LockFileEx", ERROR_NETNAME_DELETED );
+            }
         }
 
         std::string executable = enforceExeExtension( argv[2] );