select: fix processing storage error for tuple-merger

Storage select/pairs method responds with `nil, err` if something went
wrong. tuple-merger implementation of select/pairs do not support this
type of response, resulting in fail while trying to subscribe nil value.
The patch fixes this behavior by extracting second return value in case
of `nil, err` and wrapping it with errors wrapper to restore errors
metatable.

Closes #271

CHANGELOG.md

@@ -16,6 +16,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 * Optimize select with known bucket_id (#234).
 
 ### Fixed
+* Fix processing storage error for tuple-merger implementation of
+  select/pairs (#271).
 
 ## [0.10.0] - 01-12-21
 

crud/select/merger.lua

@@ -1,4 +1,5 @@
 local buffer = require('buffer')
+local errors = require('errors')
 local msgpack = require('msgpack')
 local ffi = require('ffi')
 local call = require('crud.common.call')
@@ -80,9 +81,14 @@ local function decode_metainfo(buf)
     buf.rpos = decode_response_headers(buf)
 
     -- Decode a first return value (metainfo).
-    local res
+    local res, err
     res, buf.rpos = msgpack.decode(buf.rpos, buf:size())
-    return res
+
+    -- If res is nil, decode second return value (error).
+    if res == nil then
+        err, buf.rpos = msgpack.decode(buf.rpos, buf:size())
+    end
+    return res, err
 end
 
 --- Wait for a data chunk and request for the next data chunk.
@@ -109,7 +115,11 @@ local function fetch_chunk(context, state)
     end
 
     -- Decode metainfo, leave data to be processed by the merger.
-    local cursor = decode_metainfo(buf)
+    local cursor, err = decode_metainfo(buf)
+    if cursor == nil then
+        -- Wrap net.box errors error to restore metatable.
+        error(errors.wrap(err))
+    end
 
     -- Extract stats info.
     -- Stats extracted with callback here and not passed

test/integration/select_test.lua

@@ -1654,3 +1654,39 @@ pgroup.test_select_no_map_reduce = function(g)
     local diff_2 = map_reduces_after_2 - map_reduces_after_1
     t.assert_equals(diff_2, 0, 'Select request was not a map reduce')
 end
+
+pgroup.before_test('test_storage_nil_err_is_processed', function(g)
+    helpers.call_on_storages(g.cluster, function(server)
+        server.net_box:eval([[
+            local errors = require('errors')
+            local StorageError = errors.new_class("StorageError")
+
+            local _crud = rawget(_G, '_crud')
+            rawset(_G, '_real_crud_select', _crud.select_on_storage)
+
+            local return_err = function()
+                return nil, StorageError:new('My storage error')
+            end
+            _crud.select_on_storage = return_err
+        ]])
+    end)
+end)
+
+pgroup.after_test('test_storage_nil_err_is_processed', function(g)
+    helpers.call_on_storages(g.cluster, function(server)
+        server.net_box:eval([[
+            local _crud = rawget(_G, '_crud')
+            _crud.select_on_storage = rawget(_G, '_real_crud_select')
+            rawset(_G, '_real_crud_select', nil)
+        ]])
+    end)
+end)
+
+pgroup.test_storage_nil_err_is_processed = function(g)
+    local obj, err = g.cluster.main_server.net_box:call('crud.select', {
+        'customers', {{'==', 'age', 101}}
+    })
+    t.assert_equals(obj, nil)
+    t.assert_str_contains(err.str, 'StorageError', 'Storage error class is preserved')
+    t.assert_str_contains(err.str, 'My storage error', 'Storage error msg is preserved')
+end