Fix deadlock in galeramon

The parameter extraction caused a recursive lock of the server
spinlock. To work around this, an unlocked version of server_get_parameter
is needed.

Ideally, a lock-free setup would be used but due to this being a bug fix,
it will have to be done later on.

include/maxscale/server.h

@@ -358,6 +358,7 @@ extern void server_clear_status_nolock(SERVER *server, uint64_t bit);
 extern void server_transfer_status(SERVER *dest_server, const SERVER *source_server);
 extern void server_add_mon_user(SERVER *server, const char *user, const char *passwd);
 extern size_t server_get_parameter(const SERVER *server, const char *name, char* out, size_t size);
+extern size_t server_get_parameter_nolock(const SERVER *server, const char *name, char* out, size_t size);
 extern void server_update_credentials(SERVER *server, const char *user, const char *passwd);
 extern DCB* server_get_persistent(SERVER *server, const char *user, const char* ip, const char *protocol, int id);
 extern void server_update_address(SERVER *server, const char *address);

server/core/server.cc

@@ -957,20 +957,15 @@ static void server_parameter_free(SERVER_PARAM *tofree)
 }
 
 /**
- * Retrieve a parameter value from a server
+ * Same as server_get_parameter but doesn't lock the server
  *
- * @param server The server we are looking for a parameter of
+ * @note Should only be called when the server is already locked
- * @param name   The name of the parameter we require
- * @param out    Buffer where value is stored, use NULL to check if the parameter exists
- * @param size   Size of @c out, ignored if @c out is NULL
- *
- * @return Length of the parameter value or 0 if parameter was not found
  */
-size_t server_get_parameter(const SERVER *server, const char *name, char* out, size_t size)
+size_t server_get_parameter_nolock(const SERVER *server, const char *name, char* out, size_t size)
 {
-    int len = 0;
+    ss_dassert(SPINLOCK_IS_LOCKED(server->lock));
+    size_t len = 0;
     SERVER_PARAM *param = server->parameters;
-    spinlock_acquire(&server->lock);
 
     while (param)
     {
@@ -982,6 +977,23 @@ size_t server_get_parameter(const SERVER *server, const char *name, char* out, s
         param = param->next;
     }
 
+    return len;
+}
+
+/**
+ * Retrieve a parameter value from a server
+ *
+ * @param server The server we are looking for a parameter of
+ * @param name   The name of the parameter we require
+ * @param out    Buffer where value is stored, use NULL to check if the parameter exists
+ * @param size   Size of @c out, ignored if @c out is NULL
+ *
+ * @return Length of the parameter value or 0 if parameter was not found
+ */
+size_t server_get_parameter(const SERVER *server, const char *name, char* out, size_t size)
+{
+    spinlock_acquire(&server->lock);
+    size_t len = server_get_parameter_nolock(server, name, out, size);
     spinlock_release(&server->lock);
     return len;
 }

server/modules/monitor/galeramon/galeramon.c

@@ -695,7 +695,7 @@ static MXS_MONITORED_SERVER *get_candidate_master(MXS_MONITOR* mon)
 
             moitor_servers->server->depth = 0;
             char buf[50]; // Enough to hold most numbers
-            if (handle->use_priority && server_get_parameter(moitor_servers->server, "priority", buf, sizeof(buf)))
+            if (handle->use_priority && server_get_parameter_nolock(moitor_servers->server, "priority", buf, sizeof(buf)))
             {
                 /** The server has a priority  */
                 if ((currval = atoi(buf)) > 0)
@@ -712,7 +712,7 @@ static MXS_MONITORED_SERVER *get_candidate_master(MXS_MONITOR* mon)
                      (!handle->use_priority || /** Server priority disabled*/
                       candidate_master == NULL || /** No candidate chosen */
                        /** Candidate has no priority */
-                      !server_get_parameter(moitor_servers->server, "priority", buf, sizeof(buf))))
+                      !server_get_parameter_nolock(moitor_servers->server, "priority", buf, sizeof(buf))))
             {
                 if (min_id < 0 || moitor_servers->server->node_id < min_id)
                 {
@@ -848,7 +848,7 @@ static void update_sst_donor_nodes(MXS_MONITOR *mon, int is_cluster)
              */
 
             if (handle->use_priority &&
-                server_get_parameter(ptr->server, "priority", NULL, 0))
+                server_get_parameter_nolock(ptr->server, "priority", NULL, 0))
             {
                 ignore_priority = false;
             }
@@ -983,8 +983,8 @@ static int compare_node_priority (const void *a, const void *b)
     const MXS_MONITORED_SERVER *s_b = *(MXS_MONITORED_SERVER * const *)b;
     char pri_a[50];
     char pri_b[50];
-    bool have_a = server_get_parameter(s_a->server, "priority", pri_a, sizeof(pri_a));
+    bool have_a = server_get_parameter_nolock(s_a->server, "priority", pri_a, sizeof(pri_a));
-    bool have_b = server_get_parameter(s_b->server, "priority", pri_b, sizeof(pri_b));
+    bool have_b = server_get_parameter_nolock(s_b->server, "priority", pri_b, sizeof(pri_b));
 
     /**
      * Check priority parameter: