error: export error_unref() function

C struct error objects can be created directly only in C.
C-side increments their reference counter when pushes to the Lua
stack.

It is not going to be so convenient soon. error_unpack() function
will be used in netbox to decode error object via Lua FFI.

Such error object will have 0 refs and no Lua GC callback
established. Because it won't be pushed on Lua stack natually,
from Lua C. To make such errors alive their reference counter
will be incremented and error_unref() will be set as GC callback.

Follow up for #4398

extra/exports

@@ -242,6 +242,7 @@ box_error_last
 box_error_clear
 box_error_set
 error_set_prev
+error_unref
 box_latch_new
 box_latch_delete
 box_latch_lock

src/lib/core/diag.c

@@ -31,6 +31,26 @@
 #include "diag.h"
 #include "fiber.h"
 
+void
+error_unref(struct error *e)
+{
+	assert(e->refs > 0);
+	struct error *to_delete = e;
+	while (--to_delete->refs == 0) {
+		/* Unlink error from lists completely.*/
+		struct error *cause = to_delete->cause;
+		assert(to_delete->effect == NULL);
+		if (to_delete->cause != NULL) {
+			to_delete->cause->effect = NULL;
+			to_delete->cause = NULL;
+		}
+		to_delete->destroy(to_delete);
+		if (cause == NULL)
+			return;
+		to_delete = cause;
+	}
+}
+
 int
 error_set_prev(struct error *e, struct error *prev)
 {

src/lib/core/diag.h

@@ -118,25 +118,8 @@ error_ref(struct error *e)
 	e->refs++;
 }
 
-static inline void
-error_unref(struct error *e)
-{
-	assert(e->refs > 0);
-	struct error *to_delete = e;
-	while (--to_delete->refs == 0) {
-		/* Unlink error from lists completely.*/
-		struct error *cause = to_delete->cause;
-		assert(to_delete->effect == NULL);
-		if (to_delete->cause != NULL) {
-			to_delete->cause->effect = NULL;
-			to_delete->cause = NULL;
-		}
-		to_delete->destroy(to_delete);
-		if (cause == NULL)
-			return;
-		to_delete = cause;
-	}
-}
+void
+error_unref(struct error *e);
 
 /**
  * Unlink error from its effect. For instance: