7 files changed, 67 insertions, 16 deletions
diff --git a/source/blender/blenkernel/BKE_blender.h b/source/blender/blenkernel/BKE_blender.h
index 940dec2fcd5..1f0919bb3e6 100644
--- a/source/blender/blenkernel/BKE_blender.h
+++ b/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);
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);
 
diff --git a/source/blender/blenkernel/intern/blender.c b/source/blender/blenkernel/intern/blender.c
index 947cdc0c8bf..23cf368af01 100644
--- a/source/blender/blenkernel/intern/blender.c
+++ b/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;
+}
+
 /** \} */
 
 /* -------------------------------------------------------------------- */
diff --git a/source/blender/blenkernel/intern/blendfile.c b/source/blender/blenkernel/intern/blendfile.c
index ba208f688ee..6546659f6cd 100644
--- a/source/blender/blenkernel/intern/blendfile.c
+++ b/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);
diff --git a/source/blender/blenkernel/intern/lib_override.cc b/source/blender/blenkernel/intern/lib_override.cc
index e2f991a4026..04679b72b2d 100644
--- a/source/blender/blenkernel/intern/lib_override.cc
+++ b/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)
diff --git a/source/blender/blenkernel/intern/main.c b/source/blender/blenkernel/intern/main.c
index 239aacf28d6..3e8ff957d53 100644
--- a/source/blender/blenkernel/intern/main.c
+++ b/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;
 }
 
diff --git a/source/blender/blenloader/intern/versioning_280.c b/source/blender/blenloader/intern/versioning_280.c
index 3e26516cd69..a9048037513 100644
--- a/source/blender/blenloader/intern/versioning_280.c
+++ b/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. */