diff --git a/source/blender/blenkernel/BKE_anim_data.hh b/source/blender/blenkernel/BKE_anim_data.hh
index 667ef2c9ead..f5586b61efd 100644
--- a/source/blender/blenkernel/BKE_anim_data.hh
+++ b/source/blender/blenkernel/BKE_anim_data.hh
@@ -8,6 +8,8 @@
  * \ingroup bke
  */
 
+#include <optional>
+
 #include "BLI_sys_types.h" /* for bool */
 
 struct AnimData;
@@ -15,6 +17,7 @@ struct BlendDataReader;
 struct BlendLibReader;
 struct BlendWriter;
 struct ID;
+struct Library;
 struct LibraryForeachIDData;
 struct Main;
 struct ReportList;
@@ -84,12 +87,29 @@ void BKE_animdata_foreach_id(AnimData *adt, LibraryForeachIDData *data);
 
 /**
  * Make a copy of the given AnimData - to be used when copying data-blocks.
- * \param flag: Control ID pointers management,
- * see LIB_ID_CREATE_.../LIB_ID_COPY_... flags in BKE_lib_id.hh
+ *
+ * \note: Regarding handling of IDs managed by the #AnimData struct, this function follows the
+ * behaviors of the generic #BKE_id_copy_ex, please see its documetation for more details.
+ *
+ * \param flag: Control ID pointers management, see LIB_ID_CREATE_.../LIB_ID_COPY_... flags in
+ * #BKE_lib_id.hh
+ *
  * \return The copied animdata.
  */
 AnimData *BKE_animdata_copy(Main *bmain, AnimData *adt, int flag);
 
+/**
+ * Same as #BKE_animdata_copy, but allows to duplicate Action IDs into a library.
+ *
+ * \param owner_library the Library to 'assign' the newly created ID to. Use `nullptr` to make ID
+ * not use any library (i.e. become a local ID). Use `std::nullopt` for default behavior (i.e.
+ * behavior of the #BKE_animdata_copy function).
+ */
+AnimData *BKE_animdata_copy_in_lib(Main *bmain,
+                                   std::optional<Library *> owner_library,
+                                   AnimData *adt,
+                                   int flag);
+
 /**
  * \param flag: Control ID pointers management,
  * see LIB_ID_CREATE_.../LIB_ID_COPY_... flags in BKE_lib_id.hh
diff --git a/source/blender/blenkernel/BKE_idtype.hh b/source/blender/blenkernel/BKE_idtype.hh
index 25d2a5ef22d..f10d0365a50 100644
--- a/source/blender/blenkernel/BKE_idtype.hh
+++ b/source/blender/blenkernel/BKE_idtype.hh
@@ -10,6 +10,8 @@
  * ID type structure, helping to factorize common operations and data for all data-block types.
  */
 
+#include <optional>
+
 #include "BLI_sys_types.h"
 
 struct AssetTypeInfo;
@@ -18,6 +20,7 @@ struct BlendDataReader;
 struct BlendLibReader;
 struct BlendWriter;
 struct ID;
+struct Library;
 struct LibraryForeachIDData;
 struct Main;
 
@@ -63,7 +66,8 @@ bool BKE_idtype_cache_key_cmp(const void *key_a_v, const void *key_b_v);
 using IDTypeInitDataFunction = void (*)(ID *id);
 
 /** \param flag: Copying options (see BKE_lib_id.hh's LIB_ID_COPY_... flags for more). */
-using IDTypeCopyDataFunction = void (*)(Main *bmain, ID *id_dst, const ID *id_src, int flag);
+using IDTypeCopyDataFunction = void (*)(
+    Main *bmain, std::optional<Library *> owner_library, ID *id_dst, const ID *id_src, int flag);
 
 using IDTypeFreeDataFunction = void (*)(ID *id);
 
diff --git a/source/blender/blenkernel/BKE_lib_id.hh b/source/blender/blenkernel/BKE_lib_id.hh
index 78e1172f15c..52b3decedd4 100644
--- a/source/blender/blenkernel/BKE_lib_id.hh
+++ b/source/blender/blenkernel/BKE_lib_id.hh
@@ -30,6 +30,8 @@
  * specific cases requiring advanced (and potentially dangerous) handling.
  */
 
+#include <optional>
+
 #include "BLI_compiler_attrs.h"
 #include "BLI_set.hh"
 #include "BLI_utildefines.h"
@@ -57,13 +59,29 @@ size_t BKE_libblock_get_alloc_info(short type, const char **name);
  */
 void *BKE_libblock_alloc_notest(short type) ATTR_WARN_UNUSED_RESULT;
 /**
- * Allocates and returns a block of the specified type, with the specified name
+ * Allocates and returns an ID block of the specified type, with the specified name
  * (adjusted as necessary to ensure uniqueness), and appended to the specified list.
  * The user count is set to 1, all other content (apart from name and links) being
  * initialized to zero.
+ *
+ * \note: By default, IDs allocated in a Main database will get the current library of the Main,
+ * i.e. usually (besides in readfile case), they will have a `nullptr` `lib` pointer and be local
+ * data. IDs allocated outside of a Main database will always get a `nullptr` `lib` pointer.
  */
 void *BKE_libblock_alloc(Main *bmain, short type, const char *name, int flag)
     ATTR_WARN_UNUSED_RESULT;
+/**
+ * Same as for #BKE_libblock_alloc, but allows creating a data-block for a given owner library.
+ *
+ * \param owner_library the Library to 'assign' the newly created ID to. Use `nullptr` to make ID
+ * not use any library (i.e. become a local ID). Use `std::nullopt` for default behavior (i.e.
+ * behavior of the #BKE_libblock_alloc function).
+ */
+void *BKE_libblock_alloc_in_lib(Main *bmain,
+                                std::optional<Library *> owner_library,
+                                short type,
+                                const char *name,
+                                int flag) ATTR_WARN_UNUSED_RESULT;
 /**
  * Initialize an ID of given type, such that it has valid 'empty' data.
  * ID is assumed to be just calloc'ed.
@@ -102,9 +120,24 @@ void BKE_lib_libblock_session_uid_renew(ID *id);
 /**
  * Generic helper to create a new empty data-block of given type in given \a bmain database.
  *
+ * \note: By default, IDs created in a Main database will get the current library of the Main,
+ * i.e. usually (besides in readfile case), they will have a `nullptr` `lib` pointer and be local
+ * data. IDs created outside of a Main database will always get a `nullptr` `lib` pointer.
+
  * \param name: can be NULL, in which case we get default name for this ID type.
  */
 void *BKE_id_new(Main *bmain, short type, const char *name);
+/**
+ * Same as for #BKE_id_new, but allows creating a data-block for (whithin) a given owner library.
+ *
+ * \param owner_library the Library to 'assign' the newly created ID to. Use `nullptr` to make ID
+ * not use any library (i.e. become a local ID). Use `std::nullopt` for default behavior (i.e.
+ * behavior of the #BKE_id_new function).
+ */
+void *BKE_id_new_in_lib(Main *bmain,
+                        std::optional<Library *> owner_library,
+                        short type,
+                        const char *name);
 /**
  * Generic helper to create a new temporary empty data-block of given type,
  * *outside* of any Main database.
@@ -163,8 +196,6 @@ enum {
   /* *** Ideally we should not have those, but we need them for now... *** */
   /** EXCEPTION! Deep-copy actions used by animation-data of copied ID. */
   LIB_ID_COPY_ACTIONS = 1 << 24,
-  /** Keep the library pointer when copying data-block outside of bmain. */
-  LIB_ID_COPY_KEEP_LIB = 1 << 25,
   /** EXCEPTION! Deep-copy shape-keys used by copied obdata ID. */
   LIB_ID_COPY_SHAPEKEY = 1 << 26,
   /** EXCEPTION! Specific deep-copy of node trees used e.g. for rendering purposes. */
@@ -188,8 +219,28 @@ enum {
 };
 
 void BKE_libblock_copy_ex(Main *bmain, const ID *id, ID **r_newid, int orig_flag);
+/**
+ * Same as #BKE_libblock_copy_ex, but allows copying data into a library, and not as local data
+ * only.
+ *
+ * \param owner_library the Library to 'assign' the newly created ID to. Use `nullptr` to make ID
+ * not use any library (i.e. become a local ID). Use std::nullopt for default behavior (i.e.
+ * behavior of the #BKE_libblock_copy_ex function).
+ */
+void BKE_libblock_copy_in_lib(Main *bmain,
+                              std::optional<Library *> owner_library,
+                              const ID *id,
+                              ID **r_newid,
+                              int orig_flag);
+
 /**
  * Used everywhere in blenkernel.
+ *
+ * \note Typically, the newly copied ID will be a local data (its `lib` pointer will be `nullptr`).
+ * In practice, ID copying follows the same behavior as ID creation (see #BKE_libblock_alloc
+ * documentation), with one special case: when the special flag #LIB_ID_CREATE_NO_ALLOCATE is
+ * specified, the copied ID will have the same library as the source ID.
+ *
  */
 void *BKE_libblock_copy(Main *bmain, const ID *id) ATTR_WARN_UNUSED_RESULT ATTR_NONNULL();
 
@@ -436,6 +487,11 @@ bool BKE_id_copy_is_allowed(const ID *id);
  *
  * \note User-count of new copy is always set to 1.
  *
+ * \note Typically, the newly copied ID will be a local data (its `lib` pointer will be `nullptr`).
+ * In practice, ID copying follows the same behavior as ID creation (see #BKE_libblock_alloc
+ * documentation), with one special case: when the special flag #LIB_ID_CREATE_NO_ALLOCATE is
+ * specified, the copied ID will have the same library as the source ID.
+ *
  * \param bmain: Main database, may be NULL only if LIB_ID_CREATE_NO_MAIN is specified.
  * \param id: Source data-block.
  * \param r_newid: Pointer to new (copied) ID pointer, may be NULL.
@@ -445,6 +501,17 @@ bool BKE_id_copy_is_allowed(const ID *id);
  * \return NULL when copying that ID type is not supported, the new copy otherwise.
  */
 ID *BKE_id_copy_ex(Main *bmain, const ID *id, ID **r_newid, int flag);
+/**
+ * Enable coying non-local data into libraries.
+ *
+ * See #BKE_id_copy_ex for details.
+ *
+ * \param owner_library the Library to 'assign' the newly created ID to. Use `nullptr` to make ID
+ * not use any library (i.e. become a local ID). Use std::nullopt for default behavior (i.e.
+ * behavior of the #BKE_id_copy_ex function).
+ */
+struct ID *BKE_id_copy_in_lib(
+    Main *bmain, std::optional<Library *> owner_library, const ID *id, ID **r_newid, int flag);
 /**
  * Invoke the appropriate copy method for the block and return the new id as result.
  *
diff --git a/source/blender/blenkernel/intern/action.cc b/source/blender/blenkernel/intern/action.cc
index 87aa3d03390..eb41ba1b206 100644
--- a/source/blender/blenkernel/intern/action.cc
+++ b/source/blender/blenkernel/intern/action.cc
@@ -10,6 +10,7 @@
 #include <cstddef>
 #include <cstdlib>
 #include <cstring>
+#include <optional>
 
 #include "MEM_guardedalloc.h"
 
@@ -97,7 +98,11 @@ static CLG_LogRef LOG = {"bke.action"};
  *
  * \param flag: Copying options (see BKE_lib_id.hh's LIB_ID_COPY_... flags for more).
  */
-static void action_copy_data(Main * /*bmain*/, ID *id_dst, const ID *id_src, const int flag)
+static void action_copy_data(Main * /*bmain*/,
+                             std::optional<Library *> /*owner_library*/,
+                             ID *id_dst,
+                             const ID *id_src,
+                             const int flag)
 {
   bAction *action_dst = (bAction *)id_dst;
   const bAction *action_src = (const bAction *)id_src;
diff --git a/source/blender/blenkernel/intern/anim_data.cc b/source/blender/blenkernel/intern/anim_data.cc
index 1811c58bd65..d4658967b67 100644
--- a/source/blender/blenkernel/intern/anim_data.cc
+++ b/source/blender/blenkernel/intern/anim_data.cc
@@ -8,6 +8,7 @@
 #include "MEM_guardedalloc.h"
 
 #include <cstring>
+#include <optional>
 
 #include "BKE_action.h"
 #include "BKE_anim_data.hh"
@@ -303,7 +304,10 @@ void BKE_animdata_foreach_id(AnimData *adt, LibraryForeachIDData *data)
 
 /* Copying -------------------------------------------- */
 
-AnimData *BKE_animdata_copy(Main *bmain, AnimData *adt, const int flag)
+AnimData *BKE_animdata_copy_in_lib(Main *bmain,
+                                   std::optional<Library *> owner_library,
+                                   AnimData *adt,
+                                   const int flag)
 {
   AnimData *dadt;
 
@@ -333,8 +337,10 @@ AnimData *BKE_animdata_copy(Main *bmain, AnimData *adt, const int flag)
                                  flag;
     BLI_assert(bmain != nullptr);
     BLI_assert(dadt->action == nullptr || dadt->action != dadt->tmpact);
-    dadt->action = (bAction *)BKE_id_copy_ex(bmain, (ID *)dadt->action, nullptr, id_copy_flag);
-    dadt->tmpact = (bAction *)BKE_id_copy_ex(bmain, (ID *)dadt->tmpact, nullptr, id_copy_flag);
+    dadt->action = reinterpret_cast<bAction *>(BKE_id_copy_in_lib(
+        bmain, owner_library, reinterpret_cast<ID *>(dadt->action), nullptr, id_copy_flag));
+    dadt->tmpact = reinterpret_cast<bAction *>(BKE_id_copy_in_lib(
+        bmain, owner_library, reinterpret_cast<ID *>(dadt->tmpact), nullptr, id_copy_flag));
   }
   else if (do_id_user) {
     id_us_plus((ID *)dadt->action);
@@ -355,6 +361,11 @@ AnimData *BKE_animdata_copy(Main *bmain, AnimData *adt, const int flag)
   return dadt;
 }
 
+AnimData *BKE_animdata_copy(Main *bmain, AnimData *adt, const int flag)
+{
+  return BKE_animdata_copy_in_lib(bmain, std::nullopt, adt, flag);
+}
+
 bool BKE_animdata_copy_id(Main *bmain, ID *id_to, ID *id_from, const int flag)
 {
   AnimData *adt;
diff --git a/source/blender/blenkernel/intern/armature.cc b/source/blender/blenkernel/intern/armature.cc
index 6573ae5e9b2..cd1c624083b 100644
--- a/source/blender/blenkernel/intern/armature.cc
+++ b/source/blender/blenkernel/intern/armature.cc
@@ -13,6 +13,7 @@
 #include <cstdlib>
 #include <cstring>
 #include <limits>
+#include <optional>
 
 #include "MEM_guardedalloc.h"
 
@@ -130,7 +131,11 @@ static void copy_bone_collection(bArmature *armature_dst,
  *
  * \param flag: Copying options (see BKE_lib_id.hh's LIB_ID_COPY_... flags for more).
  */
-static void armature_copy_data(Main * /*bmain*/, ID *id_dst, const ID *id_src, const int flag)
+static void armature_copy_data(Main * /*bmain*/,
+                               std::optional<Library *> /*owner_library*/,
+                               ID *id_dst,
+                               const ID *id_src,
+                               const int flag)
 {
   bArmature *armature_dst = (bArmature *)id_dst;
   const bArmature *armature_src = (const bArmature *)id_src;
diff --git a/source/blender/blenkernel/intern/brush.cc b/source/blender/blenkernel/intern/brush.cc
index 6806806d088..c9d73e40933 100644
--- a/source/blender/blenkernel/intern/brush.cc
+++ b/source/blender/blenkernel/intern/brush.cc
@@ -5,6 +5,7 @@
 /** \file
  * \ingroup bke
  */
+#include <optional>
 
 #include "MEM_guardedalloc.h"
 
@@ -55,7 +56,11 @@ static void brush_init_data(ID *id)
   BKE_brush_curve_preset(brush, CURVE_PRESET_SMOOTH);
 }
 
-static void brush_copy_data(Main * /*bmain*/, ID *id_dst, const ID *id_src, const int flag)
+static void brush_copy_data(Main * /*bmain*/,
+                            std::optional<Library *> /*owner_library*/,
+                            ID *id_dst,
+                            const ID *id_src,
+                            const int flag)
 {
   Brush *brush_dst = (Brush *)id_dst;
   const Brush *brush_src = (const Brush *)id_src;
diff --git a/source/blender/blenkernel/intern/cachefile.cc b/source/blender/blenkernel/intern/cachefile.cc
index c8c4c667752..6c4b179eb88 100644
--- a/source/blender/blenkernel/intern/cachefile.cc
+++ b/source/blender/blenkernel/intern/cachefile.cc
@@ -7,6 +7,7 @@
  */
 
 #include <cstring>
+#include <optional>
 
 #include "DNA_cachefile_types.h"
 #include "DNA_object_types.h"
@@ -59,6 +60,7 @@ static void cache_file_init_data(ID *id)
 }
 
 static void cache_file_copy_data(Main * /*bmain*/,
+                                 std::optional<Library *> /*owner_library*/,
                                  ID *id_dst,
                                  const ID *id_src,
                                  const int /*flag*/)
diff --git a/source/blender/blenkernel/intern/camera.cc b/source/blender/blenkernel/intern/camera.cc
index aaabf3bed1d..5d8fad6e3d1 100644
--- a/source/blender/blenkernel/intern/camera.cc
+++ b/source/blender/blenkernel/intern/camera.cc
@@ -8,6 +8,7 @@
 
