Fix: GPencil drawing sessions now respect limits for maximum undo steps

When drawing with Grease Pencil "continous drawing" for a long time
(i.e. basically, drawing a very large number of strokes), it could be
possible to cause lower-specced machines to run out of RAM and start
swapping. This was because there was no limit on the number of undo
states that the GP undo code was storing; since the undo states grow
exponentially on each stroke (i.e. each stroke results in another undo
state which contains all the existing strokes AND the newest stroke), this
could cause issues when taken to the extreme.

1 files changed, 23 insertions, 0 deletions

diff --git a/source/blender/editors/gpencil/gpencil_undo.c b/source/blender/editors/gpencil/gpencil_undo.c
index 196aec894b9..7a9ad2b32c0 100644
--- a/source/blender/editors/gpencil/gpencil_undo.c
+++ b/source/blender/editors/gpencil/gpencil_undo.c
@@ -155,6 +155,29 @@ void gpencil_undo_push(bGPdata *gpd)
 		}
 	}
 	
+	/* limit number of undo steps to the maximum undo steps
+	 *  - to prevent running out of memory during **really** 
+	 *    long drawing sessions (triggering swapping)
+	 */
+	/* TODO: Undo-memory constraint is not respected yet, but can be added if we have any need for it */
+	if (U.undosteps && !BLI_listbase_is_empty(&undo_nodes)) {
+		/* remove anything older than n-steps before cur_node */
+		int steps = 0;
+		
+		undo_node = (cur_node) ? cur_node : undo_nodes.last;
+		while (undo_node) {
+			bGPundonode *prev_node = undo_node->prev;
+			
+			if (steps >= U.undosteps) {
+				gpencil_undo_free_node(undo_node);
+				BLI_freelinkN(&undo_nodes, undo_node);
+			}
+			
+			steps++;
+			undo_node = prev_node;
+		}
+	}
+	
 	/* create new undo node */
 	undo_node = MEM_callocN(sizeof(bGPundonode), "gpencil undo node");
 	undo_node->gpd = BKE_gpencil_data_duplicate(G.main, gpd, true);