box: always promote error created via box.error() to diag

This patch makes box.error() always promote error to the diagnostic
area despite of passed arguments.

Closes #4829

@TarantoolBot document
Title: always promote error created via box.error() to diag

box.error() is able to accept two types of argument: either pair of code
and reason (box.error{code = 555, reason = 'Arbitrary message'}) or error
object (box.error(err)). In the first case error is promoted to
diagnostic area, meanwhile in the latter - it is not:
```
e1 = box.error.new({code = 111, reason = "cause"})
box.error({code = 111, reason = "err"})
- error: err
box.error.last()
- err
box.error(e1)
- error: cause
box.error.last()
- err
```
From now box.error(e1) sets error to diagnostic area as well:
```
box.error(e1)
- error: cause
box.error.last()
- cause
```

src/box/lua/error.cc

@@ -114,9 +114,16 @@ luaT_error_call(lua_State *L)
 			return luaT_error(L);
 		return 0;
 	}
-	if (lua_gettop(L) == 2 && luaL_iserror(L, 2))
-		return lua_error(L);
-	struct error *e = luaT_error_create(L, 2);
+	struct error *e = NULL;
+	if (lua_gettop(L) == 2) {
+		e = luaL_iserror(L, 2);
+		if (e != NULL) {
+			/* Re-set error to diag area. */
+			diag_set_error(&fiber()->diag, e);
+			return lua_error(L);
+		}
+	}
+	e = luaT_error_create(L, 2);
 	if (e == NULL)
 		return luaL_error(L, "box.error(): bad arguments");
 	diag_set_error(&fiber()->diag, e);

test/box/error.result

@@ -808,3 +808,25 @@ assert(e2.prev == e3)
  | ---
  | - true
  | ...
+
+-- gh-4829: always promote error created via box.error() to
+-- diagnostic area.
+e1 = box.error.new({code = 111, reason = "cause"})
+ | ---
+ | ...
+box.error({code = 111, reason = "err"})
+ | ---
+ | - error: err
+ | ...
+box.error.last()
+ | ---
+ | - err
+ | ...
+box.error(e1)
+ | ---
+ | - error: cause
+ | ...
+assert(box.error.last() == e1)
+ | ---
+ | - true
+ | ...

test/box/error.test.lua

@@ -221,3 +221,11 @@ e2:set_prev(e3)
 box.error.set(e2)
 assert(e1.prev == nil)
 assert(e2.prev == e3)
+
+-- gh-4829: always promote error created via box.error() to
+-- diagnostic area.
+e1 = box.error.new({code = 111, reason = "cause"})
+box.error({code = 111, reason = "err"})
+box.error.last()
+box.error(e1)
+assert(box.error.last() == e1)