 #include <cstddef>
 #include <cstdlib>
+#include <optional>
 
 /* Allow using deprecated functionality for .blend file I/O. */
 #define DNA_DEPRECATED_ALLOW
@@ -68,7 +69,11 @@ static void camera_init_data(ID *id)
  *
  * \param flag: Copying options (see BKE_lib_id.hh's LIB_ID_COPY_... flags for more).
  */
-static void camera_copy_data(Main * /*bmain*/, ID *id_dst, const ID *id_src, const int flag)
+static void camera_copy_data(Main * /*bmain*/,
+                             std::optional<Library *> /*owner_library*/,
+                             ID *id_dst,
+                             const ID *id_src,
+                             const int flag)
 {
   Camera *cam_dst = (Camera *)id_dst;
   const Camera *cam_src = (const Camera *)id_src;
diff --git a/source/blender/blenkernel/intern/collection.cc b/source/blender/blenkernel/intern/collection.cc
index e2b03a973ab..95395a9626d 100644
--- a/source/blender/blenkernel/intern/collection.cc
+++ b/source/blender/blenkernel/intern/collection.cc
@@ -12,6 +12,7 @@
 #include "CLG_log.h"
 
 #include <cstring>
+#include <optional>
 
 #include "BLI_blenlib.h"
 #include "BLI_iterator.h"
@@ -132,7 +133,11 @@ static void collection_init_data(ID *id)
  *
  * \param flag: Copying options (see BKE_lib_id.hh's LIB_ID_COPY_... flags for more).
  */
-static void collection_copy_data(Main *bmain, ID *id_dst, const ID *id_src, const int flag)
+static void collection_copy_data(Main *bmain,
+                                 std::optional<Library *> /*owner_library*/,
+                                 ID *id_dst,
+                                 const ID *id_src,
+                                 const int flag)
 {
   Collection *collection_dst = (Collection *)id_dst;
   const Collection *collection_src = (const Collection *)id_src;
@@ -929,13 +934,15 @@ Collection *BKE_collection_master_add(Scene *scene)
   BLI_assert(scene != nullptr && scene->master_collection == nullptr);
 
   /* Not an actual datablock, but owned by scene. */
-  Collection *master_collection = static_cast<Collection *>(
-      BKE_libblock_alloc(nullptr, ID_GR, BKE_SCENE_COLLECTION_NAME, LIB_ID_CREATE_NO_MAIN));
+  Collection *master_collection = static_cast<Collection *>(BKE_libblock_alloc_in_lib(
+      nullptr, scene->id.lib, ID_GR, BKE_SCENE_COLLECTION_NAME, LIB_ID_CREATE_NO_MAIN));
   master_collection->id.flag |= LIB_EMBEDDED_DATA;
   master_collection->owner_id = &scene->id;
   master_collection->flag |= COLLECTION_IS_MASTER;
   master_collection->color_tag = COLLECTION_COLOR_NONE;
 
+  BLI_assert(scene->id.lib == master_collection->id.lib);
+
   return master_collection;
 }
 
diff --git a/source/blender/blenkernel/intern/curve.cc b/source/blender/blenkernel/intern/curve.cc
index 14adcdb2496..55ad75bd48a 100644
--- a/source/blender/blenkernel/intern/curve.cc
+++ b/source/blender/blenkernel/intern/curve.cc
@@ -9,6 +9,7 @@
 #include <cmath> /* floor */
 #include <cstdlib>
 #include <cstring>
+#include <optional>
 
 #include "MEM_guardedalloc.h"
 
@@ -79,7 +80,11 @@ static void curve_init_data(ID *id)
   MEMCPY_STRUCT_AFTER(curve, DNA_struct_default_get(Curve), id);
 }
 
-static void curve_copy_data(Main *bmain, ID *id_dst, const ID *id_src, const int flag)
+static void curve_copy_data(Main *bmain,
+                            std::optional<Library *> owner_library,
+                            ID *id_dst,
+                            const ID *id_src,
+                            const int flag)
 {
   Curve *curve_dst = (Curve *)id_dst;
   const Curve *curve_src = (const Curve *)id_src;
@@ -97,7 +102,7 @@ static void curve_copy_data(Main *bmain, ID *id_dst, const ID *id_src, const int
   curve_dst->bevel_profile = BKE_curveprofile_copy(curve_src->bevel_profile);
 
   if (curve_src->key && (flag & LIB_ID_COPY_SHAPEKEY)) {
-    BKE_id_copy_ex(bmain, &curve_src->key->id, (ID **)&curve_dst->key, flag);
+    BKE_id_copy_in_lib(bmain, owner_library, &curve_src->key->id, (ID **)&curve_dst->key, flag);
     /* XXX This is not nice, we need to make BKE_id_copy_ex fully re-entrant... */
     curve_dst->key->from = &curve_dst->id;
   }
diff --git a/source/blender/blenkernel/intern/curves.cc b/source/blender/blenkernel/intern/curves.cc
index 55bd587d0a4..a87ab0366f2 100644
--- a/source/blender/blenkernel/intern/curves.cc
+++ b/source/blender/blenkernel/intern/curves.cc
@@ -8,6 +8,7 @@
 
 #include <cmath>
 #include <cstring>
+#include <optional>
 
 #include "MEM_guardedalloc.h"
 
@@ -61,7 +62,11 @@ static void curves_init_data(ID *id)
   new (&curves->geometry) blender::bke::CurvesGeometry();
 }
 
-static void curves_copy_data(Main * /*bmain*/, ID *id_dst, const ID *id_src, const int /*flag*/)
+static void curves_copy_data(Main * /*bmain*/,
+                             std::optional<Library *> /*owner_library*/,
+                             ID *id_dst,
+                             const ID *id_src,
+                             const int /*flag*/)
 {
   Curves *curves_dst = (Curves *)id_dst;
   const Curves *curves_src = (const Curves *)id_src;
diff --git a/source/blender/blenkernel/intern/gpencil_legacy.cc b/source/blender/blenkernel/intern/gpencil_legacy.cc
index 649de3a27d9..3628aa1c7ac 100644
--- a/source/blender/blenkernel/intern/gpencil_legacy.cc
+++ b/source/blender/blenkernel/intern/gpencil_legacy.cc
@@ -11,6 +11,7 @@
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
+#include <optional>
 
 #include "CLG_log.h"
 
@@ -59,6 +60,7 @@
 static CLG_LogRef LOG = {"bke.gpencil"};
 
 static void greasepencil_copy_data(Main * /*bmain*/,
+                                   std::optional<Library *> /*owner_library*/,
                                    ID *id_dst,
                                    const ID *id_src,
                                    const int /*flag*/)
@@ -1059,7 +1061,7 @@ bGPdata *BKE_gpencil_data_duplicate(Main *bmain, const bGPdata *gpd_src, bool in
   }
 
   /* Copy internal data (layers, etc.) */
-  greasepencil_copy_data(bmain, &gpd_dst->id, &gpd_src->id, 0);
+  greasepencil_copy_data(bmain, std::nullopt, &gpd_dst->id, &gpd_src->id, 0);
 
   /* return new */
   return gpd_dst;
diff --git a/source/blender/blenkernel/intern/grease_pencil.cc b/source/blender/blenkernel/intern/grease_pencil.cc
index bbdf542887f..6f115a62d1b 100644
--- a/source/blender/blenkernel/intern/grease_pencil.cc
+++ b/source/blender/blenkernel/intern/grease_pencil.cc
@@ -7,6 +7,7 @@
  */
 
 #include <iostream>
+#include <optional>
 
 #include "BKE_action.h"
 #include "BKE_anim_data.hh"
@@ -88,6 +89,7 @@ static void grease_pencil_init_data(ID *id)
 }
 
 static void grease_pencil_copy_data(Main * /*bmain*/,
+                                    std::optional<Library *> /*owner_library*/,
                                     ID *id_dst,
                                     const ID *id_src,
                                     const int /*flag*/)
diff --git a/source/blender/blenkernel/intern/image.cc b/source/blender/blenkernel/intern/image.cc
index d10355ed1eb..35f265bd329 100644
--- a/source/blender/blenkernel/intern/image.cc
+++ b/source/blender/blenkernel/intern/image.cc
@@ -12,6 +12,7 @@
 #include <cstring>
 #include <ctime>
 #include <fcntl.h>
+#include <optional>
 #ifndef WIN32
 #  include <unistd.h>
 #else
@@ -156,7 +157,11 @@ static void image_init_data(ID *id)
   }
 }
 
