BLO: Functions for temporarily loading a single datablock

This introduces two functions to the blenloader module, here shown as
calls for brevity:

* `temp_lib_ctx = BLO_library_temp_load_id(real_main, blend_file_path, idcode, idname, reports);`
* Now the data in `temp_lib_ctx->temp_id` can be used (but ought not to
  be not assigned to non-temp datablocks).
* `BLO_library_temp_free(temp_lib_ctx);`

The first loads a datablock from a blend file, and returns it as part of
a `struct TempLibraryContext`. This struct contains the temp-loaded ID,
as well as enough information to correctly free everything again.

Differential Revision: https://developer.blender.org/D10736

3 files changed, 80 insertions, 0 deletions

diff --git a/source/blender/blenloader/BLO_readfile.h b/source/blender/blenloader/BLO_readfile.h
index 09f4c405613..d8c613ba900 100644
--- a/source/blender/blenloader/BLO_readfile.h
+++ b/source/blender/blenloader/BLO_readfile.h
@@ -18,6 +18,7 @@
  */
 #pragma once
 
+#include "BLI_listbase.h"
 #include "BLI_sys_types.h"
 
 /** \file
@@ -221,6 +222,26 @@ void BLO_library_link_end(struct Main *mainl,
 
 int BLO_library_link_copypaste(struct Main *mainl, BlendHandle *bh, const uint64_t id_types_mask);
 
+/**
+ * Struct for temporarily loading datablocks from a blend file.
+ */
+typedef struct TempLibraryContext {
+  struct Main *temp_main;
+  struct BlendHandle *blendhandle;
+  struct LibraryLink_Params liblink_params;
+  struct Library *lib;
+
+  /* The ID datablock that was loaded. Is NULL if loading failed. */
+  struct ID *temp_id;
+} TempLibraryContext;
+
+TempLibraryContext *BLO_library_temp_load_id(struct Main *real_main,
+                                             const char *blend_file_path,
+                                             const short idcode,
+                                             const char *idname,
+                                             struct ReportList *reports);
+void BLO_library_temp_free(TempLibraryContext *temp_lib_ctx);
+
 /** \} */
 
 void *BLO_library_read_struct(struct FileData *fd, struct BHead *bh, const char *blockname);
diff --git a/source/blender/blenloader/CMakeLists.txt b/source/blender/blenloader/CMakeLists.txt
index b65a3c4d58e..ee9b9a49768 100644
--- a/source/blender/blenloader/CMakeLists.txt
+++ b/source/blender/blenloader/CMakeLists.txt
@@ -50,6 +50,7 @@ set(SRC
   intern/blend_validate.c
   intern/readblenentry.c
   intern/readfile.c
+  intern/readfile_tempload.c
   intern/undofile.c
   intern/versioning_250.c
   intern/versioning_260.c
diff --git a/source/blender/blenloader/intern/readfile_tempload.c b/source/blender/blenloader/intern/readfile_tempload.c
new file mode 100644
index 00000000000..691d8f542ab
--- /dev/null
+++ b/source/blender/blenloader/intern/readfile_tempload.c
@@ -0,0 +1,58 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ */
+
+/** \file
+ * \ingroup blenloader
+ */
+#include "BLO_readfile.h"
+
+#include "MEM_guardedalloc.h"
+
+#include "BKE_report.h"
+
+#include "DNA_ID.h"
+
+TempLibraryContext *BLO_library_temp_load_id(struct Main *real_main,
+                                             const char *blend_file_path,
+                                             const short idcode,
+                                             const char *idname,
+                                             struct ReportList *reports)
+{
+  TempLibraryContext *temp_lib_ctx = MEM_callocN(sizeof(*temp_lib_ctx), __func__);
+
+  temp_lib_ctx->blendhandle = BLO_blendhandle_from_file(blend_file_path, reports);
+
+  BLO_library_link_params_init(&temp_lib_ctx->liblink_params, real_main, 0, LIB_TAG_TEMP_MAIN);
+
+  temp_lib_ctx->temp_main = BLO_library_link_begin(
+      &temp_lib_ctx->blendhandle, blend_file_path, &temp_lib_ctx->liblink_params);
+
+  temp_lib_ctx->temp_id = BLO_library_link_named_part(temp_lib_ctx->temp_main,
+                                                      &temp_lib_ctx->blendhandle,
+                                                      idcode,
+                                                      idname,
+                                                      &temp_lib_ctx->liblink_params);
+
+  return temp_lib_ctx;
+}
+
+void BLO_library_temp_free(TempLibraryContext *temp_lib_ctx)
+{
+  BLO_library_link_end(
+      temp_lib_ctx->temp_main, &temp_lib_ctx->blendhandle, &temp_lib_ctx->liblink_params);
+  BLO_blendhandle_close(temp_lib_ctx->blendhandle);
+  MEM_freeN(temp_lib_ctx);
+}