In Rails 5, including `ActionView::Context` can have a significant and
hidden performance penalty because this module also includes
`ActionView::CompiledTemplates`. This means that any module that
includes ActionView::Context becomes a descendant of
`CompiledTemplates`.
When a partial is rendered for the first time, it runs
`ActionView::CompiledTemplates#module_eval`, which will evaluate a
string that defines a new method for that partial. For example, the
source of partial might be this string:
```
def _app_views_project_show_html_haml___12345(local_assigns, output)
"hello world"
end
```
When this string is evaluated, the Ruby interpreter will define the
method and clear the global method cache for all descendants of
`ActionView::CompiledTemplates`. Previous to this change, we
inadvertently made a number of modules fall into this category:
* GroupChildEntity
* NoteUserEntity
* Notify
* MergeRequestUserEntity
* AnalyticsCommitEntity
* CommitEntity
* UserEntity
* Kaminari::Helpers::Paginator
* CurrentUserEntity
* ActionView::Base
* ActionDispatch::DebugExceptions::DebugView
* MarkupHelper
* MergeRequestPresenter
After this change:
* Kaminari::Helpers::Paginator
* ActionView::Base
* ActionDispatch::DebugExceptions::DebugView
Each time a partial is rendered for the first time, all methods for
those modules will have to be redefined. This can exact a significant
performance penalty.
How bad is this penalty? Using the following benchmark script, we can
use DTrace to sample the Ruby interpreter:
```
Benchmark.bm do |x|
x.report do
1000.times do
ActionView::CompiledTemplates.module_eval("def testme\nend")
end
end
end
```
This revealed a 11x jump in the time spent in `core#define_method`
alone.
Rails 6 fixes this behavior by moving the `include CompiledTemplates`
into ActionView::Base so that including `ActionView::Context` doesn't
quietly affect other modules in this way.
Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/11198
