BKE_Main: Add clear separation between 'temp' mains and global main.

Blender is using more and more temporary Main data-base (historically
for reading linked data, but also now when resyncing liboverrides, for
temp data in asset code, etc.).

This commit aims at making this a bit more formal and defined, by:
* Adding a dedicated flag in Main struct to mark a Main as global.
* Adding some API to replace, or temporarily swap the current global
  Main (`G_MAIN`) by another one.

NOTE: Having to temporarily replace `G_MAIN` is a workaround for the
limitation of current RNA, ideally this should be fixed in RNA itself,
but for now at least having an API helps tracking those cases (since
this is potentially risky operation).

This work is also a preparation for more usages of temp mains in the near
future (Asset Brushes and its presets system e.g. will most likely use
temp mains too).

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D15977

1 files changed, 14 insertions, 0 deletions

diff --git a/source/blender/blenkernel/BKE_main.h b/source/blender/blenkernel/BKE_main.h
index 0048ad4dde5..7c3a64f1cad 100644
--- a/source/blender/blenkernel/BKE_main.h
+++ b/source/blender/blenkernel/BKE_main.h
@@ -138,6 +138,14 @@ typedef struct Main {
    */
   bool is_locked_for_linking;
 
+  /**
+   * True if this main is the 'GMAIN' of current Blender.
+   *
+   * \note There should always be only one global main, all others generated temporarily for
+   * various data management process must have this property set to false..
+   */
+  bool is_global_main;
+
   BlendThumbnail *blen_thumb;
 
   struct Library *curlib;
@@ -202,6 +210,12 @@ typedef struct Main {
   struct MainLock *lock;
 } Main;
 
+/**
+ * Create a new Main data-base.
+ *
+ * \note Always generate a non-global Main, use #BKE_blender_globals_main_replace to put a newly
+ * created one in `G_MAIN`.
+ */
 struct Main *BKE_main_new(void);
 void BKE_main_free(struct Main *mainvar);