graphql: support default variables (#920)

See: https://graphql.org/learn/queries/#default-variables
Note: there is a difference between explicitly passed "null" value
and just omitted variable. See graphql/graphql-spec#418

Closes #866

Author: olegrok

cartridge/graphql/execute.lua

@@ -240,16 +240,26 @@ local function getFieldEntry(objectType, object, fields, context)
     argumentMap[argument.name.value] = argument
   end
 
+  local defaultValues = {}
+  if context.operation.variableDefinitions ~= nil then
+    for _, value in ipairs(context.operation.variableDefinitions) do
+      if value.defaultValue ~= nil then
+        local variableType = query_util.typeFromAST(value.type, context.schema)
+        defaultValues[value.variable.name.value] = util.coerceValue(value.defaultValue, variableType)
+      end
+    end
+  end
+
   local arguments = util.map(fieldType.arguments or {}, function(argument, name)
     local supplied = argumentMap[name] and argumentMap[name].value
 
     supplied = util.coerceValue(supplied, argument, context.variables,
       {strict_non_null = true})
-    if supplied ~= nil then
+    if type(supplied) ~= 'nil' then
       return supplied
     end
 
-    return argument.defaultValue
+    return defaultValues[name]
   end)
 
   --[[

cartridge/graphql/rules.lua

@@ -410,7 +410,8 @@ function rules.variableDefaultValuesHaveCorrectType(node, context)
       if definition.type.kind == 'nonNullType' and definition.defaultValue then
         error('Non-null variables can not have default values')
       elseif definition.defaultValue then
-        util.coerceValue(definition.defaultValue, context.schema:getType(definition.type.name.value))
+        local variableType = query_util.typeFromAST(definition.type, context.schema)
+        util.coerceValue(definition.defaultValue, variableType)
       end
     end
   end

test/integration/graphql_test.lua

@@ -779,3 +779,134 @@ function g.test_middleware()
         }
     )
 end
+
+g.test_default_values = function()
+    local server = cluster.main_server
+
+    server.net_box:eval([[
+        package.loaded['test'] = package.loaded['test'] or {}
+        package.loaded['test']['test_default_value'] = function(_, args)
+            if args.arg == nil then
+                return 'nil'
+            end
+            return args.arg
+        end
+
+        package.loaded['test']['test_default_list'] = function(_, args)
+            if args.arg == nil then
+                return 'nil'
+            end
+            return args.arg[1]
+        end
+
+        package.loaded['test']['test_default_object'] = function(_, args)
+            if args.arg == nil then
+                return 'nil'
+            end
+            return args.arg.field
+        end
+
+        local graphql = require('cartridge.graphql')
+        local types = require('cartridge.graphql.types')
+
+        graphql.add_callback({
+            name = 'test_default_value',
+            args = {
+                arg = types.string,
+            },
+            kind = types.string,
+            callback = 'test.test_default_value',
+        })
+
+        graphql.add_callback({
+            name = 'test_default_list',
+            args = {
+                arg = types.list(types.string),
+            },
+            kind = types.string,
+            callback = 'test.test_default_list',
+        })
+
+        local input_object = types.inputObject({
+            name = 'default_input_object',
+            fields = {
+                field = types.string,
+            }
+        })
+
+        graphql.add_callback({
+            name = 'test_default_object',
+            args = {
+                arg = input_object,
+            },
+            kind = types.string,
+            callback = 'test.test_default_object',
+        })
+    ]])
+
+    t.assert_equals(
+        server:graphql({
+            query = [[
+                query($arg: String = "default_value") {
+                    test_default_value(arg: $arg)
+                }
+            ]],
+        variables = {}}
+        ).data.test_default_value, 'default_value'
+    )
+
+    t.assert_equals(
+        server:graphql({
+            query = [[
+                query($arg: String = "default_value") {
+                    test_default_value(arg: $arg)
+                }
+            ]],
+        variables = {arg = box.NULL}}
+        ).data.test_default_value, 'nil'
+    )
+
+    t.assert_equals(
+        server:graphql({
+            query = [[
+                query($arg: [String] = ["default_value"]) {
+                    test_default_list(arg: $arg)
+                }
+            ]],
+        variables = {}}
+        ).data.test_default_list, 'default_value'
+    )
+
+    t.assert_equals(
+        server:graphql({
+            query = [[
+                query($arg: [String] = ["default_value"]) {
+                    test_default_list(arg: $arg)
+                }
+            ]],
+        variables = {arg = box.NULL}}
+        ).data.test_default_list, 'nil'
+    )
+
+    t.assert_equals(
+        server:graphql({
+            query = [[
+                query($arg: default_input_object = {field: "default_value"}) {
+                    test_default_object(arg: $arg)
+                }
+            ]],
+        variables = {}}
+        ).data.test_default_object, 'default_value'
+    )
+
+    t.assert_equals(
+        server:graphql({
+            query = [[
+                query($arg: default_input_object = {field: "default_value"}) {
+                    test_default_object(arg: $arg)
+                }
+            ]],
+        variables = {arg = box.NULL}}
+        ).data.test_default_object, 'nil'
+    )
+end