-static void image_copy_data(Main * /*bmain*/, ID *id_dst, const ID *id_src, const int flag)
+static void image_copy_data(Main * /*bmain*/,
+                            std::optional<Library *> /*owner_library*/,
+                            ID *id_dst,
+                            const ID *id_src,
+                            const int flag)
 {
   Image *image_dst = (Image *)id_dst;
   const Image *image_src = (const Image *)id_src;
diff --git a/source/blender/blenkernel/intern/key.cc b/source/blender/blenkernel/intern/key.cc
index 4749fc1138e..b40ec9e3644 100644
--- a/source/blender/blenkernel/intern/key.cc
+++ b/source/blender/blenkernel/intern/key.cc
@@ -9,6 +9,7 @@
 #include <cmath>
 #include <cstddef>
 #include <cstring>
+#include <optional>
 
 #include "MEM_guardedalloc.h"
 
@@ -54,7 +55,11 @@
 
 #include "BLO_read_write.hh"
 
-static void shapekey_copy_data(Main * /*bmain*/, ID *id_dst, const ID *id_src, const int /*flag*/)
+static void shapekey_copy_data(Main * /*bmain*/,
+                               std::optional<Library *> /*owner_library*/,
+                               ID *id_dst,
+                               const ID *id_src,
+                               const int /*flag*/)
 {
   Key *key_dst = (Key *)id_dst;
   const Key *key_src = (const Key *)id_src;
diff --git a/source/blender/blenkernel/intern/lattice.cc b/source/blender/blenkernel/intern/lattice.cc
index 3865391ef7c..fde95d5aa45 100644
--- a/source/blender/blenkernel/intern/lattice.cc
+++ b/source/blender/blenkernel/intern/lattice.cc
@@ -56,7 +56,11 @@ static void lattice_init_data(ID *id)
   BKE_lattice_resize(lattice, 2, 2, 2, nullptr); /* creates a uniform lattice */
 }
 
-static void lattice_copy_data(Main *bmain, ID *id_dst, const ID *id_src, const int flag)
+static void lattice_copy_data(Main *bmain,
+                              std::optional<Library *> owner_library,
+                              ID *id_dst,
+                              const ID *id_src,
+                              const int flag)
 {
   Lattice *lattice_dst = (Lattice *)id_dst;
   const Lattice *lattice_src = (const Lattice *)id_src;
@@ -64,7 +68,8 @@ static void lattice_copy_data(Main *bmain, ID *id_dst, const ID *id_src, const i
   lattice_dst->def = static_cast<BPoint *>(MEM_dupallocN(lattice_src->def));
 
   if (lattice_src->key && (flag & LIB_ID_COPY_SHAPEKEY)) {
-    BKE_id_copy_ex(bmain, &lattice_src->key->id, (ID **)&lattice_dst->key, flag);
+    BKE_id_copy_in_lib(
+        bmain, owner_library, &lattice_src->key->id, (ID **)&lattice_dst->key, flag);
     /* XXX This is not nice, we need to make BKE_id_copy_ex fully re-entrant... */
     lattice_dst->key->from = &lattice_dst->id;
   }
diff --git a/source/blender/blenkernel/intern/lib_id.cc b/source/blender/blenkernel/intern/lib_id.cc
index 6b906db4af2..8b7cc831ec6 100644
--- a/source/blender/blenkernel/intern/lib_id.cc
+++ b/source/blender/blenkernel/intern/lib_id.cc
@@ -626,19 +626,28 @@ bool BKE_id_copy_is_allowed(const ID *id)
 #undef LIB_ID_TYPES_NOCOPY
 }
 
-ID *BKE_id_copy_ex(Main *bmain, const ID *id, ID **r_newid, const int flag)
+ID *BKE_id_copy_in_lib(Main *bmain,
+                       std::optional<Library *> owner_library,
+                       const ID *id,
+                       ID **r_newid,
+                       const int flag)
 {
   ID *newid = (r_newid != nullptr) ? *r_newid : nullptr;
+  BLI_assert_msg(newid || (flag & LIB_ID_CREATE_NO_ALLOCATE) == 0,
+                 "Copying with 'no allocate' behavior should always get a non-null new ID buffer");
+
   /* Make sure destination pointer is all good. */
   if ((flag & LIB_ID_CREATE_NO_ALLOCATE) == 0) {
     newid = nullptr;
   }
   else {
-    if (newid != nullptr) {
-      /* Allow some garbage non-initialized memory to go in, and clean it up here. */
-      const size_t size = BKE_libblock_get_alloc_info(GS(id->name), nullptr);
-      memset(newid, 0, size);
+    if (!newid) {
+      /* Invalid case, already caught by the assert above. */
+      return nullptr;
     }
+    /* Allow some garbage non-initialized memory to go in, and clean it up here. */
+    const size_t size = BKE_libblock_get_alloc_info(GS(id->name), nullptr);
+    memset(newid, 0, size);
   }
 
   /* Early output if source is nullptr. */
@@ -653,16 +662,21 @@ ID *BKE_id_copy_ex(Main *bmain, const ID *id, ID **r_newid, const int flag)
       return nullptr;
     }
 
-    BKE_libblock_copy_ex(bmain, id, &newid, flag);
+    BKE_libblock_copy_in_lib(bmain, owner_library, id, &newid, flag);
 
     if (idtype_info->copy_data != nullptr) {
-      idtype_info->copy_data(bmain, newid, id, flag);
+      idtype_info->copy_data(bmain, owner_library, newid, id, flag);
     }
   }
   else {
     BLI_assert_msg(0, "IDType Missing IDTypeInfo");
   }
 
+  BLI_assert_msg(newid, "Could not get an allocated new ID to copy into");
+  if (!newid) {
+    return nullptr;
+  }
+
   /* Update ID refcount, remap pointers to self in new ID. */
   IDCopyLibManagementData data{};
   data.id_src = id;
@@ -673,11 +687,20 @@ ID *BKE_id_copy_ex(Main *bmain, const ID *id, ID **r_newid, const int flag)
   /* Do not make new copy local in case we are copying outside of main...
    * XXX TODO: is this behavior OK, or should we need own flag to control that? */
   if ((flag & LIB_ID_CREATE_NO_MAIN) == 0) {
-    BLI_assert((flag & LIB_ID_COPY_KEEP_LIB) == 0);
-    lib_id_copy_ensure_local(bmain, id, newid, 0);
+    BLI_assert(!owner_library || newid->lib == *owner_library);
+    if (!ID_IS_LINKED(newid)) {
+      lib_id_copy_ensure_local(bmain, id, newid, 0);
+    }
   }
   else {
-    newid->lib = id->lib;
+    /* NOTE: Do not call `ensure_local` for IDs copied outside of Main, even if they do become
+     * local.
+     *
+     * Most of the time, this would not be the desired behavior currently.
+     *
+     * In the few cases where this is actually needed (e.g. from liboverride resync code, see
+     * #lib_override_library_create_from), calling code is responsible for this. */
+    newid->lib = owner_library ? *owner_library : id->lib;
   }
 
   if (r_newid != nullptr) {
@@ -687,9 +710,14 @@ ID *BKE_id_copy_ex(Main *bmain, const ID *id, ID **r_newid, const int flag)
   return newid;
 }
 
+ID *BKE_id_copy_ex(Main *bmain, const ID *id, ID **r_newid, const int flag)
+{
+  return BKE_id_copy_in_lib(bmain, std::nullopt, id, r_newid, flag);
+}
+
 ID *BKE_id_copy(Main *bmain, const ID *id)
 {
-  return BKE_id_copy_ex(bmain, id, nullptr, LIB_ID_COPY_DEFAULT);
+  return BKE_id_copy_in_lib(bmain, std::nullopt, id, nullptr, LIB_ID_COPY_DEFAULT);
 }
 
 ID *BKE_id_copy_for_duplicate(Main *bmain,
@@ -1220,7 +1248,11 @@ void *BKE_libblock_alloc_notest(short type)
   return nullptr;
 }
 
-void *BKE_libblock_alloc(Main *bmain, short type, const char *name, const int flag)
+void *BKE_libblock_alloc_in_lib(Main *bmain,
+                                std::optional<Library *> owner_library,
+                                short type,
+                                const char *name,
+                                const int flag)
 {
   BLI_assert((flag & LIB_ID_CREATE_NO_ALLOCATE) == 0);
   BLI_assert((flag & LIB_ID_CREATE_NO_MAIN) != 0 || bmain != nullptr);
@@ -1250,20 +1282,44 @@ void *BKE_libblock_alloc(Main *bmain, short type, const char *name, const int fl
       BLI_assert(bmain->is_locked_for_linking == false || ELEM(type, ID_WS, ID_GR, ID_NT));
       ListBase *lb = which_libbase(bmain, type);
 
