Add caching of sharding function

The ddl.bucket_id() function needs to know a sharding function.
It is costly to obtain the function declaration /
definition stored in the _ddl_sharding_func space.

Cache contains:
raw_tuple - raw sharding metadata, used for get_schema()
parsed_func_name - parsed dot notation (like {'foo', 'bar'})
callable - function ready to call, this offloads using of loadstring()
error - string with an error: not nil only if setting callable fails

Cache will be rebuilded if:
* _ddl_sharding_func space changed: cache sets _ddl_sharding_func:on_replace
  trigger
* schema changed: cache checks box.internal.schema_version changes

This patch does not serve hot reload techniques.
This entails an on_replace trigger duplication if hot reload occurs.
Hot reload support will be done in separate task:
#87

Benchmarks - 10000000 bucket_id() calls (file test/bench_cache.lua):
Baseline (no DDL):     3.38s user 0.01s system 99% cpu 3.389 total
After this patch:
DDL with function body: 3.81s user 0.01s system 99% cpu 3.818 total
DDL with function name: 5.49s user 0.00s system 99% cpu 5.495 total
Before patch:
DDL with function body: 55.95s user 0.40s system 99% cpu 56.354 total
DDL with function name: 13.68s user 0.13s system 99% cpu 13.807 total

Closes #82

.github/workflows/test.yml

@@ -66,3 +66,20 @@ jobs:
       # Cleanup cached paths
       - run: tarantoolctl rocks remove cartridge
       - run: tarantoolctl rocks remove ddl
+  benchmark:
+    strategy:
+      fail-fast: false
+      matrix:
+        tarantool: ['1.10', '2.5', '2.6', '2.7']
+        coveralls: [false]
+        include:
+          - tarantool: '2.8'
+            coveralls: true
+    runs-on: [ubuntu-latest]
+    steps:
+      - uses: actions/checkout@v2
+      - uses: tarantool/setup-tarantool@v1
+        with:
+          tarantool-version: ${{ matrix.tarantool }}
+
+      - run: tarantool ./test/bench_cache.lua

README.md

@@ -89,6 +89,11 @@ bucket identifier (number)
     - Calculate bucket id for a specified space and sharding key.
     Method uses sharding function specified in DDL schema.
 
+    Method is not transactional in the sense that it catches up
+    _ddl_sharding_func changes immediatelly: it may see changes that're
+    not committed yet and may see a state from another transaction,
+    which should not be visible in the current transaction.
+
     Return values: bucket_id if no error, otherwise return `nil, err`
 
 ## Input data format

ddl/cache.lua

@@ -0,0 +1,124 @@
+local cache = nil
+
+local SPACE_NAME_IDX      = 1
+local SHARD_FUNC_NAME_IDX = 2
+local SHARD_FUNC_BODY_IDX = 3
+
+-- Build cache.
+--
+-- We don't need to call this function with any type of locking:
+-- _ddl_sharding_func is memtx space, so calling :pairs() on it
+-- is atomic
+--
+-- Cache structure format:
+--
+-- cache = {
+--   spaces = {
+--     [space_name] = {
+--       -- raw sharding metadata, used for ddl.get()
+--       raw_tuple = <tuple object> (<nil> at error),
+--       -- parsed dot notation (like {'foo', 'bar'})
+--       parsed_func_name = <table> or <nil>
+--       -- a function ready to call
+--       callable = <function> or <nil>,
+--       -- string with an error: not nil only if setting callable fails
+--       error = <string> or <nil>,
+--     }
+--   },
+--   -- current schema version
+--   schema_version = <...>,
+-- }
+
+-- function returns nothing
+local function cache_build()
+    -- clear cache
+    cache.spaces = {}
+
+    if box.space._ddl_sharding_func == nil then
+        return
+    end
+
+    for _, tuple in box.space._ddl_sharding_func:pairs() do
+        local space_name = tuple[SPACE_NAME_IDX]
+        local func_name = tuple[SHARD_FUNC_NAME_IDX]
+        local func_body = tuple[SHARD_FUNC_BODY_IDX]
+
+        cache.spaces[space_name] = {
+            raw_tuple = tuple
+        }
+
+        if func_body ~= nil then
+            local sharding_func, err = loadstring('return ' .. func_body)
+            if sharding_func == nil then
+                cache.spaces[space_name].error =
+                    string.format("Body is incorrect in sharding_func for space (%s): %s",
+                    space_name, err)
+            else
+                cache.spaces[space_name].callable = sharding_func()
+            end
+        elseif func_name ~= nil then
+            -- we cannot save the function itself into the cache,
+            -- because the function can be changed in runtime and
+            -- there is no way to catch this change
+            local chunks = string.split(func_name, '.')
+            cache.spaces[space_name].parsed_func_name = chunks
+        end
+    end
+
+    cache.schema_version = box.internal.schema_version()
+
+end
+
+-- Rebuild cache if _ddl_sharding_func space changed.
+local function cache_set_trigger()
+    if box.space._ddl_sharding_func == nil then
+        return
+    end
+
+    local trigger_found = false
+
+    for _, func in pairs(box.space._ddl_sharding_func:on_replace()) do
+        if func == cache_build then
+            trigger_found = true
+            break
+        end
+    end
+
+    if not trigger_found then
+        box.space._ddl_sharding_func:on_replace(cache_build)
+    end
+end
+
+-- Get data from cache.
+-- Returns all cached data for "space_name" or nil.
+local function cache_get(space_name)
+    if space_name == nil then
+        return nil
+    end
+
+    -- using tarantool internal API.
+    -- this is not reliable, but it is the only way to track
+    -- schema_version changes. Fix it if a public method appears:
+    -- https://github.com/tarantool/tarantool/issues/6544
+    local schema_version = box.internal.schema_version()
+
+    if not cache then
+        cache = {}
+        cache_build()
+        cache_set_trigger()
+    end
+
+    -- rebuild cache if database schema changed
+    if schema_version ~= cache.schema_version then
+        cache_build()
+        cache_set_trigger()
+    end
+
+    return cache.spaces[space_name]
+end
+
+return {
+    internal = {
+        get = cache_get,
+    }
+}

