Restore Redis 4.3.0 accidental AUTH fallback behavior with a deprecation warning

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Unreleased
 
+* Restore the accidential auth behavior of redis-rb 4.3.0 with a warning. If provided with the `default` user password, but a wrong username,
+  redis-rb will first try to connect as the provided user, but then will fallback to connect as the `default` user with the provided password.
+  This behavior is deprecated and will be removed in Redis 4.6.0. Fix #1038.
+
 # 4.5.0
 
 * Handle parts of the command using incompatible encodings. See #1037.

lib/redis/client.rb

@@ -122,6 +122,16 @@ def connect
             rescue CommandError => err # Likely on Redis < 6
               if err.message.match?(/ERR wrong number of arguments for \'auth\' command/)
                 call [:auth, password]
+              elsif err.message.match?(/WRONGPASS invalid username-password pair/)
+                begin
+                  call [:auth, password]
+                rescue CommandError
+                  raise err
+                end
+                ::Kernel.warn(
+                  "[redis-rb] The Redis connection was configured with username #{username.inspect}, but" \
+                  " the provided password was for the default user. This will start failing in redis-rb 4.6."
+                )
               else
                 raise
               end

test/connection_test.rb

@@ -9,6 +9,34 @@ def test_provides_a_meaningful_inspect
     assert_equal "#<Redis client v#{Redis::VERSION} for redis://127.0.0.1:#{PORT}/15>", r.inspect
   end
 
+  def test_connection_with_user_and_password
+    target_version "6.0" do
+      with_acl do |username, password|
+        redis = Redis.new(OPTIONS.merge(username: username, password: password))
+        assert_equal "PONG", redis.ping
+      end
+    end
+  end
+
+  def test_connection_with_default_user_and_password
+    target_version "6.0" do
+      with_default_user_password do |_username, password|
+        redis = Redis.new(OPTIONS.merge(password: password))
+        assert_equal "PONG", redis.ping
+      end
+    end
+  end
+
+  def test_connection_with_wrong_user_and_password
+    target_version "6.0" do
+      with_default_user_password do |_username, password|
+        Kernel.expects(:warn).once
+        redis = Redis.new(OPTIONS.merge(username: "does-not-exist", password: password))
+        assert_equal "PONG", redis.ping
+      end
+    end
+  end
+
   def test_connection_information
     assert_equal "127.0.0.1",                 r.connection.fetch(:host)
     assert_equal 6381,                        r.connection.fetch(:port)

test/helper.rb

@@ -172,9 +172,19 @@ def with_acl
                 '+ping', '+select', '+command', '+cluster|slots', '+cluster|nodes',
                 '>mysecret')
       yield('johndoe', 'mysecret')
+    ensure
       admin.acl('DELUSER', 'johndoe')
       admin.close
     end
+
+    def with_default_user_password
+      admin = _new_client
+      admin.acl('SETUSER', 'default', '>mysecret')
+      yield('default', 'mysecret')
+    ensure
+      admin.acl('SETUSER', 'default', 'nopass')
+      admin.close
+    end
   end
 
   module Client