+      /* This is important in 'readfile doversion after liblink' context mainly, but is a good
+       * behavior for consistency in general: ID created for a Main should get that main's current
+       * library pointer.
+       *
+       * NOTE: A bit convoluted.
+       *   - When Main has a defined `curlib`, it is assumed to be a split main containing only IDs
+       *     from that library. In that case, the library can be set later, and it avoids
+       *     synchronization issues in the namemap between the one of that temp 'library' Main and
+       *     the library ID runtime namemap itself. In a way, the ID can be assumed local to the
+       *     current Main, for its assignment to this Main.
+       *   - In all other cases, the Main is assumed 'complete', i.e. containing all local and
+       *     linked IDs, In that case, it is critical that the ID gets the correct library assigned
+       *     now, to ensure that the call to #BKE_id_new_name_validate gives a fully valid result
+       *     once it has been assigned to the current Main.
+       */
+      if (bmain->curlib) {
+        id->lib = nullptr;
+      }
+      else {
+        id->lib = owner_library ? *owner_library : nullptr;
+      }
+
       BKE_main_lock(bmain);
       BLI_addtail(lb, id);
-      BKE_id_new_name_validate(bmain, lb, id, name, false);
+      BKE_id_new_name_validate(bmain, lb, id, name, true);
       bmain->is_memfile_undo_written = false;
       /* alphabetic insertion: is in new_id */
       BKE_main_unlock(bmain);
 
+      /* Split Main case, now the ID should get the Main's #curlib. */
+      if (bmain->curlib) {
+        BLI_assert(!owner_library || *owner_library == bmain->curlib);
+        id->lib = bmain->curlib;
+      }
+
       /* This assert avoids having to keep name_map consistency when changing the library of an ID,
        * if this check is not true anymore it will have to be done here too. */
       BLI_assert(bmain->curlib == nullptr || bmain->curlib->runtime.name_map == nullptr);
-      /* This is important in 'readfile doversion after liblink' context mainly, but is a good
-       * consistency change in general: ID created for a Main should get that main's current
-       * library pointer. */
-      id->lib = bmain->curlib;
 
       /* TODO: to be removed from here! */
       if ((flag & LIB_ID_CREATE_NO_DEG_TAG) == 0) {
@@ -1272,6 +1328,7 @@ void *BKE_libblock_alloc(Main *bmain, short type, const char *name, const int fl
     }
     else {
       BLI_strncpy(id->name + 2, name, sizeof(id->name) - 2);
+      id->lib = owner_library ? *owner_library : nullptr;
     }
 
     /* We also need to ensure a valid `session_uid` for some non-main data (like embedded IDs).
@@ -1285,6 +1342,11 @@ void *BKE_libblock_alloc(Main *bmain, short type, const char *name, const int fl
   return id;
 }
 
+void *BKE_libblock_alloc(Main *bmain, short type, const char *name, const int flag)
+{
+  return BKE_libblock_alloc_in_lib(bmain, std::nullopt, type, name, flag);
+}
+
 void BKE_libblock_init_empty(ID *id)
 {
   const IDTypeInfo *idtype_info = BKE_idtype_get_info_from_id(id);
@@ -1329,7 +1391,11 @@ void BKE_lib_libblock_session_uid_renew(ID *id)
   BKE_lib_libblock_session_uid_ensure(id);
 }
 
-void *BKE_id_new(Main *bmain, const short type, const char *name)
+void *BKE_id_new_in_lib(Main *bmain,
+                        std::optional<Library *> owner_library,
+                        const short type,
+                        const char *name)
+
 {
   BLI_assert(bmain != nullptr);
 
@@ -1337,12 +1403,17 @@ void *BKE_id_new(Main *bmain, const short type, const char *name)
     name = DATA_(BKE_idtype_idcode_to_name(type));
   }
 
-  ID *id = static_cast<ID *>(BKE_libblock_alloc(bmain, type, name, 0));
+  ID *id = static_cast<ID *>(BKE_libblock_alloc_in_lib(bmain, owner_library, type, name, 0));
   BKE_libblock_init_empty(id);
 
   return id;
 }
 
+void *BKE_id_new(Main *bmain, const short type, const char *name)
+{
+  return BKE_id_new_in_lib(bmain, std::nullopt, type, name);
+}
+
 void *BKE_id_new_nomain(const short type, const char *name)
 {
   if (name == nullptr) {
@@ -1359,7 +1430,11 @@ void *BKE_id_new_nomain(const short type, const char *name)
   return id;
 }
 
-void BKE_libblock_copy_ex(Main *bmain, const ID *id, ID **r_newid, const int orig_flag)
+void BKE_libblock_copy_in_lib(Main *bmain,
+                              std::optional<Library *> owner_library,
+                              const ID *id,
+                              ID **r_newid,
+                              const int orig_flag)
 {
   ID *new_id = *r_newid;
   int flag = orig_flag;
@@ -1388,10 +1463,12 @@ void BKE_libblock_copy_ex(Main *bmain, const ID *id, ID **r_newid, const int ori
     STRNCPY(new_id->name, id->name);
     new_id->us = 0;
     new_id->tag |= LIB_TAG_NOT_ALLOCATED | LIB_TAG_NO_MAIN | LIB_TAG_NO_USER_REFCOUNT;
+    new_id->lib = owner_library ? *owner_library : id->lib;
     /* TODO: Do we want/need to copy more from ID struct itself? */
   }
   else {
-    new_id = static_cast<ID *>(BKE_libblock_alloc(bmain, GS(id->name), id->name + 2, flag));
+    new_id = static_cast<ID *>(
+        BKE_libblock_alloc_in_lib(bmain, owner_library, GS(id->name), id->name + 2, flag));
   }
   BLI_assert(new_id != nullptr);
 
@@ -1451,7 +1528,7 @@ void BKE_libblock_copy_ex(Main *bmain, const ID *id, ID **r_newid, const int ori
        * in their anim data *are* in bmain... super-mega-hooray. */
       BLI_assert((copy_data_flag & LIB_ID_COPY_ACTIONS) == 0 ||
                  (copy_data_flag & LIB_ID_CREATE_NO_MAIN) == 0);
-      iat->adt = BKE_animdata_copy(bmain, iat->adt, copy_data_flag);
+      iat->adt = BKE_animdata_copy_in_lib(bmain, owner_library, iat->adt, copy_data_flag);
     }
     else {
       iat->adt = nullptr;
@@ -1465,11 +1542,16 @@ void BKE_libblock_copy_ex(Main *bmain, const ID *id, ID **r_newid, const int ori
   *r_newid = new_id;
 }
 
+void BKE_libblock_copy_ex(Main *bmain, const ID *id, ID **r_newid, const int orig_flag)
+{
+  BKE_libblock_copy_in_lib(bmain, std::nullopt, id, r_newid, orig_flag);
+}
+
 void *BKE_libblock_copy(Main *bmain, const ID *id)
 {
   ID *idn;
 
-  BKE_libblock_copy_ex(bmain, id, &idn, 0);
+  BKE_libblock_copy_in_lib(bmain, std::nullopt, id, &idn, 0);
 
   return idn;
 }
diff --git a/source/blender/blenkernel/intern/lib_override.cc b/source/blender/blenkernel/intern/lib_override.cc
index d0927375f16..7df00b7c0ef 100644
--- a/source/blender/blenkernel/intern/lib_override.cc
+++ b/source/blender/blenkernel/intern/lib_override.cc
@@ -253,11 +253,17 @@ static ID *lib_override_library_create_from(Main *bmain,
                                             const int lib_id_copy_flags)
 {
   /* NOTE: do not copy possible override data from the reference here. */
-  ID *local_id = BKE_id_copy_ex(bmain,
-                                reference_id,
-                                nullptr,
-                                LIB_ID_COPY_DEFAULT | LIB_ID_COPY_NO_LIB_OVERRIDE |
-                                    lib_id_copy_flags);
+  ID *local_id = BKE_id_copy_in_lib(
+      bmain,
+      owner_library,
+      reference_id,
+      nullptr,
+      (LIB_ID_COPY_DEFAULT | LIB_ID_COPY_NO_LIB_OVERRIDE | lib_id_copy_flags));
+  if (local_id == nullptr) {
+    return nullptr;
+  }
+  BLI_assert(local_id->lib == owner_library);
+  id_us_min(local_id);
 
   /* In case we could not get an override ID with the exact same name as its linked reference,
    * ensure we at least get a uniquely named override ID over the whole current Main data, to
@@ -271,16 +277,14 @@ static ID *lib_override_library_create_from(Main *bmain,
     id_sort_by_name(which_libbase(bmain, GS(local_id->name)), local_id, nullptr);
   }
 
