box/error: don't set error created via box.error.new to diag

To achieve this let's refactor luaT_error_create() to return error
object instead of setting it via box_error_set().
luaT_error_create() is used both to handle box.error() and
box.error.new() invocations, and box.error() is still expected to set
error to diagnostic area. So, luaT_error_call() which implements
box.error() processing at the end calls diag_set_error().
It is worth mentioning that net.box module relied on the fact that
box.error.new() set error to diagnostic area: otherwise request errors
don't get to diagnostic area on client side.

Needed for #1148
Closes #4778

@TarantoolBot document
Title: Don't promote error created via box.error.new to diagnostic area

Now box.error.new() only creates error object, but doesn't set it to
Tarantool's diagnostic area:
```
box.error.clear()
e = box.error.new({code = 111, reason = "cause"})
assert(box.error.last() == nil)
---
- true
...
```
To set error in diagnostic area explicitly box.error.set() has been
introduced. It accepts error object which is set as last system error
(i.e. becomes available via box.error.last()).
Finally, box.error.new() does not longer accept error object as an
argument (this was undocumented feature).
Note that patch does not affect box.error(), which still pushes error to
diagnostic area. This fact is reflected in docs:
'''
Emulate a request error, with text based on one of the pre-defined
Tarantool errors...
'''

src/box/error.cc

@@ -86,6 +86,22 @@ box_error_set(const char *file, unsigned line, uint32_t code,
 	return -1;
 }
 
+struct error *
+box_error_new(const char *file, unsigned line, uint32_t code,
+	      const char *fmt, ...)
+{
+	struct error *e = BuildClientError(file, line, ER_UNKNOWN);
+	ClientError *client_error = type_cast(ClientError, e);
+	if (client_error != NULL) {
+		client_error->m_errcode = code;
+		va_list ap;
+		va_start(ap, fmt);
+		error_vformat_msg(e, fmt, ap);
+		va_end(ap);
+	}
+	return e;
+}
+
 /* }}} */
 
 struct rmean *rmean_error = NULL;

src/box/error.h

@@ -137,6 +137,14 @@ box_error_set(const char *file, unsigned line, uint32_t code,
 
 /** \endcond public */
 
+/**
+ * Construct error object without setting it in the diagnostics
+ * area. On the memory allocation fail returns NULL.
+ */
+struct error *
+box_error_new(const char *file, unsigned line, uint32_t code,
+	      const char *fmt, ...);
+
 extern const struct type_info type_ClientError;
 extern const struct type_info type_XlogError;
 extern const struct type_info type_AccessDeniedError;

src/box/lua/error.cc

@@ -42,7 +42,14 @@ extern "C" {
 #include "lua/utils.h"
 #include "box/error.h"
 
-static void
+/**
+ * Parse Lua arguments (they can come as single table
+ * f({code : number, reason : string}) or as separate members
+ * f(code, reason)) and construct struct error with given values.
+ * In case one of arguments is missing its corresponding field
+ * in struct error is filled with default value.
+ */
+static struct error *
 luaT_error_create(lua_State *L, int top_base)
 {
 	uint32_t code = 0;
@@ -69,25 +76,19 @@ luaT_error_create(lua_State *L, int top_base)
 			reason = lua_tostring(L, -1);
 		} else if (strchr(reason, '%') != NULL) {
 			/* Missing arguments to format string */
-			luaL_error(L, "box.error(): bad arguments");
-		}
-	} else if (top == top_base) {
-		if (lua_istable(L, top_base)) {
-			/* A special case that rethrows raw error (used by net.box) */
-			lua_getfield(L, top_base, "code");
-			code = lua_tonumber(L, -1);
-			lua_pop(L, 1);
-			lua_getfield(L, top_base, "reason");
-			reason = lua_tostring(L, -1);
-			if (reason == NULL)
-				reason = "";
-			lua_pop(L, 1);
-		} else if (luaL_iserror(L, top_base)) {
-			lua_error(L);
-			return;
+			return NULL;
 		}
+	} else if (top == top_base && lua_istable(L, top_base)) {
+		lua_getfield(L, top_base, "code");
+		code = lua_tonumber(L, -1);
+		lua_pop(L, 1);
+		lua_getfield(L, top_base, "reason");
+		reason = lua_tostring(L, -1);
+		if (reason == NULL)
+			reason = "";
+		lua_pop(L, 1);
 	} else {
-		luaL_error(L, "box.error(): bad arguments");
+		return NULL;
 	}
 
