Fix T100026: crash with zero-sized attributes

The problem was that zero-sized and non-existant attributes were
handled the same in some parts of the attribute API, which led to
unexpected behavior.

The solution is to properly differentiate the case when an attribute
does not exist and when it is just empty (because the geometry
is empty).

Differential Revision: https://developer.blender.org/D15557

1 files changed, 56 insertions, 40 deletions

diff --git a/source/blender/blenkernel/intern/attribute_access.cc b/source/blender/blenkernel/intern/attribute_access.cc
index 8d21c6fe792..1af3cde1821 100644
--- a/source/blender/blenkernel/intern/attribute_access.cc
+++ b/source/blender/blenkernel/intern/attribute_access.cc
@@ -214,36 +214,33 @@ static bool add_custom_data_layer_from_attribute_init(const AttributeIDRef &attr
                                                       const int domain_num,
                                                       const AttributeInit &initializer)
 {
+  const int old_layer_num = custom_data.totlayer;
   switch (initializer.type) {
     case AttributeInit::Type::Default: {
-      void *data = add_generic_custom_data_layer(
+      add_generic_custom_data_layer(
           custom_data, data_type, CD_DEFAULT, nullptr, domain_num, attribute_id);
-      return data != nullptr;
+      break;
     }
     case AttributeInit::Type::VArray: {
       void *data = add_generic_custom_data_layer(
           custom_data, data_type, CD_DEFAULT, nullptr, domain_num, attribute_id);
-      if (data == nullptr) {
-        return false;
+      if (data != nullptr) {
+        const GVArray &varray = static_cast<const AttributeInitVArray &>(initializer).varray;
+        varray.materialize_to_uninitialized(varray.index_range(), data);
       }
-      const GVArray &varray = static_cast<const AttributeInitVArray &>(initializer).varray;
-      varray.materialize_to_uninitialized(varray.index_range(), data);
-      return true;
+      break;
     }
     case AttributeInit::Type::MoveArray: {
       void *source_data = static_cast<const AttributeInitMove &>(initializer).data;
       void *data = add_generic_custom_data_layer(
           custom_data, data_type, CD_ASSIGN, source_data, domain_num, attribute_id);
-      if (data == nullptr) {
+      if (source_data != nullptr && data == nullptr) {
         MEM_freeN(source_data);
-        return false;
       }
-      return true;
+      break;
     }
   }
-
-  BLI_assert_unreachable();
-  return false;
+  return old_layer_num < custom_data.totlayer;
 }
 
 static bool custom_data_layer_matches_attribute_id(const CustomDataLayer &layer,
@@ -265,17 +262,25 @@ GVArray BuiltinCustomDataLayerProvider::try_get_for_read(const void *owner) cons
     return {};
   }
 
-  const void *data;
-  if (stored_as_named_attribute_) {
-    data = CustomData_get_layer_named(custom_data, stored_type_, name_.c_str());
-  }
-  else {
-    data = CustomData_get_layer(custom_data, stored_type_);
+  const void *data = nullptr;
+  bool found_attribute = false;
+  for (const CustomDataLayer &layer : Span(custom_data->layers, custom_data->totlayer)) {
+    if (stored_as_named_attribute_) {
+      if (layer.name == name_) {
+        data = layer.data;
+        found_attribute = true;
+        break;
+      }
+    }
+    else if (layer.type == stored_type_) {
+      data = layer.data;
+      found_attribute = true;
+      break;
+    }
   }
-  if (data == nullptr) {
+  if (!found_attribute) {
     return {};
   }
-
   const int element_num = custom_data_access_.get_element_num(owner);
   return as_read_attribute_(data, element_num);
 }
@@ -291,31 +296,42 @@ GAttributeWriter BuiltinCustomDataLayerProvider::try_get_for_write(void *owner)
   }
   const int element_num = custom_data_access_.get_element_num(owner);
 
-  void *data;
-  if (stored_as_named_attribute_) {
-    data = CustomData_get_layer_named(custom_data, stored_type_, name_.c_str());
-  }
-  else {
-    data = CustomData_get_layer(custom_data, stored_type_);
+  void *data = nullptr;
+  bool found_attribute = false;
+  for (const CustomDataLayer &layer : Span(custom_data->layers, custom_data->totlayer)) {
+    if (stored_as_named_attribute_) {
+      if (layer.name == name_) {
+        data = layer.data;
+        found_attribute = true;
+        break;
+      }
+    }
+    else if (layer.type == stored_type_) {
+      data = layer.data;
+      found_attribute = true;
+      break;
+    }
   }
-  if (data == nullptr) {
+  if (!found_attribute) {
     return {};
   }
 
-  void *new_data;
-  if (stored_as_named_attribute_) {
-    new_data = CustomData_duplicate_referenced_layer_named(
-        custom_data, stored_type_, name_.c_str(), element_num);
-  }
-  else {
-    new_data = CustomData_duplicate_referenced_layer(custom_data, stored_type_, element_num);
-  }
+  if (data != nullptr) {
+    void *new_data;
+    if (stored_as_named_attribute_) {
+      new_data = CustomData_duplicate_referenced_layer_named(
+          custom_data, stored_type_, name_.c_str(), element_num);
+    }
+    else {
+      new_data = CustomData_duplicate_referenced_layer(custom_data, stored_type_, element_num);
+    }
 
-  if (data != new_data) {
-    if (custom_data_access_.update_custom_data_pointers) {
-      custom_data_access_.update_custom_data_pointers(owner);
+    if (data != new_data) {
+      if (custom_data_access_.update_custom_data_pointers) {
+        custom_data_access_.update_custom_data_pointers(owner);
+      }
+      data = new_data;
     }
-    data = new_data;
   }
 
   std::function<void()> tag_modified_fn;