MXS-1392 Manage DCB lifetime using refcounts

Now it is also possible to ensure that a DCB stays alive while
a task referring to it is posted from one worker to another.
That will be implemented in a subsequent commit.

server/core/maxscale/dcb.h

@@ -0,0 +1,75 @@
+#pragma once
+/*
+ * Copyright (c) 2016 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl11.
+ *
+ * Change Date: 2020-01-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+#include <maxscale/dcb.h>
+
+MXS_BEGIN_DECLS
+
+void dcb_free_all_memory(DCB *dcb);
+
+/**
+ * @brief Increase the reference count of the DCB
+ *
+ * @param dcb  The dcb whose reference count should be increased.
+ */
+static inline void dcb_inc_ref(DCB* dcb)
+{
+    // A DCB starts out with a refcount of 1.
+    ss_dassert(dcb->poll.refcount >= 1);
+
+    poll_inc_ref(&dcb->poll);
+}
+
+/**
+ * @brief Decrease the reference count of the DCB. If it reaches 0,
+ *        the DCB will be freed.
+ *
+ * @param dcb  The dcb whose reference count should be decreased.
+ *
+ * @return True, if the dcb is still usable after the call, otherwise false.
+ *
+ * @note If the function returns false, the caller cannot use the dcb
+ *       for anything.
+ */
+static inline bool dcb_dec_ref(DCB* dcb)
+{
+    // A DCB starts out with a refcount of 1.
+    ss_dassert(dcb->poll.refcount >= 1);
+
+    bool rv = true;
+    if (poll_dec_ref(&dcb->poll) == 1)
+    {
+        dcb_free_all_memory(dcb);
+        rv = false;
+    }
+    return rv;
+}
+
+/**
+ * @brief Increase the reference count of the DCB.
+ *
+ * Convenience function for the situation where a received dcb is stored
+ * and the reference count needs to be increased.
+ *
+ * @param dcb  The dcb whose reference count should be increased.
+ *
+ * @return The dcb provided as argument.
+ */
+static inline DCB* dcb_get_ref(DCB* dcb)
+{
+    dcb_inc_ref(dcb);
+    return dcb;
+}
+
+MXS_END_DECLS