error: add __concat method to error object

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.

The case of error "error_mt.__concat(): neither of args is an error"
is not covered by tests because of #4723

Closes #4489

(cherry picked from commit 935db173)

src/lua/error.lua

@@ -150,9 +150,20 @@ local function error_index(err, key)
     return error_methods[key]
 end
 
+local function error_concat(lhs, rhs)
+    if ffi.istype('struct error', lhs) then
+        return tostring(lhs) .. rhs
+    elseif ffi.istype('struct error', rhs) then
+        return lhs .. tostring(rhs)
+    else
+       error('error_mt.__concat(): neither of args is an error')
+    end
+end
+
 local error_mt = {
     __index = error_index;
     __tostring = error_message;
+    __concat = error_concat;
 };
 
 ffi.metatype('struct error', error_mt);

test/box/misc.result

@@ -196,6 +196,79 @@ 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 local ''rhs'' (a nil value)'
+...
+nil .. e
+---
+- error: 'builtin/error.lua: attempt to concatenate local ''lhs'' (a nil value)'
+...
+e .. box.NULL
+---
+- error: 'builtin/error.lua: attempt to concatenate ''string'' and ''void *'''
+...
+box.NULL .. e
+---
+- error: 'builtin/error.lua: attempt to concatenate ''void *'' and ''string'''
+...
+123 .. e
+---
+- 123Unknown error
+...
+e .. 123
+---
+- Unknown error123
+...
+e .. e
+---
+- Unknown errorUnknown error
+...
+e .. {}
+---
+- error: 'builtin/error.lua: attempt to concatenate local ''rhs'' (a table value)'
+...
+{} .. e
+---
+- error: 'builtin/error.lua: attempt to concatenate local ''lhs'' (a table value)'
+...
+-1ULL .. e
+---
+- error: 'builtin/error.lua: attempt to concatenate ''uint64_t'' and ''string'''
+...
+e .. -1ULL
+---
+- error: 'builtin/error.lua: attempt to concatenate ''string'' and ''uint64_t'''
+...
+1LL .. e
+---
+- error: 'builtin/error.lua: attempt to concatenate ''int64_t'' and ''string'''
+...
+e .. 1LL
+---
+- error: 'builtin/error.lua: attempt to concatenate ''string'' and ''int64_t'''
+...
+e = nil
+---
+...
 ----------------
 -- # box.stat
 ----------------
@@ -1032,7 +1105,7 @@ error()
 ---
 - error: null
 ...
---  A test case for bitwise operations 
+--  A test case for bitwise operations
 bit.lshift(1, 32)
 ---
 - 1

test/box/misc.test.lua

@@ -59,6 +59,28 @@ 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 .. e
+e .. {}
+{} .. e
+-1ULL .. e
+e .. -1ULL
+1LL .. e
+e .. 1LL
+e = nil
+
 ----------------
 -- # box.stat
 ----------------
@@ -271,7 +293,7 @@ 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)