Fix: handle writes of length 0

[lttng-ust.git] / libringbuffer / ring_buffer_frontend.c

diff --git a/libringbuffer/ring_buffer_frontend.c b/libringbuffer/ring_buffer_frontend.c
index 728cbf9b72665036ccc0e275b9249cb00e591a01..650affad047a34f006cabbe0cf40a21c1690c867 100644 (file)
--- a/libringbuffer/ring_buffer_frontend.c
+++ b/libringbuffer/ring_buffer_frontend.c
@@ -1,7 +1,22 @@
 /*
  * ring_buffer_frontend.c
  *
- * (C) Copyright 2005-2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ * Copyright (C) 2005-2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; only
+ * version 2.1 of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ *
  *
  * Ring buffer wait-free buffer synchronization. Producer-consumer and flight
  * recorder (overwrite) modes. See thesis:
@@ -34,8 +49,6 @@
  *   - splice one subbuffer worth of data to a pipe
  *   - splice the data from pipe to disk/network
  *   - put_subbuf
- *
- * Dual LGPL v2.1/GPL v2 license.
  */
 
 #define _GNU_SOURCE
@@ -1196,6 +1209,19 @@ int lib_ring_buffer_try_switch_slow(enum switch_mode mode,
         */
        if (mode == SWITCH_FLUSH || off > 0) {
                if (caa_unlikely(off == 0)) {
+                       /*
+                        * A final flush that encounters an empty
+                        * sub-buffer cannot switch buffer if a
+                        * reader is located within this sub-buffer.
+                        * Anyway, the purpose of final flushing of a
+                        * sub-buffer at offset 0 is to handle the case
+                        * of entirely empty stream.
+                        */
+                       if (caa_unlikely(subbuf_trunc(offsets->begin, chan)
+                                        - subbuf_trunc((unsigned long)
+                                            uatomic_read(&buf->consumed), chan)
+                                       >= chan->backend.buf_size))
+                               return -1;
                        /*
                         * The client does not save any header information.
                         * Don't switch empty subbuffer on finalize, because it