 raise:
@@ -101,8 +102,7 @@ luaT_error_create(lua_State *L, int top_base)
 		}
 		line = info.currentline;
 	}
-	say_debug("box.error() at %s:%i", file, line);
-	box_error_set(file, line, code, "%s", reason);
+	return box_error_new(file, line, code, "%s", reason);
 }
 
 static int
@@ -111,10 +111,15 @@ luaT_error_call(lua_State *L)
 	if (lua_gettop(L) <= 1) {
 		/* Re-throw saved exceptions if any. */
 		if (box_error_last())
-			luaT_error(L);
+			return luaT_error(L);
 		return 0;
 	}
-	luaT_error_create(L, 2);
+	if (lua_gettop(L) == 2 && luaL_iserror(L, 2))
+		return lua_error(L);
+	struct error *e = luaT_error_create(L, 2);
+	if (e == NULL)
+		return luaL_error(L, "box.error(): bad arguments");
+	diag_set_error(&fiber()->diag, e);
 	return luaT_error(L);
 }
 
@@ -139,9 +144,12 @@ luaT_error_new(lua_State *L)
 {
 	if (lua_gettop(L) == 0)
 		return luaL_error(L, "Usage: box.error.new(code, args)");
-	luaT_error_create(L, 1);
+	struct error *e = luaT_error_create(L, 1);
+	if (e == NULL)
+		return luaL_error(L, "Usage: box.error.new(code, args)");
 	lua_settop(L, 0);
-	return luaT_error_last(L);
+	luaT_pusherror(L, e);
+	return 1;
 }
 
 static int

test/box/error.result

@@ -445,7 +445,7 @@ err = box.error.new({code = 111, reason = "cause"})
  | ...
 assert(box.error.last() ~= err)
  | ---
- | - error: assertion failed!
+ | - true
  | ...
 box.error.set(err)
  | ---
@@ -471,6 +471,36 @@ assert(box.error.last() == err)
  | ---
  | - true
  | ...
+-- Check that box.error.new() does not set error to diag.
+--
+box.error.clear()
+ | ---
+ | ...
+err = box.error.new(1, "cause")
+ | ---
+ | ...
+assert(box.error.last() == nil)
+ | ---
+ | - true
+ | ...
+
+-- box.error.new() does not accept error objects.
+--
+box.error.new(err)
+ | ---
+ | - error: 'Usage: box.error.new(code, args)'
+ | ...
+
+-- box.error() is supposed to re-throw last diagnostic error.
+-- Make sure it does not fail if there's no errors at all
+-- (in diagnostics area).
+--
+box.error.clear()
+ | ---
+ | ...
+box.error()
+ | ---
+ | ...
 
 space:drop()
  | ---

test/box/error.test.lua

@@ -91,5 +91,21 @@ box.error.set(1)
 box.error.set(nil)
 box.error.set(box.error.last())
 assert(box.error.last() == err)
+-- Check that box.error.new() does not set error to diag.
+--
+box.error.clear()
+err = box.error.new(1, "cause")
+assert(box.error.last() == nil)
+
+-- box.error.new() does not accept error objects.
+--
+box.error.new(err)
+
+-- box.error() is supposed to re-throw last diagnostic error.
+-- Make sure it does not fail if there's no errors at all
+-- (in diagnostics area).
+--
+box.error.clear()
+box.error()
 
 space:drop()