Dash comments should match read query regexp

As well as allowing queries of the form

    /* some comment */
    SELECT ...

we should also allow queries of the form

    -- some comment
    SELECT ...

Additionally, I've added tests for both comment forms into the general
adapter test case, along with some additional checks to ensure that the
regexp does not return a false-positive match if the read query
signifier keyword is actually a part of the comment.

This expands on the change already implemented in
12b964c50f.

activerecord/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Allow double-dash comment syntax when querying read-only databases
+
+    *James Adam*
+
 *   Add `values_at` method.
 
     Returns an array containing the values associated with the given methods.

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -37,7 +37,7 @@ module ActiveRecord
       include Savepoints
 
       SIMPLE_INT = /\A\d+\z/
-      COMMENT_REGEX = %r{/\*(?:[^\*]|\*[^/])*\*/}m
+      COMMENT_REGEX = %r{(?:\-\-.*\n)*|/\*(?:[^\*]|\*[^/])*\*/}m
 
       attr_accessor :pool
       attr_reader :visitor, :owner, :logger, :lock

activerecord/test/cases/adapter_test.rb

@@ -235,6 +235,72 @@ module ActiveRecord
       end
     end
 
+    def test_doesnt_error_when_a_select_query_starting_with_a_slash_star_comment_is_called_while_preventing_writes
+      @connection.insert("INSERT INTO subscribers(nick) VALUES ('138853948594')")
+
+      @connection_handler.while_preventing_writes do
+        result = @connection.select_all("/* some comment */ SELECT subscribers.* FROM subscribers WHERE nick = '138853948594'")
+        assert_equal 1, result.length
+      end
+    end
+
+    def test_errors_when_an_insert_query_prefixed_by_a_slash_star_comment_is_called_while_preventing_writes
+      @connection_handler.while_preventing_writes do
+        @connection.transaction do
+          assert_no_queries do
+            assert_raises(ActiveRecord::ReadOnlyError) do
+              @connection.insert("/* some comment */ INSERT INTO subscribers(nick) VALUES ('138853948594')", nil, false)
+            end
+          end
+        end
+      end
+    end
+
+    def test_doesnt_error_when_a_select_query_starting_with_double_dash_comments_is_called_while_preventing_writes
+      @connection.insert("INSERT INTO subscribers(nick) VALUES ('138853948594')")
+
+      @connection_handler.while_preventing_writes do
+        result = @connection.select_all("-- some comment\n-- comment about INSERT\nSELECT subscribers.* FROM subscribers WHERE nick = '138853948594'")
+        assert_equal 1, result.length
+      end
+    end
+
+    def test_errors_when_an_insert_query_prefixed_by_a_double_dash_comment_is_called_while_preventing_writes
+      @connection_handler.while_preventing_writes do
+        @connection.transaction do
+          assert_no_queries do
+            assert_raises(ActiveRecord::ReadOnlyError) do
+              @connection.insert("-- some comment\nINSERT INTO subscribers(nick) VALUES ('138853948594')", nil, false)
+            end
+          end
+        end
+      end
+    end
+
+    def test_errors_when_an_insert_query_prefixed_by_a_slash_star_comment_containing_read_command_is_called_while_preventing_writes
+      @connection_handler.while_preventing_writes do
+        @connection.transaction do
+          assert_no_queries do
+            assert_raises(ActiveRecord::ReadOnlyError) do
+              @connection.insert("/* SELECT */ INSERT INTO subscribers(nick) VALUES ('138853948594')", nil, false)
+            end
+          end
+        end
+      end
+    end
+
+    def test_errors_when_an_insert_query_prefixed_by_a_double_dash_comment_containing_read_command_is_called_while_preventing_writes
+      @connection_handler.while_preventing_writes do
+        @connection.transaction do
+          assert_no_queries do
+            assert_raises(ActiveRecord::ReadOnlyError) do
+              @connection.insert("-- SELECT\nINSERT INTO subscribers(nick) VALUES ('138853948594')", nil, false)
+            end
+          end
+        end
+      end
+    end
+
     if ActiveRecord::Base.connection.supports_common_table_expressions?
       def test_doesnt_error_when_a_read_query_with_a_cte_is_called_while_preventing_writes
         @connection.insert("INSERT INTO subscribers(nick) VALUES ('138853948594')")