Better fix for T47787 - remove extra user due to 'user_one' as soon as we increase 'real' user count.

This has several benefits:
* User count remains coherent, regardless of the order in which you use 'user_one' & real refcounting users
  (i.e. if you add to group, and then link in scene, or the reverse, you now always get same final user count).
* Avoids the need to check for potential 'user_one' extra user in count in several places in code (e.g. when
  making IDs single users...).
* Users won't wonder why they cannot make 'single user' and ID even though its user count shows '2'!
* readfile.c now always uses code from BKE's library.c when modifying id->us. Which means we can consider
  (asside from assignment during initialization) that id->us is read-only outside of library.c context.

Note that this commit reverts previous one (rB6b1d77a8052b) - please **do not** backport this one in 2.77.

2 files changed, 21 insertions, 2 deletions

diff --git a/source/blender/blenkernel/BKE_library.h b/source/blender/blenkernel/BKE_library.h
index 606edd9cc3a..21585a160dd 100644
--- a/source/blender/blenkernel/BKE_library.h
+++ b/source/blender/blenkernel/BKE_library.h
@@ -73,6 +73,7 @@ void id_lib_extern(struct ID *id);
 void BKE_library_filepath_set(struct Library *lib, const char *filepath);
 void id_us_ensure_real(struct ID *id);
 void id_us_clear_real(struct ID *id);
+void id_us_plus_no_lib(struct ID *id);
 void id_us_plus(struct ID *id);
 void id_us_min(struct ID *id);
 void id_fake_user_set(struct ID *id);
diff --git a/source/blender/blenkernel/intern/library.c b/source/blender/blenkernel/intern/library.c
index c04eda00904..6d7a61534a8 100644
--- a/source/blender/blenkernel/intern/library.c
+++ b/source/blender/blenkernel/intern/library.c
@@ -194,11 +194,29 @@ void id_us_clear_real(ID *id)
 	}
 }
 
+/* Same as \a id_us_plus, but does not handle lib indirect -> extern.
+ * Only used by readfile.c so far, but simpler/safer to keep it here nontheless. */
+void id_us_plus_no_lib(ID *id)
+{
+	if (id) {
+		if ((id->tag & LIB_TAG_EXTRAUSER) && (id->tag & LIB_TAG_EXTRAUSER_SET)) {
+			BLI_assert(id->us >= 1);
+			/* No need to increase count, just tag extra user as no more set.
+			 * Avoids annoying & inconsistent +1 in user count. */
+			id->tag &= ~LIB_TAG_EXTRAUSER_SET;
+		}
+		else {
+			BLI_assert(id->us >= 0);
+			id->us++;
+		}
+	}
+}
+
+
 void id_us_plus(ID *id)
 {
 	if (id) {
-		BLI_assert(id->us >= 0);
-		id->us++;
+		id_us_plus_no_lib(id);
 		id_lib_extern(id);
 	}
 }