Add ability to prevent writes to a database

This PR adds the ability to prevent writes to a database even if the
database user is able to write (ie the database is a primary and not a
replica).

This is useful for a few reasons: 1) when converting your database from
a single db to a primary/replica setup - you can fix all the writes on
reads early on, 2) when we implement automatic database switching or
when an app is manually switching connections this feature can be used
to ensure reads are reading and writes are writing. We want to make sure
we raise if we ever try to write in read mode, regardless of database
type and 3) for local development if you don't want to set up multiple
databases but do want to support rw/ro queries.

This should be used in conjunction with `connected_to` in write mode.
For example:

```
ActiveRecord::Base.connected_to(role: :writing) do
  Dog.connection.while_preventing_writes do
    Dog.create! # will raise because we're preventing writes
  end
end

ActiveRecord::Base.connected_to(role: :reading) do
  Dog.connection.while_preventing_writes do
    Dog.first # will not raise because we're not writing
  end
end
```

1 files changed, 56 insertions, 0 deletions

diff --git a/activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb b/activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb
index cbb6cd42b5..59ddb8457e 100644
--- a/activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb
+++ b/activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb
@@ -376,6 +376,62 @@ module ActiveRecord
         end
       end
 
+      def test_errors_when_an_insert_query_is_called_while_preventing_writes
+        with_example_table do
+          assert_raises(ActiveRecord::StatementInvalid) do
+            @connection.while_preventing_writes do
+              @connection.execute("INSERT INTO ex (data) VALUES ('138853948594')")
+            end
+          end
+        end
+      end
+
+      def test_errors_when_an_update_query_is_called_while_preventing_writes
+        with_example_table do
+          @connection.execute("INSERT INTO ex (data) VALUES ('138853948594')")
+
+          assert_raises(ActiveRecord::StatementInvalid) do
+            @connection.while_preventing_writes do
+              @connection.execute("UPDATE ex SET data = '9989' WHERE data = '138853948594'")
+            end
+          end
+        end
+      end
+
+      def test_errors_when_a_delete_query_is_called_while_preventing_writes
+        with_example_table do
+          @connection.execute("INSERT INTO ex (data) VALUES ('138853948594')")
+
+          assert_raises(ActiveRecord::StatementInvalid) do
+            @connection.while_preventing_writes do
+              @connection.execute("DELETE FROM ex where data = '138853948594'")
+            end
+          end
+        end
+      end
+
+      def test_doesnt_error_when_a_select_query_is_called_while_preventing_writes
+        with_example_table do
+          @connection.execute("INSERT INTO ex (data) VALUES ('138853948594')")
+
+          @connection.while_preventing_writes do
+            assert_equal 1, @connection.execute("SELECT * FROM ex WHERE data = '138853948594'").entries.count
+          end
+        end
+      end
+
+      def test_doesnt_error_when_a_show_query_is_called_while_preventing_writes
+        @connection.while_preventing_writes do
+          assert_equal 1, @connection.execute("SHOW TIME ZONE").entries.count
+        end
+      end
+
+      def test_doesnt_error_when_a_set_query_is_called_while_preventing_writes
+        @connection.while_preventing_writes do
+          assert_equal [], @connection.execute("SET standard_conforming_strings = on").entries
+        end
+      end
+
       private
 
         def with_example_table(definition = "id serial primary key, number integer, data character varying(255)", &block)