Fix #129385: Bake Action operator not working with slots

This fixes the issue by traversing the new data structure for actions.
* When a slot is assigned, the action is baked into the slot.
* if no slot is assigned, it creates a new slot and bakes into that. However since no slot was assigned, the animation will be static.

Pull Request: https://projects.blender.org/blender/blender/pulls/129525

scripts/modules/bpy_extras/anim_utils.py

@@ -11,7 +11,7 @@ __all__ = (
 )
 
 import bpy
-from bpy.types import Action
+from bpy.types import Action, ActionSlot
 from dataclasses import dataclass
 
 from collections.abc import (
@@ -73,6 +73,26 @@ class BakeOptions:
     """Bake custom properties."""
 
 
+def _get_channelbag_for_slot(action: Action, slot: ActionSlot):
+    # This is on purpose limited to the first layer and strip. To support more
+    # than 1 layer, a rewrite of this operator is needed which ideally would
+    # happen in C++.
+    for layer in action.layers:
+        for strip in layer.strips:
+            channelbag = strip.channels(slot.handle)
+            return channelbag
+
+
+def _ensure_channelbag_exists(action: Action, slot: ActionSlot):
+    channelbag = _get_channelbag_for_slot(action, slot)
+    if channelbag:
+        return channelbag
+
+    for layer in action.layers:
+        for strip in layer.strips:
+            return strip.channelbags.new(slot)
+
+
 def bake_action(
         obj,
         *,
@@ -357,6 +377,11 @@ def bake_action_iter(
     is_new_action = action is None
     if is_new_action:
         action = bpy.data.actions.new("Action")
+    else:
+        # When baking into the current action, a slot needs to be assigned.
+        if not atd.action_slot:
+            slot = action.slots.new(for_id=obj)
+            atd.action_slot = slot
 
     # Only leave tweak mode if we actually need to modify the action (#57159)
     if action != atd.action:
@@ -377,7 +402,13 @@ def bake_action_iter(
     # Apply transformations to action
 
     # pose
-    lookup_fcurves = {(fcurve.data_path, fcurve.array_index): fcurve for fcurve in action.fcurves}
+    lookup_fcurves = {}
+    assert action.is_action_layered
+    channelbag = _get_channelbag_for_slot(action, atd.action_slot)
+    if channelbag:
+        # channelbag can be None if no layers or strips exist in the action.
+        lookup_fcurves = {(fcurve.data_path, fcurve.array_index): fcurve for fcurve in channelbag.fcurves}
+
     if bake_options.do_pose:
         for f, armature_custom_properties in armature_info:
             bake_custom_properties(obj, custom_props=armature_custom_properties,
@@ -470,7 +501,8 @@ def bake_action_iter(
             if is_new_action:
                 keyframes.insert_keyframes_into_new_action(total_new_keys, action, name)
             else:
-                keyframes.insert_keyframes_into_existing_action(lookup_fcurves, total_new_keys, action, name)
+                keyframes.insert_keyframes_into_existing_action(
+                    lookup_fcurves, total_new_keys, action, atd.action_slot)
 
     # object. TODO. multiple objects
     if bake_options.do_object:
@@ -536,7 +568,8 @@ def bake_action_iter(
         if is_new_action:
             keyframes.insert_keyframes_into_new_action(total_new_keys, action, name)
         else:
-            keyframes.insert_keyframes_into_existing_action(lookup_fcurves, total_new_keys, action, name)
+            keyframes.insert_keyframes_into_existing_action(
+                lookup_fcurves, total_new_keys, action, atd.action_slot)
 
         if bake_options.do_parents_clear:
             obj.parent = None
@@ -652,7 +685,7 @@ class KeyframesCo:
         lookup_fcurves: Mapping[FCurveKey, bpy.types.FCurve],
         total_new_keys: int,
         action: Action,
-        action_group_name: str,
+        action_slot: ActionSlot,
     ) -> None:
         """
         Assumes the action already exists, that it might already have F-curves. Otherwise, the
@@ -675,9 +708,12 @@ class KeyframesCo:
             fcurve = lookup_fcurves.get(fc_key, None)
             if fcurve is None:
                 data_path, array_index = fc_key
-                fcurve = action.fcurves.new(
-                    data_path, index=array_index, action_group=action_group_name
-                )
+                assert action.is_action_layered
+                channelbag = _ensure_channelbag_exists(action, action_slot)
+                if not channelbag:
+                    # Can happen if no layers or strips exist on the action.
+                    continue
+                fcurve = channelbag.fcurves.new(data_path, index=array_index)
 
             keyframe_points = fcurve.keyframe_points