GUACAMOLE-377: Stream/object sentinel must be assigned BEFORE returni…

…ng index to pool.

Doing otherwise results in a race condition where the index of a valid
stream is changed to -1 by a different thread, breaking assertions and
causing the connection to disconnect.

src/libguac/client.c

@@ -146,12 +146,13 @@ guac_stream* guac_client_alloc_stream(guac_client* client) {
 
 void guac_client_free_stream(guac_client* client, guac_stream* stream) {
 
-    /* Release index to pool */
-    guac_pool_free_int(client->__stream_pool, (stream->index - 1) / 2);
-
     /* Mark stream as closed */
+    int freed_index = stream->index;
     stream->index = GUAC_CLIENT_CLOSED_STREAM_INDEX;
 
+    /* Release index to pool */
+    guac_pool_free_int(client->__stream_pool, (freed_index - 1) / 2);
+
 }
 
 /**

src/libguac/user.c

@@ -126,12 +126,13 @@ guac_stream* guac_user_alloc_stream(guac_user* user) {
 
 void guac_user_free_stream(guac_user* user, guac_stream* stream) {
 
-    /* Release index to pool */
-    guac_pool_free_int(user->__stream_pool, stream->index / 2);
-
     /* Mark stream as closed */
+    int freed_index = stream->index;
     stream->index = GUAC_USER_CLOSED_STREAM_INDEX;
 
+    /* Release index to pool */
+    guac_pool_free_int(user->__stream_pool, freed_index / 2);
+
 }
 
 guac_object* guac_user_alloc_object(guac_user* user) {
@@ -157,12 +158,13 @@ guac_object* guac_user_alloc_object(guac_user* user) {
 
 void guac_user_free_object(guac_user* user, guac_object* object) {
 
-    /* Release index to pool */
-    guac_pool_free_int(user->__object_pool, object->index);
-
     /* Mark object as undefined */
+    int freed_index = object->index;
     object->index = GUAC_USER_UNDEFINED_OBJECT_INDEX;
 
+    /* Release index to pool */
+    guac_pool_free_int(user->__object_pool, freed_index);
+
 }
 
 int guac_user_handle_instruction(guac_user* user, const char* opcode, int argc, char** argv) {