Fix merger support recognition on tarantool 2.10+

We cannot use just string comparison as before, because '2.10.0-<...>'
less than, say, '2.3' lexicographically. The module had to use the old
select implementation on tarantool 2.10 due to this problem.

Implemented more accurate checks, whether built-in and external
tuple-merger are supported. Say, it'll not enable the built-in merger on
tarantool 2.4.1, where the module has a known problem (see comments in
the code).

Fixed the problem of the same kind in a test case.

When I replaced `_G._TARANTOOL` with just `_TARANTOOL`, the luacheck
linter said me:

 | <...>/test/helper.lua:307:31: accessing undefined field match of
 | global _TARANTOOL

On the line:

 | local major_minor_patch = _TARANTOOL:split('-', 1)[1]

But when I added '_TARANTOOL' into globals, it stops to complain.
Strange, but okay. I prefer just `_TARANTOOL` without `_G`.

Author: Totktonada

.luacheckrc

@@ -1,5 +1,5 @@
 redefined = false
-globals = {'box', 'utf8', 'checkers'}
+globals = {'box', 'utf8', 'checkers', '_TARANTOOL'}
 include_files = {'**/*.lua', '*.luacheckrc', '*.rockspec'}
 exclude_files = {'**/*.rocks/', 'tmp/', 'tarantool-enterprise/'}
 max_line_length = 120

CHANGELOG.md

@@ -13,6 +13,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ### Fixed
 
+* Use tuple-merger backed select implementation on tarantool 2.10+ (it gives
+  less pressure on Lua GC).
+
 ## [0.9.0] - 20-10-21
 
 ### Added

crud/common/utils.lua

@@ -210,6 +210,35 @@ local function determine_enabled_features()
     -- since Tarantool 2.6.3 / 2.7.2 / 2.8.1
     enabled_tarantool_features.jsonpath_indexes = major >= 3 or (major >= 2 and ((minor >= 6 and patch >= 3)
         or (minor >= 7 and patch >= 2) or (minor >= 8 and patch >= 1) or minor >= 9))
+
+    -- The merger module was implemented in 2.2.1, see [1].
+    -- However it had the critical problem [2], which leads to
+    -- segfault at attempt to use the module from a fiber serving
+    -- iproto request. So we don't use it in versions before the
+    -- fix.
+    --
+    -- [1]: https://github.com/tarantool/tarantool/issues/3276
+    -- [2]: https://github.com/tarantool/tarantool/issues/4954
+    enabled_tarantool_features.builtin_merger =
+        (major == 2 and minor == 3 and patch >= 3) or
+        (major == 2 and minor == 4 and patch >= 2) or
+        (major == 2 and minor == 5 and patch >= 1) or
+        (major >= 2 and minor >= 6) or
+        (major >= 3)
+
+    -- The external merger module leans on a set of relatively
+    -- new APIs in tarantool. So it works only on tarantool
+    -- versions, which offer those APIs.
+    --
+    -- See README of the module:
+    -- https://github.com/tarantool/tuple-merger
+    enabled_tarantool_features.external_merger =
+        (major == 1 and minor == 10 and patch >= 8) or
+        (major == 2 and minor == 4 and patch >= 3) or
+        (major == 2 and minor == 5 and patch >= 2) or
+        (major == 2 and minor == 6 and patch >= 1) or
+        (major == 2 and minor >= 7) or
+        (major >= 3)
 end
 
 function utils.tarantool_supports_fieldpaths()
@@ -236,6 +265,22 @@ function utils.tarantool_supports_jsonpath_indexes()
     return enabled_tarantool_features.jsonpath_indexes
 end
 
+function utils.tarantool_has_builtin_merger()
+    if enabled_tarantool_features.builtin_merger == nil then
+        determine_enabled_features()
+    end
+
+    return enabled_tarantool_features.builtin_merger
+end
+
+function utils.tarantool_supports_external_merger()
+    if enabled_tarantool_features.external_merger == nil then
+        determine_enabled_features()
+    end
+
+    return enabled_tarantool_features.external_merger
+end
+
 local function add_nullable_fields_recursive(operations, operations_map, space_format, tuple, id)
     if id < 2 or tuple[id - 1] ~= box.NULL then
         return operations

crud/select.lua

@@ -1,5 +1,6 @@
 local errors = require('errors')
 
+local utils = require('crud.common.utils')
 local select_executor = require('crud.select.executor')
 local select_filters = require('crud.select.filters')
 local dev_checks = require('crud.common.dev_checks')
@@ -9,18 +10,12 @@ local SelectError = errors.new_class('SelectError')
 
 local select_module
 
-if '2' <= _TARANTOOL and _TARANTOOL <= '2.3.3' then
-    -- "merger" segfaults here
-    -- See https://github.com/tarantool/tarantool/issues/4954
-    select_module = require('crud.select.compat.select_old')
-elseif not package.search('tuple.merger') and package.loaded['merger'] == nil then
-    -- we don't use pcall(require, modile_name) here because it
-    -- leads to ignoring errors other than 'No LuaRocks module found'
-
-    -- "merger" isn't supported here
-    select_module = require('crud.select.compat.select_old')
-else
+local has_merger = (utils.tarantool_supports_external_merger() and
+    package.search('tuple.merger')) or utils.tarantool_has_builtin_merger()
+if has_merger then
     select_module = require('crud.select.compat.select')
+else
+    select_module = require('crud.select.compat.select_old')
 end
 
 local function make_cursor(data)

test/helper.lua

@@ -300,4 +300,27 @@ function helpers.get_other_storage_bucket_id(cluster, bucket_id)
     ]], {bucket_id})
 end
 
+function helpers.tarantool_version_at_least(wanted_major, wanted_minor,
+        wanted_patch)
+    -- Borrowed from `determine_enabled_features()` from
+    -- crud/common/utils.lua.
+    local major_minor_patch = _TARANTOOL:split('-', 1)[1]
+    local major_minor_patch_parts = major_minor_patch:split('.', 2)
+
+    local major = tonumber(major_minor_patch_parts[1])
+    local minor = tonumber(major_minor_patch_parts[2])
+    local patch = tonumber(major_minor_patch_parts[3])
+
+    if major < (wanted_major or 0) then return false end
+    if major > (wanted_major or 0) then return true end
+
+    if minor < (wanted_minor or 0) then return false end
+    if minor > (wanted_minor or 0) then return true end
+
+    if patch < (wanted_patch or 0) then return false end
+    if patch > (wanted_patch or 0) then return true end
+
+    return true
+end
+
 return helpers

test/integration/simple_operations_test.lua

@@ -432,11 +432,11 @@ pgroup.test_intermediate_nullable_fields_update = function(g)
     -- However since 2.8 Tarantool could update intermediate nullable fields
     -- (https://github.com/tarantool/tarantool/issues/3378).
     -- So before 2.8 update returns an error but after it update is correct.
-    if _TARANTOOL > "2.8" then
+    if helpers.tarantool_version_at_least(2, 8) then
         local _, err = g.cluster.main_server.net_box:call('crud.update',
             {'developers', 1, {{'=', '[5].a.b[1]', 3}, {'=', 'extra_5', 'extra_value_5'}}})
         t.assert_equals(err, nil)
-    elseif _TARANTOOL >= "2.3" then
+    elseif helpers.tarantool_version_at_least(2, 3) then
         local _, err = g.cluster.main_server.net_box:call('crud.update',
             {'developers', 1, {{'=', '[5].a.b[1]', 3}, {'=', 'extra_5', 'extra_value_5'}}})
         t.assert_equals(err.err, "Failed to update: Field ''extra_5'' was not found in the tuple")