From c04cceb40ed5574dcba8a55cfe97a1132c869895 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Sybren=20A=2E=20St=C3=BCvel?= <sybren>
Date: Tue, 6 Jul 2021 15:36:27 +0300
Subject: Fix T89435: Reordering FCurves can cause crash or corruption

Correctly reset `prev` and `next` pointers of action group FCurves when
separating them into distinct `ListBase`s per `bActionGroup`.

These `NULL` pointers are necessary to temporarily demarcate the start &
end of the `bActionGroup::channels` list. Having them still point to
other FCurves caused ordering issues when moving curves towards the
start/end of a group.

This commit corrects the above issue and adds versioning code to rectify
any ordering issues that may have been caused. For this purpose the
`BKE_action_groups_reconstruct()` function is rewritten to avoid relying
on the `bAction::curves` list order or `prev` link integrity.

Differential Revision: https://developer.blender.org/D11811
---
 source/blender/blenkernel/intern/action.c | 38 ++++++++++++++++++-------------
 1 file changed, 22 insertions(+), 16 deletions(-)

(limited to 'source/blender/blenkernel/intern/action.c')

diff --git a/source/blender/blenkernel/intern/action.c b/source/blender/blenkernel/intern/action.c
index 13ca5ecf23c..d55f023d209 100644
--- a/source/blender/blenkernel/intern/action.c
+++ b/source/blender/blenkernel/intern/action.c
@@ -497,9 +497,8 @@ void action_groups_add_channel(bAction *act, bActionGroup *agrp, FCurve *fcurve)
 }
 
 /* Reconstruct group channel pointers.
- * Assumes that the channels are still in the proper order, i.e. that channels of the same group
- * are adjacent in the act->channels list. It also assumes that the groups
- * referred to by the FCurves are already in act->groups.
+ * Assumes that the groups referred to by the FCurves are already in act->groups.
+ * Reorders the main channel list to match group order.
  */
 void BKE_action_groups_reconstruct(bAction *act)
 {
@@ -514,23 +513,30 @@ void BKE_action_groups_reconstruct(bAction *act)
     BLI_listbase_clear(&group->channels);
   }
 
-  bActionGroup *grp;
-  bActionGroup *last_grp = NULL;
-  LISTBASE_FOREACH (FCurve *, fcurve, &act->curves) {
-    if (fcurve->grp == NULL) {
-      continue;
-    }
+  /* Sort the channels into the group lists, destroying the act->curves list. */
+  ListBase ungrouped = {NULL, NULL};
 
-    grp = fcurve->grp;
-    if (last_grp != grp) {
-      /* If this is the first time we see this group, this must be the first channel. */
-      grp->channels.first = fcurve;
+  LISTBASE_FOREACH_MUTABLE (FCurve *, fcurve, &act->curves) {
+    if (fcurve->grp) {
+      BLI_assert(BLI_findindex(&act->groups, fcurve->grp) >= 0);
+
+      BLI_addtail(&fcurve->grp->channels, fcurve);
+    }
+    else {
+      BLI_addtail(&ungrouped, fcurve);
     }
+  }
+
+  /* Recombine into the main list. */
+  BLI_listbase_clear(&act->curves);
 
-    /* This is the last channel, until it's overwritten by a later iteration. */
-    grp->channels.last = fcurve;
-    last_grp = grp;
+  LISTBASE_FOREACH (bActionGroup *, group, &act->groups) {
+    /* Copy the list header to preserve the pointers in the group. */
+    ListBase tmp = group->channels;
+    BLI_movelisttolist(&act->curves, &tmp);
   }
+
+  BLI_movelisttolist(&act->curves, &ungrouped);
 }
 
 /* Remove the given channel from all groups */
-- 
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