Handle unique index constraint violation in shard

Fixes #188.

Author: Totktonada

graphql/accessor_shard.lua

@@ -4,6 +4,7 @@
 
 local json = require('json')
 local yaml = require('yaml')
+local digest = require('digest')
 local utils = require('graphql.utils')
 local shard = utils.optional_require('shard')
 local accessor_general = require('graphql.accessor_general')
@@ -22,6 +23,18 @@ local index_info_cache = {}
 
 local function shard_check_error(func_name, result, err)
     if result ~= nil then return end
+
+    -- avoid json encoding of an error message (when the error is in the known
+    -- format)
+    if type(err) == 'table' and type(err.error) == 'string' then
+        error({
+            message = err.error,
+            extensions = {
+                shard_error = err,
+            }
+        })
+    end
+
     error(('%s: %s'):format(func_name, json.encode(err)))
 end
 
@@ -215,6 +228,12 @@ local function space_operation(collection_name, nodes, operation, ...)
     return master_result
 end
 
+local function get_shard_key_hash(key)
+    local shards_n = #shard.shards
+    local num = type(key) == 'number' and key or digest.crc32(key)
+    return 1 + digest.guava(num, shards_n)
+end
+
 -- }}}
 
 --- Check whether a collection (it is sharded space for that accessor) exists.
@@ -395,6 +414,46 @@ end
 
 --- Update a tuple with an update statements.
 ---
+--- In case when the update should change the storage where the tuple stored
+--- we perform insert to the new storage and delete from the old one. The
+--- order must be 'first insert, then delete', because insert can report an
+--- error in case of unique index constraints violation and we must not
+--- perform delete in the case.
+---
+--- This function emulates (more or less preciselly, see below) behaviour of
+--- update as if it would be performed on a local tarantool instance. In case
+--- when the tuple resides on the same storage the update operation performs
+--- a unique index constraints check within the storage, but not on the overall
+--- cluster. In case when the tuple changes its storage the insert operation
+--- performs the check within the target storage.
+---
+--- We can consider this as relaxing of the constraints: the function can
+--- silently violate cluster-wide uniqueness constraints or report a
+--- violation that was introduced by some previous operation, but cannot
+--- report a violation when a local tarantool space would not.
+---
+--- 'Insert, then delete' approach is applicable and do not lead to a false
+--- positive unique index constraint violation when storage nodes are different
+--- and do not contain same tuples. We check the first condition in the
+--- function and the second is guaranteed by the shard module.
+---
+--- Note: if one want to use this function as basis for a similar one, but
+--- allowing update of a primary key the following details should be noticed. A
+--- primary key update that **changes a storage** where the tuple saved can be
+--- performed with the 'insert, then delete' approach. An update **within one
+--- storage** cannot be performed in the following ways:
+---
+--- * as update (because tarantool forbids update of a primary key),
+--- * 'insert, then delete' way (because insert can report a unique index
+---   constraint violation due to values in the old version of the tuple),
+--- * 'tuple:update(), then replace' (at least because old tuple resides in the
+---   storage and because an other tuple can be silently rewritten).
+---
+--- To support primary key update for **one storage** case one can use 'delete,
+--- then insert' way and perform the rollback action (insert old tuple) in case
+--- when insert of the new tuple reports an error. There are other ways, e.g.
+--- manual unique constraints check.
+---
 --- @tparam table self accessor_general instance
 ---
 --- @tparam string collection_name
@@ -420,16 +479,6 @@ local function update_tuple(self, collection_name, key, statements, opts)
 
     shard_check_status(func_name)
 
-    local is_shard_key_to_be_updated = false
-    for _, statement in ipairs(statements) do
-        -- statement is {operator, field_no, value}
-        local field_no = statement[2]
-        if field_no == SHARD_KEY_FIELD_NO then
-            is_shard_key_to_be_updated = true
-            break
-        end
-    end
-
     -- We follow tarantool convention and disallow update of primary key parts.
     local primary_index_info = get_index_info(collection_name, 0)
     for _, statement in ipairs(statements) do
@@ -443,14 +492,37 @@ local function update_tuple(self, collection_name, key, statements, opts)
         end
     end
 
+    local is_shard_key_to_be_updated = false
+    local new_shard_key_value
+    for _, statement in ipairs(statements) do
+        -- statement is {operator, field_no, value}
+        local field_no = statement[2]
+        if field_no == SHARD_KEY_FIELD_NO then
+            is_shard_key_to_be_updated = true
+            new_shard_key_value = statement[3]
+            break
+        end
+    end
+
+    local tuple = opts.tuple or get_tuple(self, collection_name, key)
+
+    local is_storage_to_be_changed = false
     if is_shard_key_to_be_updated then
-        local tuple = self.funcs.delete_tuple(self, collection_name, key,
-            {tuple = opts.tuple})
-        tuple = tuple:update(statements)
-        return self.funcs.insert_tuple(self, collection_name, tuple)
+        local old_shard_key_value = tuple[1]
+        local old_shard_key_hash = get_shard_key_hash(old_shard_key_value)
+        local new_shard_key_hash = get_shard_key_hash(new_shard_key_value)
+        is_storage_to_be_changed = old_shard_key_hash ~= new_shard_key_hash
+    end
+
+    if is_storage_to_be_changed then
+        -- different storages case
+        local old_tuple = opts.tuple or get_tuple(self, collection_name, key)
+        local new_tuple = old_tuple:update(statements)
+        self.funcs.insert_tuple(self, collection_name, new_tuple)
+        self.funcs.delete_tuple(self, collection_name, key, {tuple = old_tuple})
+        return new_tuple
     else
-        local tuple = opts.tuple or get_tuple(self, collection_name,
-            key)
+        -- one storage case
         local nodes = shard.shard(tuple[SHARD_KEY_FIELD_NO])
         local tuple = space_operation(collection_name, nodes, 'update', key,
             statements)