Reset roles by "SET ROLE NONE"

[chernser]

Summary

Closes #1647

Checklist

Delete items not relevant to your PR:

• Unit and integration tests covering the common scenarios were added
• A human-readable description of the changes was provided to include in CHANGELOG

[CLAassistant]

All committers have signed the CLA.

[slvrtrn]

The following simulation of an app with the impersonation feature based on row policies could be an OK integration test.

Setup:

CREATE OR REPLACE TABLE test_table
(
    `s` String
)
ENGINE = MergeTree
ORDER BY tuple();
INSERT INTO test_table VALUES ('a'), ('b');

CREATE USER test_user NOT IDENTIFIED;
CREATE ROLE row_a;
CREATE ROLE row_b;
GRANT row_a, row_b TO test_user;

GRANT SELECT ON default.test_table TO test_user;
CREATE ROW POLICY OR REPLACE policy_row_b ON test_table FOR SELECT USING s = 'b' TO row_b;
CREATE ROW POLICY OR REPLACE policy_row_a ON test_table FOR SELECT USING s = 'a' TO row_a;

Test:

➜ curl -H "Authorization: Basic dGVzdF91c2VyOg==" "http://localhost:8123" --data-binary "SELECT * FROM test_table"
a
b
➜ curl -H "Authorization: Basic dGVzdF91c2VyOg==" "http://localhost:8123?role=row_b" --data-binary "SELECT * FROM test_table"
b
➜ curl -H "Authorization: Basic dGVzdF91c2VyOg==" "http://localhost:8123?role=row_a" --data-binary "SELECT * FROM test_table"
a
➜ curl -H "Authorization: Basic dGVzdF91c2VyOg==" "http://localhost:8123?role=row_a&role=row_b" --data-binary "SELECT * FROM test_table"
a
b

In Java code, that could be:

-- no roles set - a select query will get both rows A and B
SET ROLE row_a;
-- gets only row A
SET ROLE row_b;
-- gets both rows A and B
SET ROLE NONE;
-- gets both rows A and B
SET ROLE row_b;
-- gets only row B

[mzitnik]

Why we are setting this, it is the default

[chernser]

I've forgot this after testing. Will remove.