[Tarantool-patches] [PATCH v2] error: Add __concat method to error object

[Oleg Babin <olegrok@tarantool.org>]

From: Oleg Babin <babinoleg@mail.ru>

Usually functions return pair {nil, err} and expected that err is string.
Let's make the behaviour of error object closer to string
and define __concat metamethod.

Closes #4489
---

Changes in v2:
  - Added tests

---
 src/lua/error.lua      |  8 +++++++
 test/box/misc.result   | 47 +++++++++++++++++++++++++++++++++++++++++-
 test/box/misc.test.lua | 17 ++++++++++++++-
 3 files changed, 70 insertions(+), 2 deletions(-)

diff --git a/src/lua/error.lua b/src/lua/error.lua
index 28fc0377d..b44fab7b6 100644
--- a/src/lua/error.lua
+++ b/src/lua/error.lua
@@ -150,9 +150,17 @@ local function error_index(err, key)
     return error_methods[key]
 end
 
+local function error_concat(lhs, rhs)
+    if lhs == nil or rhs == nil then
+        error("attempt to concatenate struct error and nil")
+    end
+    return tostring(lhs) .. tostring(rhs)
+end
+
 local error_mt = {
     __index = error_index;
     __tostring = error_message;
+    __concat = error_concat;
 };
 
 ffi.metatype('struct error', error_mt);
diff --git a/test/box/misc.result b/test/box/misc.result
index b2930515b..db184c037 100644
--- a/test/box/misc.result
+++ b/test/box/misc.result
@@ -196,6 +196,51 @@ box.error.new()
 ---
 - error: 'Usage: box.error.new(code, args)'
 ...
+--
+-- gh-4489: box.error has __concat metamethod
+--
+test_run:cmd("push filter '(.builtin/.*.lua):[0-9]+' to '\\1'")
+---
+- true
+...
+e = box.error.new(box.error.UNKNOWN)
+---
+...
+'left side: ' .. e
+---
+- 'left side: Unknown error'
+...
+e .. ': right side'
+---
+- 'Unknown error: right side'
+...
+e .. nil
+---
+- error: 'builtin/error.lua: attempt to concatenate struct error and nil'
+...
+nil .. e
+---
+- error: 'builtin/error.lua: attempt to concatenate struct error and nil'
+...
+e .. box.NULL
+---
+- error: 'builtin/error.lua: attempt to concatenate struct error and nil'
+...
+box.NULL .. e
+---
+- error: 'builtin/error.lua: attempt to concatenate struct error and nil'
+...
+123 .. e
+---
+- 123Unknown error
+...
+e .. 123
+---
+- Unknown error123
+...
+e = nil
+---
+...
 ----------------
 -- # box.stat
 ----------------
@@ -1037,7 +1082,7 @@ error()
 ---
 - error: null
 ...
---  A test case for bitwise operations 
+--  A test case for bitwise operations
 bit.lshift(1, 32)
 ---
 - 1
diff --git a/test/box/misc.test.lua b/test/box/misc.test.lua
index cc223b2ef..6104b935b 100644
--- a/test/box/misc.test.lua
+++ b/test/box/misc.test.lua
@@ -59,6 +59,21 @@ e = box.error.new(box.error.CREATE_SPACE, "space", "error")
 e
 box.error.new()
 
+--
+-- gh-4489: box.error has __concat metamethod
+--
+test_run:cmd("push filter '(.builtin/.*.lua):[0-9]+' to '\\1'")
+e = box.error.new(box.error.UNKNOWN)
+'left side: ' .. e
+e .. ': right side'
+e .. nil
+nil .. e
+e .. box.NULL
+box.NULL .. e
+123 .. e
+e .. 123
+e = nil
+
 ----------------
 -- # box.stat
 ----------------
@@ -271,7 +286,7 @@ dostring('return abc')
 dostring('return ...', 1, 2, 3)
 --  A test case for Bug#1043804 lua error() -> server crash
 error()
---  A test case for bitwise operations 
+--  A test case for bitwise operations
 bit.lshift(1, 32)
 bit.band(1, 3)
 bit.bor(1, 2)
-- 
2.23.0