ddl/get.lua

@@ -1,4 +1,5 @@
 local utils = require('ddl.utils')
+local cache = require('ddl.cache')
 local ddl_check = require('ddl.check')
 
 local function _get_index_field_path(space, index_part)
@@ -66,11 +67,18 @@ local function get_metadata(space_name, metadata_name)
 end
 
 local function get_sharding_func(space_name)
-    local record = get_metadata(space_name, "sharding_func")
-    if not record then
+    return cache.internal.get(space_name)
+end
+
+local function get_sharding_func_raw(space_name)
+    local record = cache.internal.get(space_name)
+
+    if not record or not record.raw_tuple then
         return nil
     end
 
+    record = record.raw_tuple
+
     if record.sharding_func_body ~= nil then
         return {body = record.sharding_func_body}
     end
@@ -97,7 +105,7 @@ local function get_space_schema(space_name)
     space_ddl.engine = box_space.engine
     space_ddl.format = box_space:format()
     space_ddl.sharding_key = get_sharding_key(space_name)
-    space_ddl.sharding_func = get_sharding_func(space_name)
+    space_ddl.sharding_func = get_sharding_func_raw(space_name)
     for _, field in ipairs(space_ddl.format) do
         if field.is_nullable == nil then
             field.is_nullable = false
@@ -115,21 +123,20 @@ local function get_space_schema(space_name)
 end
 
 local function prepare_sharding_func_for_call(space_name, sharding_func_def)
-    if type(sharding_func_def) == 'string' then
-        local sharding_func = utils.get_G_function(sharding_func_def)
+    if sharding_func_def.error ~= nil then
+        return nil, sharding_func_def.error
+    end
+
+    if sharding_func_def.parsed_func_name ~= nil then
+        local sharding_func = utils.get_G_function(sharding_func_def.parsed_func_name)
         if sharding_func ~= nil and
            ddl_check.internal.is_callable(sharding_func) == true then
             return sharding_func
         end
     end
 
-    if type(sharding_func_def) == 'table' then
-        local sharding_func, err = loadstring('return ' .. sharding_func_def.body)
-        if sharding_func == nil then
-            return nil, string.format(
-                "Body is incorrect in sharding_func for space (%s): %s", space_name, err)
-        end
-        return sharding_func()
+    if sharding_func_def.callable ~= nil then
+        return sharding_func_def.callable
     end
 
     return nil, string.format(

ddl/utils.lua

@@ -189,9 +189,19 @@ end
 -- split sharding func name in dot notation by dot
 -- foo.bar.baz -> chunks: foo bar baz
 -- foo -> chunks: foo
+--
+-- func_name parameter may be a string in dot notation or table
+-- if func_name type is of type table it is assumed that it is already split
 local function get_G_function(func_name)
-    local chunks = string.split(func_name, '.')
     local sharding_func = _G
+    local chunks
+
+    if type(func_name) == 'string' then
+        chunks = string.split(func_name, '.')
+    else
+        chunks = func_name
+    end
+
     -- check is the each chunk an identifier
     for _, chunk in pairs(chunks) do
         if not check_name_isident(chunk) or sharding_func == nil then