PyAPI: add bpy.types.BlendFile.temp_data for temporary library loading

This adds support for creating a `BlendFile` (internally called `Main`),
which is limited to a context.

Temporary data can now be created which can then use
`.libraries.load()` the same as with `bpy.data`.

To prevent errors caused by mixing the temporary ID's with data in
`bpy.data` they are tagged as temporary so they can't be assigned
to properties, however they can be passed as arguments to functions.

Reviewed By: mont29, sybren

Ref D10612

2 files changed, 14 insertions, 3 deletions

diff --git a/source/blender/blenloader/intern/readfile.c b/source/blender/blenloader/intern/readfile.c
index aa3362aa211..de7353d827a 100644
--- a/source/blender/blenloader/intern/readfile.c
+++ b/source/blender/blenloader/intern/readfile.c
@@ -2424,7 +2424,9 @@ static void direct_link_id_common(
     id->session_uuid = MAIN_ID_SESSION_UUID_UNSET;
   }
 
-  BKE_lib_libblock_session_uuid_ensure(id);
+  if ((tag & LIB_TAG_TEMP_MAIN) == 0) {
+    BKE_lib_libblock_session_uuid_ensure(id);
+  }
 
   id->lib = current_library;
   id->us = ID_FAKE_USERS(id);
@@ -3168,7 +3170,9 @@ static ID *create_placeholder(Main *mainvar, const short idcode, const char *idn
   BLI_addtail(lb, ph_id);
   id_sort_by_name(lb, ph_id, NULL);
 
-  BKE_lib_libblock_session_uuid_ensure(ph_id);
+  if ((tag & LIB_TAG_TEMP_MAIN) == 0) {
+    BKE_lib_libblock_session_uuid_ensure(ph_id);
+  }
 
   return ph_id;
 }
@@ -4998,6 +5002,11 @@ static Main *library_link_begin(
 {
   Main *mainl;
 
+  /* Only allow specific tags to be set as extra,
+   * otherwise this could conflict with library loading logic.
+   * Other flags can be added here, as long as they are safe. */
+  BLI_assert((id_tag_extra & ~LIB_TAG_TEMP_MAIN) == 0);
+
   (*fd)->id_tag_extra = id_tag_extra;
 
   (*fd)->mainlist = MEM_callocN(sizeof(ListBase), "FileData.mainlist");
diff --git a/source/blender/blenloader/intern/versioning_250.c b/source/blender/blenloader/intern/versioning_250.c
index 983fdce15f1..467fd8b0399 100644
--- a/source/blender/blenloader/intern/versioning_250.c
+++ b/source/blender/blenloader/intern/versioning_250.c
@@ -449,7 +449,9 @@ static void versions_gpencil_add_main(ListBase *lb, ID *id, const char *name)
   BKE_id_new_name_validate(lb, id, name);
   /* alphabetic insertion: is in BKE_id_new_name_validate */
 
-  BKE_lib_libblock_session_uuid_ensure(id);
+  if ((id->tag & LIB_TAG_TEMP_MAIN) == 0) {
+    BKE_lib_libblock_session_uuid_ensure(id);
+  }
 
   if (G.debug & G_DEBUG) {
     printf("Converted GPencil to ID: %s\n", id->name + 2);