-  if (local_id == nullptr) {
-    return nullptr;
-  }
-  id_us_min(local_id);
-
-  /* TODO: Handle this properly in LIB_NO_MAIN case as well (i.e. resync case). Or offload to
-   * generic ID copy code? Would probably be better to have a version of #BKE_id_copy_ex that takes
-   * an extra `target_lib` parameter. */
-  local_id->lib = owner_library;
-  if ((lib_id_copy_flags & LIB_ID_CREATE_NO_MAIN) != 0 && owner_library == nullptr) {
+  /* In `NO_MAIN` case, generic `BKE_id_copy` code won't call this.
+   * In liboverride resync case however, the currently not-in-Main new IDs will be added back to
+   * Main later, so ensure that their linked dependencies and paths are properly handled here.
+   *
+   * NOTE: This is likely not the best place to do this. Ideally, #BKE_libblock_management_main_add
+   * e.g. should take care of this. But for the time being, this works and has been battle-proofed.
+   */
+  if ((lib_id_copy_flags & LIB_ID_CREATE_NO_MAIN) != 0 && !ID_IS_LINKED(local_id)) {
     lib_id_copy_ensure_local(bmain, reference_id, local_id, 0);
   }
 
diff --git a/source/blender/blenkernel/intern/light.cc b/source/blender/blenkernel/intern/light.cc
index 7a17c5e0e29..7d475b5da03 100644
--- a/source/blender/blenkernel/intern/light.cc
+++ b/source/blender/blenkernel/intern/light.cc
@@ -7,6 +7,7 @@
  */
 
 #include <cstdlib>
+#include <optional>
 
 #include "MEM_guardedalloc.h"
 
@@ -53,7 +54,11 @@ static void light_init_data(ID *id)
  *
  * \param flag: Copying options (see BKE_lib_id.hh's LIB_ID_COPY_... flags for more).
  */
-static void light_copy_data(Main *bmain, ID *id_dst, const ID *id_src, const int flag)
+static void light_copy_data(Main *bmain,
+                            std::optional<Library *> owner_library,
+                            ID *id_dst,
+                            const ID *id_src,
+                            const int flag)
 {
   Light *la_dst = (Light *)id_dst;
   const Light *la_src = (const Light *)id_src;
@@ -67,8 +72,11 @@ static void light_copy_data(Main *bmain, ID *id_dst, const ID *id_src, const int
       la_dst->nodetree = ntreeLocalize(la_src->nodetree);
     }
     else {
-      BKE_id_copy_ex(
-          bmain, (ID *)la_src->nodetree, (ID **)&la_dst->nodetree, flag_private_id_data);
+      BKE_id_copy_in_lib(bmain,
+                         owner_library,
+                         (ID *)la_src->nodetree,
+                         (ID **)&la_dst->nodetree,
+                         flag_private_id_data);
     }
     la_dst->nodetree->owner_id = &la_dst->id;
   }
diff --git a/source/blender/blenkernel/intern/linestyle.cc b/source/blender/blenkernel/intern/linestyle.cc
index 1891d7d7383..e83bfe745d3 100644
--- a/source/blender/blenkernel/intern/linestyle.cc
+++ b/source/blender/blenkernel/intern/linestyle.cc
@@ -9,6 +9,8 @@
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
+#include <optional>
+
 #include <fmt/format.h>
 
 #include "MEM_guardedalloc.h"
@@ -50,7 +52,11 @@ static void linestyle_init_data(ID *id)
   BKE_linestyle_geometry_modifier_add(linestyle, nullptr, LS_MODIFIER_SAMPLING);
 }
 
-static void linestyle_copy_data(Main *bmain, ID *id_dst, const ID *id_src, const int flag)
+static void linestyle_copy_data(Main *bmain,
+                                std::optional<Library *> owner_library,
+                                ID *id_dst,
+                                const ID *id_src,
+                                const int flag)
 {
   FreestyleLineStyle *linestyle_dst = (FreestyleLineStyle *)id_dst;
   const FreestyleLineStyle *linestyle_src = (const FreestyleLineStyle *)id_src;
@@ -68,10 +74,11 @@ static void linestyle_copy_data(Main *bmain, ID *id_dst, const ID *id_src, const
   }
 
   if (linestyle_src->nodetree) {
-    BKE_id_copy_ex(bmain,
-                   (ID *)linestyle_src->nodetree,
-                   (ID **)&linestyle_dst->nodetree,
-                   flag_private_id_data);
+    BKE_id_copy_in_lib(bmain,
+                       owner_library,
+                       (ID *)linestyle_src->nodetree,
+                       (ID **)&linestyle_dst->nodetree,
+                       flag_private_id_data);
     linestyle_dst->nodetree->owner_id = &linestyle_dst->id;
   }
 
diff --git a/source/blender/blenkernel/intern/main_namemap.cc b/source/blender/blenkernel/intern/main_namemap.cc
index 4b39f7dd41f..c5526f92817 100644
--- a/source/blender/blenkernel/intern/main_namemap.cc
+++ b/source/blender/blenkernel/intern/main_namemap.cc
@@ -401,6 +401,7 @@ bool BKE_main_namemap_get_name(Main *bmain, ID *id, char *name, const bool do_un
      * truncated name again. */
     is_name_changed = true;
   }
+
   return is_name_changed;
 }
 
diff --git a/source/blender/blenkernel/intern/mask.cc b/source/blender/blenkernel/intern/mask.cc
index b4f922d75a0..d83ce2575dd 100644
--- a/source/blender/blenkernel/intern/mask.cc
+++ b/source/blender/blenkernel/intern/mask.cc
@@ -8,6 +8,7 @@
 
 #include <cstddef>
 #include <cstring>
+#include <optional>
 
 #include "CLG_log.h"
 
@@ -49,7 +50,11 @@
 
 static CLG_LogRef LOG = {"bke.mask"};
 
-static void mask_copy_data(Main * /*bmain*/, ID *id_dst, const ID *id_src, const int /*flag*/)
+static void mask_copy_data(Main * /*bmain*/,
+                           std::optional<Library *> /*owner_library*/,
+                           ID *id_dst,
+                           const ID *id_src,
+                           const int /*flag*/)
 {
   Mask *mask_dst = (Mask *)id_dst;
   const Mask *mask_src = (const Mask *)id_src;
diff --git a/source/blender/blenkernel/intern/material.cc b/source/blender/blenkernel/intern/material.cc
index fcfdebc4058..e8a79b8343f 100644
--- a/source/blender/blenkernel/intern/material.cc
+++ b/source/blender/blenkernel/intern/material.cc
@@ -9,6 +9,7 @@
 #include <cmath>
 #include <cstddef>
 #include <cstring>
+#include <optional>
 
 #include "CLG_log.h"
 
@@ -91,7 +92,11 @@ static void material_init_data(ID *id)
   *((short *)id->name) = ID_MA;
 }
 
-static void material_copy_data(Main *bmain, ID *id_dst, const ID *id_src, const int flag)
+static void material_copy_data(Main *bmain,
+                               std::optional<Library *> owner_library,
+                               ID *id_dst,
+                               const ID *id_src,
+                               const int flag)
 {
   Material *material_dst = (Material *)id_dst;
   const Material *material_src = (const Material *)id_src;
@@ -105,10 +110,11 @@ static void material_copy_data(Main *bmain, ID *id_dst, const ID *id_src, const
       material_dst->nodetree = ntreeLocalize(material_src->nodetree);
     }
     else {
-      BKE_id_copy_ex(bmain,
-                     (ID *)material_src->nodetree,
-                     (ID **)&material_dst->nodetree,
-                     flag_private_id_data);
+      BKE_id_copy_in_lib(bmain,
+                         owner_library,
+                         (ID *)material_src->nodetree,
+                         (ID **)&material_dst->nodetree,
+                         flag_private_id_data);
     }
     material_dst->nodetree->owner_id = &material_dst->id;
   }
diff --git a/source/blender/blenkernel/intern/mball.cc b/source/blender/blenkernel/intern/mball.cc
index 8628846a5b5..8002ed8dca4 100644
--- a/source/blender/blenkernel/intern/mball.cc
+++ b/source/blender/blenkernel/intern/mball.cc
@@ -15,6 +15,7 @@
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
+#include <optional>
 
 #include "MEM_guardedalloc.h"
 
@@ -64,7 +65,11 @@ static void metaball_init_data(ID *id)
   MEMCPY_STRUCT_AFTER(metaball, DNA_struct_default_get(MetaBall), id);
 }
 
-static void metaball_copy_data(Main * /*bmain*/, ID *id_dst, const ID *id_src, const int /*flag*/)
+static void metaball_copy_data(Main * /*bmain*/,
+                               std::optional<Library *> /*owner_library*/,
+                               ID *id_dst,
+                               const ID *id_src,
+                               const int /*flag*/)
 {
   MetaBall *metaball_dst = (MetaBall *)id_dst;
   const MetaBall *metaball_src = (const MetaBall *)id_src;
diff --git a/source/blender/blenkernel/intern/mesh.cc b/source/blender/blenkernel/intern/mesh.cc
index 6c097019e23..89e968be30f 100644
--- a/source/blender/blenkernel/intern/mesh.cc
+++ b/source/blender/blenkernel/intern/mesh.cc
@@ -6,6 +6,8 @@
  * \ingroup bke
  */
 
+#include <optional>
+
 #include "MEM_guardedalloc.h"
 
