Move while_preventing_writes from conn to handler

If we put the `while_preventing_writes` on the connection then the
middleware that sends reads to the primary and ensures they can't write
will not work. The `while_preventing_writes` will only be applied to the
connection which it's called on - which in the case of the middleware is
Ar::Base.

This worked fine if you called it directly like
`OtherDbConn.connection.while_preventing_writes` but Rails didn't have a
way of knowing you wanted to call it on all the connections.

The change here moves the `while_preventing_writes` method from the
connection to the handler so that it can block writes to all queries for
that handler. This will apply to all the connections associated with
that handler.

1 files changed, 15 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb b/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
index adb5ae82a0..9b3f5260f7 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
@@ -999,15 +999,30 @@ module ActiveRecord
         end
       end
 
+      attr_reader :prevent_writes
+
       def initialize
         # These caches are keyed by spec.name (ConnectionSpecification#name).
         @owner_to_pool = ConnectionHandler.create_owner_to_pool
+        @prevent_writes = false
 
         # Backup finalizer: if the forked child never needed a pool, the above
         # early discard has not occurred
         ObjectSpace.define_finalizer self, ConnectionHandler.unowned_pool_finalizer(@owner_to_pool)
       end
 
+      # Prevent writing to the database regardless of role.
+      #
+      # In some cases you may want to prevent writes to the database
+      # even if you are on a database that can write. `while_preventing_writes`
+      # will prevent writes to the database for the duration of the block.
+      def while_preventing_writes
+        original, @prevent_writes = @prevent_writes, true
+        yield
+      ensure
+        @prevent_writes = original
+      end
+
       def connection_pool_list
         owner_to_pool.values.compact
       end