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, 10 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/mysql/database_statements.rb b/activerecord/lib/active_record/connection_adapters/mysql/database_statements.rb
index 9ea237dd81..0ec9581c42 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql/database_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql/database_statements.rb
@@ -19,8 +19,18 @@ module ActiveRecord
           execute(sql, name).to_a
         end
 
+        READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp.call(:begin, :select, :set, :show, :release, :savepoint) # :nodoc:
+
+        def write_query?(sql) # :nodoc:
+          !READ_QUERY.match?(sql)
+        end
+
         # Executes the SQL statement in the context of this connection.
         def execute(sql, name = nil)
+          if preventing_writes? && write_query?(sql)
+            raise ActiveRecord::StatementInvalid, "Write query attempted while in readonly mode: #{sql}"
+          end
+
           # make sure we carry over any changes to ActiveRecord::Base.default_timezone that have been
           # made since we established the connection
           @connection.query_options[:database_timezone] = ActiveRecord::Base.default_timezone