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

source/blender/blenkernel/BKE_blender.h

@@ -11,6 +11,9 @@
 extern "C" {
 #endif
 
+#include "BLI_compiler_attrs.h"
+
+struct Main;
 struct UserDef;
 
 /**
@@ -21,6 +24,16 @@ void BKE_blender_free(void);
 void BKE_blender_globals_init(void);
 void BKE_blender_globals_clear(void);
 
+/** Replace current global Main by the given one, freeing existing one. */
+void BKE_blender_globals_main_replace(struct Main *bmain);
+/**
+ * Replace current global Main by the given one, returning the old one.
+ *
+ * \warning Advanced, risky workaround addressing the issue that current RNA is not able to process
+ * correectly non-G_MAIN data, use with (a lot of) care.
+ */
+struct Main *BKE_blender_globals_main_swap(struct Main *new_gmain);
+
 void BKE_blender_userdef_data_swap(struct UserDef *userdef_a, struct UserDef *userdef_b);
 void BKE_blender_userdef_data_set(struct UserDef *userdef);
 void BKE_blender_userdef_data_set_and_free(struct UserDef *userdef);

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);
 

source/blender/blenkernel/intern/blender.c

@@ -62,8 +62,7 @@ void BKE_blender_free(void)
   /* Needs to run before main free as wm is still referenced for icons preview jobs. */
   BKE_studiolight_free();
 
-  BKE_main_free(G_MAIN);
-  G_MAIN = NULL;
+  BKE_blender_globals_clear();
 
   if (G.log.file != NULL) {
     fclose(G.log.file);
@@ -146,7 +145,7 @@ void BKE_blender_globals_init(void)
 
   U.savetime = 1;
 
-  G_MAIN = BKE_main_new();
+  BKE_blender_globals_main_replace(BKE_main_new());
 
   strcpy(G.ima, "//");
 
@@ -161,11 +160,34 @@ void BKE_blender_globals_init(void)
 
 void BKE_blender_globals_clear(void)
 {
+  if (G_MAIN == NULL) {
+    return;
+  }
+  BLI_assert(G_MAIN->is_global_main);
   BKE_main_free(G_MAIN); /* free all lib data */
 
   G_MAIN = NULL;
 }
 
+void BKE_blender_globals_main_replace(Main *bmain)
+{
+  BLI_assert(!bmain->is_global_main);
+  BKE_blender_globals_clear();
+  bmain->is_global_main = true;
+  G_MAIN = bmain;
+}
+
+Main *BKE_blender_globals_main_swap(Main *new_gmain)
+{
+  Main *old_gmain = G_MAIN;
+  BLI_assert(old_gmain->is_global_main);
+  BLI_assert(!new_gmain->is_global_main);
+  new_gmain->is_global_main = true;
+  G_MAIN = new_gmain;
+  old_gmain->is_global_main = false;
+  return old_gmain;
+}
+
 /** \} */
 
 /* -------------------------------------------------------------------- */

source/blender/blenkernel/intern/blendfile.c

@@ -287,11 +287,8 @@ static void setup_app_data(bContext *C,
     }
   }
 
-  /* free G_MAIN Main database */
-  //  CTX_wm_manager_set(C, NULL);
-  BKE_blender_globals_clear();
-
-  bmain = G_MAIN = bfd->main;
+  BKE_blender_globals_main_replace(bfd->main);
+  bmain = G_MAIN;
   bfd->main = NULL;
 
   CTX_data_main_set(C, bmain);

source/blender/blenkernel/intern/lib_override.cc

@@ -23,6 +23,7 @@
 
 #include "BKE_anim_data.h"
 #include "BKE_armature.h"
+#include "BKE_blender.h"
 #include "BKE_collection.h"
 #include "BKE_fcurve.h"
 #include "BKE_global.h"
@@ -3829,9 +3830,8 @@ void BKE_lib_override_library_main_update(Main *bmain)
 
   /* This temporary swap of G_MAIN is rather ugly,
    * but necessary to avoid asserts checks in some RNA assignment functions,
-   * since those always use on G_MAIN when they need access to a Main database. */
-  Main *orig_gmain = G_MAIN;
-  G_MAIN = bmain;
+   * since those always use G_MAIN when they need access to a Main database. */
+  Main *orig_gmain = BKE_blender_globals_main_swap(bmain);
 
   BLI_assert(BKE_main_namemap_validate(bmain));
 
@@ -3844,7 +3844,9 @@ void BKE_lib_override_library_main_update(Main *bmain)
 
   BLI_assert(BKE_main_namemap_validate(bmain));
 
-  G_MAIN = orig_gmain;
+  Main *tmp_gmain = BKE_blender_globals_main_swap(orig_gmain);
+  BLI_assert(tmp_gmain == bmain);
+  UNUSED_VARS_NDEBUG(tmp_gmain);
 }
 
 bool BKE_lib_override_library_id_is_user_deletable(Main *bmain, ID *id)

source/blender/blenkernel/intern/main.c

@@ -29,11 +29,12 @@
 #include "IMB_imbuf.h"
 #include "IMB_imbuf_types.h"
 
-Main *BKE_main_new(void)
+Main *BKE_main_new()
 {
   Main *bmain = MEM_callocN(sizeof(Main), "new main");
   bmain->lock = MEM_mallocN(sizeof(SpinLock), "main lock");
   BLI_spin_init((SpinLock *)bmain->lock);
+  bmain->is_global_main = false;
   return bmain;
 }
 

source/blender/blenloader/intern/versioning_280.c

@@ -53,6 +53,7 @@
 #include "DNA_world_types.h"
 
 #include "BKE_animsys.h"
+#include "BKE_blender.h"
 #include "BKE_brush.h"
 #include "BKE_cloth.h"
 #include "BKE_collection.h"
@@ -1613,12 +1614,13 @@ void do_versions_after_linking_280(Main *bmain, ReportList *UNUSED(reports))
       if (me->totface && !me->totpoly) {
         /* temporarily switch main so that reading from
          * external CustomData works */
-        Main *gmain = G_MAIN;
-        G_MAIN = bmain;
+        Main *orig_gmain = BKE_blender_globals_main_swap(bmain);
 
         BKE_mesh_do_versions_convert_mfaces_to_mpolys(me);
 
-        G_MAIN = gmain;
+        Main *tmp_gmain = BKE_blender_globals_main_swap(orig_gmain);
+        BLI_assert(tmp_gmain == bmain);
+        UNUSED_VARS_NDEBUG(tmp_gmain);
       }
 
       /* Deprecated, only kept for conversion. */