 /* Allow using deprecated functionality for .blend file I/O. */
@@ -99,7 +101,11 @@ static void mesh_init_data(ID *id)
   mesh->face_sets_color_seed = BLI_hash_int(BLI_time_now_seconds_i() & UINT_MAX);
 }
 
-static void mesh_copy_data(Main *bmain, ID *id_dst, const ID *id_src, const int flag)
+static void mesh_copy_data(Main *bmain,
+                           std::optional<Library *> owner_library,
+                           ID *id_dst,
+                           const ID *id_src,
+                           const int flag)
 {
   Mesh *mesh_dst = reinterpret_cast<Mesh *>(id_dst);
   const Mesh *mesh_src = reinterpret_cast<const Mesh *>(id_src);
@@ -193,7 +199,7 @@ static void mesh_copy_data(Main *bmain, ID *id_dst, const ID *id_src, const int
 
   /* TODO: Do we want to add flag to prevent this? */
   if (mesh_src->key && (flag & LIB_ID_COPY_SHAPEKEY)) {
-    BKE_id_copy_ex(bmain, &mesh_src->key->id, (ID **)&mesh_dst->key, flag);
+    BKE_id_copy_in_lib(bmain, owner_library, &mesh_src->key->id, (ID **)&mesh_dst->key, flag);
     /* XXX This is not nice, we need to make BKE_id_copy_ex fully re-entrant... */
     mesh_dst->key->from = &mesh_dst->id;
   }
diff --git a/source/blender/blenkernel/intern/movieclip.cc b/source/blender/blenkernel/intern/movieclip.cc
index 359192579a2..838d0ab969f 100644
--- a/source/blender/blenkernel/intern/movieclip.cc
+++ b/source/blender/blenkernel/intern/movieclip.cc
@@ -9,6 +9,7 @@
 #include <cstdio>
 #include <cstring>
 #include <fcntl.h>
+#include <optional>
 
 #ifndef WIN32
 #  include <unistd.h>
@@ -81,7 +82,11 @@ static void movie_clip_init_data(ID *id)
   BKE_color_managed_colorspace_settings_init(&movie_clip->colorspace_settings);
 }
 
-static void movie_clip_copy_data(Main * /*bmain*/, ID *id_dst, const ID *id_src, const int flag)
+static void movie_clip_copy_data(Main * /*bmain*/,
+                                 std::optional<Library *> /*owner_library*/,
+                                 ID *id_dst,
+                                 const ID *id_src,
+                                 const int flag)
 {
   MovieClip *movie_clip_dst = (MovieClip *)id_dst;
   const MovieClip *movie_clip_src = (const MovieClip *)id_src;
diff --git a/source/blender/blenkernel/intern/node.cc b/source/blender/blenkernel/intern/node.cc
index 31666ef6d79..60048248ec2 100644
--- a/source/blender/blenkernel/intern/node.cc
+++ b/source/blender/blenkernel/intern/node.cc
@@ -14,6 +14,7 @@
 #include <cstddef>
 #include <cstdlib>
 #include <cstring>
+#include <optional>
 
 /* Allow using deprecated functionality for .blend file I/O. */
 #define DNA_DEPRECATED_ALLOW
@@ -150,7 +151,11 @@ static void ntree_init_data(ID *id)
   ntree_set_typeinfo(ntree, nullptr);
 }
 
-static void ntree_copy_data(Main * /*bmain*/, ID *id_dst, const ID *id_src, const int flag)
+static void ntree_copy_data(Main * /*bmain*/,
+                            std::optional<Library *> /*owner_library*/,
+                            ID *id_dst,
+                            const ID *id_src,
+                            const int flag)
 {
   bNodeTree *ntree_dst = reinterpret_cast<bNodeTree *>(id_dst);
   const bNodeTree *ntree_src = reinterpret_cast<const bNodeTree *>(id_src);
diff --git a/source/blender/blenkernel/intern/object.cc b/source/blender/blenkernel/intern/object.cc
index 18c15d74b92..d81e76f090f 100644
--- a/source/blender/blenkernel/intern/object.cc
+++ b/source/blender/blenkernel/intern/object.cc
@@ -12,6 +12,7 @@
 #include <cmath>
 #include <cstdio>
 #include <cstring>
+#include <optional>
 
 #include "CLG_log.h"
 
@@ -188,7 +189,11 @@ static void object_init_data(ID *id)
   animviz_settings_init(&ob->avs);
 }
 
-static void object_copy_data(Main *bmain, ID *id_dst, const ID *id_src, const int flag)
+static void object_copy_data(Main *bmain,
+                             std::optional<Library *> /*owner_library*/,
+                             ID *id_dst,
+                             const ID *id_src,
+                             const int flag)
 {
   Object *ob_dst = (Object *)id_dst;
   const Object *ob_src = (const Object *)id_src;
diff --git a/source/blender/blenkernel/intern/paint.cc b/source/blender/blenkernel/intern/paint.cc
index dc26aca972f..18058a053e7 100644
--- a/source/blender/blenkernel/intern/paint.cc
+++ b/source/blender/blenkernel/intern/paint.cc
@@ -8,6 +8,7 @@
 
 #include <cstdlib>
 #include <cstring>
+#include <optional>
 
 #include "MEM_guardedalloc.h"
 
@@ -100,7 +101,11 @@ static void palette_init_data(ID *id)
   id_fake_user_set(&palette->id);
 }
 
-static void palette_copy_data(Main * /*bmain*/, ID *id_dst, const ID *id_src, const int /*flag*/)
+static void palette_copy_data(Main * /*bmain*/,
+                              std::optional<Library *> /*owner_library*/,
+                              ID *id_dst,
+                              const ID *id_src,
+                              const int /*flag*/)
 {
   Palette *palette_dst = (Palette *)id_dst;
   const Palette *palette_src = (const Palette *)id_src;
@@ -172,6 +177,7 @@ IDTypeInfo IDType_ID_PAL = {
 };
 
 static void paint_curve_copy_data(Main * /*bmain*/,
+                                  std::optional<Library *> /*owner_library*/,
                                   ID *id_dst,
                                   const ID *id_src,
                                   const int /*flag*/)
diff --git a/source/blender/blenkernel/intern/particle.cc b/source/blender/blenkernel/intern/particle.cc
index 17d2594055f..b608d908dd7 100644
--- a/source/blender/blenkernel/intern/particle.cc
+++ b/source/blender/blenkernel/intern/particle.cc
@@ -13,6 +13,7 @@
 #include <cmath>
 #include <cstdlib>
 #include <cstring>
+#include <optional>
 
 #include "MEM_guardedalloc.h"
 
@@ -99,6 +100,7 @@ static void particle_settings_init(ID *id)
 }
 
 static void particle_settings_copy_data(Main * /*bmain*/,
+                                        std::optional<Library *> /*owner_library*/,
                                         ID *id_dst,
                                         const ID *id_src,
                                         const int /*flag*/)
diff --git a/source/blender/blenkernel/intern/pointcloud.cc b/source/blender/blenkernel/intern/pointcloud.cc
index 3ebf1bd1617..aa4a3c1d58e 100644
--- a/source/blender/blenkernel/intern/pointcloud.cc
+++ b/source/blender/blenkernel/intern/pointcloud.cc
@@ -6,6 +6,8 @@
  * \ingroup bke
  */
 
+#include <optional>
+
 #include "MEM_guardedalloc.h"
 
 #include "DNA_defaults.h"
@@ -68,6 +70,7 @@ static void pointcloud_init_data(ID *id)
 }
 
 static void pointcloud_copy_data(Main * /*bmain*/,
+                                 std::optional<Library *> /*owner_library*/,
                                  ID *id_dst,
                                  const ID *id_src,
                                  const int /*flag*/)
diff --git a/source/blender/blenkernel/intern/scene.cc b/source/blender/blenkernel/intern/scene.cc
index adb52594082..eac2b61592a 100644
--- a/source/blender/blenkernel/intern/scene.cc
+++ b/source/blender/blenkernel/intern/scene.cc
@@ -12,6 +12,7 @@
 #include <cstddef>
 #include <cstdio>
 #include <cstring>
+#include <optional>
 
 #include "MEM_guardedalloc.h"
 
@@ -251,7 +252,11 @@ static void scene_copy_markers(Scene *scene_dst, const Scene *scene_src, const i
   }
 }
 
-static void scene_copy_data(Main *bmain, ID *id_dst, const ID *id_src, const int flag)
+static void scene_copy_data(Main *bmain,
+                            std::optional<Library *> owner_library,
+                            ID *id_dst,
+                            const ID *id_src,
+                            const int flag)
 {
   Scene *scene_dst = (Scene *)id_dst;
   const Scene *scene_src = (const Scene *)id_src;
@@ -266,10 +271,11 @@ static void scene_copy_data(Main *bmain, ID *id_dst, const ID *id_src, const int
 
