Fix T76897: Lamp Power Not Updating from Driver

Lamps were not tagged with `ID_RECALC_SHADING` when they were updated
from drivers. As a result, Cycles considered the lamp as unchanged. This
is resolved by having a (seemingly non-functional) callback in a new
`LIGHT_UPDATE` depsgraph node.

This patch unconditionally adds the `LIGHT_UPDATE` node + the relation
from the lamp's PARAMETERS node.

Differential Revision: https://developer.blender.org/D7822

Reviewed by: brecht

source/blender/blenkernel/BKE_light.h

@@ -30,6 +30,7 @@ extern "C" {
 
 #include "BLI_compiler_attrs.h"
 
+struct Depsgraph;
 struct Light;
 struct Main;
 
@@ -37,6 +38,8 @@ struct Light *BKE_light_add(struct Main *bmain, const char *name) ATTR_WARN_UNUS
 struct Light *BKE_light_copy(struct Main *bmain, const struct Light *la) ATTR_WARN_UNUSED_RESULT;
 struct Light *BKE_light_localize(struct Light *la) ATTR_WARN_UNUSED_RESULT;
 
+void BKE_light_eval(struct Depsgraph *depsgraph, struct Light *la);
+
 #ifdef __cplusplus
 }
 #endif

source/blender/blenkernel/intern/light.c

@@ -47,6 +47,8 @@
 
 #include "BLT_translation.h"
 
+#include "DEG_depsgraph.h"
+
 static void light_init_data(ID *id)
 {
   Light *la = (Light *)id;
@@ -167,3 +169,8 @@ Light *BKE_light_localize(Light *la)
 
   return lan;
 }
+
+void BKE_light_eval(struct Depsgraph *depsgraph, Light *la)
+{
+  DEG_debug_print_eval(depsgraph, __func__, la->id.name, la);
+}

source/blender/depsgraph/intern/builder/deg_builder_nodes.cc

@@ -83,6 +83,7 @@ extern "C" {
 #include "BKE_key.h"
 #include "BKE_lattice.h"
 #include "BKE_layer.h"
+#include "BKE_light.h"
 #include "BKE_mask.h"
 #include "BKE_material.h"
 #include "BKE_mball.h"
@@ -1452,6 +1453,12 @@ void DepsgraphNodeBuilder::build_light(Light *lamp)
   build_parameters(&lamp->id);
   /* light's nodetree */
   build_nodetree(lamp->nodetree);
+
+  Light *lamp_cow = get_cow_datablock(lamp);
+  add_operation_node(&lamp->id,
+                     NodeType::SHADING,
+                     OperationCode::LIGHT_UPDATE,
+                     function_bind(BKE_light_eval, _1, lamp_cow));
 }
 
 void DepsgraphNodeBuilder::build_nodetree(bNodeTree *ntree)

source/blender/depsgraph/intern/builder/deg_builder_relations.cc

@@ -2246,14 +2246,20 @@ void DepsgraphRelationBuilder::build_light(Light *lamp)
   build_idproperties(lamp->id.properties);
   build_animdata(&lamp->id);
   build_parameters(&lamp->id);
+
+  ComponentKey lamp_parameters_key(&lamp->id, NodeType::PARAMETERS);
+
   /* light's nodetree */
   if (lamp->nodetree != nullptr) {
     build_nodetree(lamp->nodetree);
-    ComponentKey lamp_parameters_key(&lamp->id, NodeType::PARAMETERS);
     ComponentKey nodetree_key(&lamp->nodetree->id, NodeType::SHADING);
     add_relation(nodetree_key, lamp_parameters_key, "NTree->Light Parameters");
     build_nested_nodetree(&lamp->id, lamp->nodetree);
   }
+
+  /* For allowing drivers on lamp properties. */
+  ComponentKey shading_key(&lamp->id, NodeType::SHADING);
+  add_relation(lamp_parameters_key, shading_key, "Light Shading Parameters");
 }
 
 void DepsgraphRelationBuilder::build_nodetree(bNodeTree *ntree)

source/blender/depsgraph/intern/node/deg_node_operation.cc

@@ -173,6 +173,8 @@ const char *operationCodeAsString(OperationCode opcode)
       return "SHADING";
     case OperationCode::MATERIAL_UPDATE:
       return "MATERIAL_UPDATE";
+    case OperationCode::LIGHT_UPDATE:
+      return "LIGHT_UPDATE";
     case OperationCode::WORLD_UPDATE:
       return "WORLD_UPDATE";
     /* Movie clip. */

source/blender/depsgraph/intern/node/deg_node_operation.h

@@ -170,6 +170,7 @@ enum class OperationCode {
   /* Shading. ------------------------------------------------------------- */
   SHADING,
   MATERIAL_UPDATE,
+  LIGHT_UPDATE,
   WORLD_UPDATE,
 
   /* Batch caches. -------------------------------------------------------- */