   /* Master Collection */
   if (scene_src->master_collection) {
-    BKE_id_copy_ex(bmain,
-                   (ID *)scene_src->master_collection,
-                   (ID **)&scene_dst->master_collection,
-                   flag_private_id_data);
+    BKE_id_copy_in_lib(bmain,
+                       owner_library,
+                       reinterpret_cast<ID *>(scene_src->master_collection),
+                       reinterpret_cast<ID **>(&scene_dst->master_collection),
+                       flag_private_id_data);
     scene_dst->master_collection->owner_id = &scene_dst->id;
   }
 
@@ -293,8 +299,11 @@ static void scene_copy_data(Main *bmain, ID *id_dst, const ID *id_src, const int
   BKE_keyingsets_copy(&(scene_dst->keyingsets), &(scene_src->keyingsets));
 
   if (scene_src->nodetree) {
-    BKE_id_copy_ex(
-        bmain, (ID *)scene_src->nodetree, (ID **)&scene_dst->nodetree, flag_private_id_data);
+    BKE_id_copy_in_lib(bmain,
+                       owner_library,
+                       (ID *)scene_src->nodetree,
+                       (ID **)&scene_dst->nodetree,
+                       flag_private_id_data);
     BKE_libblock_relink_ex(bmain,
                            scene_dst->nodetree,
                            (void *)(&scene_src->id),
diff --git a/source/blender/blenkernel/intern/sound.cc b/source/blender/blenkernel/intern/sound.cc
index 50ec2c9e611..3753152840b 100644
--- a/source/blender/blenkernel/intern/sound.cc
+++ b/source/blender/blenkernel/intern/sound.cc
@@ -8,6 +8,7 @@
 
 #include <cstdlib>
 #include <cstring>
+#include <optional>
 
 #include "MEM_guardedalloc.h"
 
@@ -58,7 +59,11 @@
 
 static void sound_free_audio(bSound *sound);
 
-static void sound_copy_data(Main * /*bmain*/, ID *id_dst, const ID *id_src, const int /*flag*/)
+static void sound_copy_data(Main * /*bmain*/,
+                            std::optional<Library *> /*owner_library*/,
+                            ID *id_dst,
+                            const ID *id_src,
+                            const int /*flag*/)
 {
   bSound *sound_dst = (bSound *)id_dst;
   const bSound *sound_src = (const bSound *)id_src;
diff --git a/source/blender/blenkernel/intern/text.cc b/source/blender/blenkernel/intern/text.cc
index f32ab16f037..3e53bdcefda 100644
--- a/source/blender/blenkernel/intern/text.cc
+++ b/source/blender/blenkernel/intern/text.cc
@@ -9,6 +9,7 @@
 #include <cstdlib> /* abort */
 #include <cstring> /* strstr */
 #include <cwctype>
+#include <optional>
 #include <sys/stat.h>
 #include <sys/types.h>
 
@@ -101,7 +102,11 @@ static void text_init_data(ID *id)
  *
  * \param flag: Copying options (see BKE_lib_id.hh's LIB_ID_COPY_... flags for more).
  */
-static void text_copy_data(Main * /*bmain*/, ID *id_dst, const ID *id_src, const int /*flag*/)
+static void text_copy_data(Main * /*bmain*/,
+                           std::optional<Library *> /*owner_library*/,
+                           ID *id_dst,
+                           const ID *id_src,
+                           const int /*flag*/)
 {
   Text *text_dst = (Text *)id_dst;
   const Text *text_src = (Text *)id_src;
diff --git a/source/blender/blenkernel/intern/texture.cc b/source/blender/blenkernel/intern/texture.cc
index d69e704c948..a28b566d636 100644
--- a/source/blender/blenkernel/intern/texture.cc
+++ b/source/blender/blenkernel/intern/texture.cc
@@ -10,6 +10,7 @@
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
+#include <optional>
 
 #include "MEM_guardedalloc.h"
 
@@ -65,7 +66,11 @@ static void texture_init_data(ID *id)
   BKE_imageuser_default(&texture->iuser);
 }
 
-static void texture_copy_data(Main *bmain, ID *id_dst, const ID *id_src, const int flag)
+static void texture_copy_data(Main *bmain,
+                              std::optional<Library *> owner_library,
+                              ID *id_dst,
+                              const ID *id_src,
+                              const int flag)
 {
   Tex *texture_dst = (Tex *)id_dst;
   const Tex *texture_src = (const Tex *)id_src;
@@ -90,8 +95,11 @@ static void texture_copy_data(Main *bmain, ID *id_dst, const ID *id_src, const i
       texture_dst->nodetree = ntreeLocalize(texture_src->nodetree);
     }
     else {
-      BKE_id_copy_ex(
-          bmain, (ID *)texture_src->nodetree, (ID **)&texture_dst->nodetree, flag_private_id_data);
+      BKE_id_copy_in_lib(bmain,
+                         owner_library,
+                         (ID *)texture_src->nodetree,
+                         (ID **)&texture_dst->nodetree,
+                         flag_private_id_data);
     }
     texture_dst->nodetree->owner_id = &texture_dst->id;
   }
diff --git a/source/blender/blenkernel/intern/vfont.cc b/source/blender/blenkernel/intern/vfont.cc
index d131afacbcd..06b920300a0 100644
--- a/source/blender/blenkernel/intern/vfont.cc
+++ b/source/blender/blenkernel/intern/vfont.cc
@@ -12,6 +12,7 @@
 #include <cstdlib>
 #include <cstring>
 #include <cwctype>
+#include <optional>
 
 #include "CLG_log.h"
 
@@ -82,7 +83,11 @@ static void vfont_init_data(ID *id)
   }
 }
 
-static void vfont_copy_data(Main * /*bmain*/, ID *id_dst, const ID * /*id_src*/, const int flag)
+static void vfont_copy_data(Main * /*bmain*/,
+                            std::optional<Library *> /*owner_library*/,
+                            ID *id_dst,
+                            const ID * /*id_src*/,
+                            const int flag)
 {
   VFont *vfont_dst = (VFont *)id_dst;
 
diff --git a/source/blender/blenkernel/intern/volume.cc b/source/blender/blenkernel/intern/volume.cc
index ac19226b63c..5b3ace72759 100644
--- a/source/blender/blenkernel/intern/volume.cc
+++ b/source/blender/blenkernel/intern/volume.cc
@@ -6,6 +6,8 @@
  * \ingroup bke
  */
 
+#include <optional>
+
 #include "MEM_guardedalloc.h"
 
 #include "DNA_defaults.h"
@@ -148,7 +150,11 @@ static void volume_init_data(ID *id)
   STRNCPY(volume->velocity_grid, "velocity");
 }
 
-static void volume_copy_data(Main * /*bmain*/, ID *id_dst, const ID *id_src, const int /*flag*/)
+static void volume_copy_data(Main * /*bmain*/,
+                             std::optional<Library *> /*owner_library*/,
+                             ID *id_dst,
+                             const ID *id_src,
+                             const int /*flag*/)
 {
   Volume *volume_dst = (Volume *)id_dst;
   const Volume *volume_src = (const Volume *)id_src;
diff --git a/source/blender/blenkernel/intern/world.cc b/source/blender/blenkernel/intern/world.cc
index 3f660215df7..6c6bdc57b02 100644
--- a/source/blender/blenkernel/intern/world.cc
+++ b/source/blender/blenkernel/intern/world.cc
@@ -9,6 +9,7 @@
 #include <cmath>
 #include <cstdlib>
 #include <cstring>
+#include <optional>
 
 #include "MEM_guardedalloc.h"
 
@@ -81,7 +82,11 @@ static void world_init_data(ID *id)
  *
  * \param flag: Copying options (see BKE_lib_id.hh's LIB_ID_COPY_... flags for more).
  */
-static void world_copy_data(Main *bmain, ID *id_dst, const ID *id_src, const int flag)
+static void world_copy_data(Main *bmain,
+                            std::optional<Library *> owner_library,
+                            ID *id_dst,
+                            const ID *id_src,
+                            const int flag)
 {
   World *wrld_dst = (World *)id_dst;
   const World *wrld_src = (const World *)id_src;
@@ -95,8 +100,11 @@ static void world_copy_data(Main *bmain, ID *id_dst, const ID *id_src, const int
       wrld_dst->nodetree = ntreeLocalize(wrld_src->nodetree);
     }
     else {
-      BKE_id_copy_ex(
-          bmain, (ID *)wrld_src->nodetree, (ID **)&wrld_dst->nodetree, flag_private_id_data);
+      BKE_id_copy_in_lib(bmain,
+                         owner_library,
+                         (ID *)wrld_src->nodetree,
+                         (ID **)&wrld_dst->nodetree,
+                         flag_private_id_data);
     }
     wrld_dst->nodetree->owner_id = &wrld_dst->id;
   }