Merge branch 'ml-add-doc-re-negative-predicate-methods' into 'master'

Document when to use negative predicate methods in E2E tests

See merge request gitlab-org/gitlab!43370

doc/development/testing_guide/end_to_end/best_practices.md

@@ -310,3 +310,56 @@ end
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 ```
+
+## Ensure `expect` statements wait efficiently
+
+In general, we use an `expect` statement to check that something _is_ as we expect it. For example:
+
+```ruby
+Page::Project::Pipeline::Show.perform do |pipeline|
+  expect(pipeline).to have_job("a_job")
+end
+```
+
+### Ensure `expect` checks for negation efficiently
+
+However, sometimes we want to check that something is _not_ as we _don't_ want it to be. In other
+words, we want to make sure something is absent. In such a case we should use an appropriate
+predicate method that returns quickly, rather than waiting for a state that won't appear.
+
+It's most efficient to use a predicate method that returns immediately when there is no job, or waits
+until it disappears:
+
+```ruby
+# Good
+Page::Project::Pipeline::Show.perform do |pipeline|
+  expect(pipeline).to have_no_job("a_job")
+end
+```
+
+### Problematic alternatives
+
+Alternatively, if we want to check that a job doesn't exist it might be tempting to use `not_to`:
+
+```ruby
+# Bad
+Page::Project::Pipeline::Show.perform do |pipeline|
+  expect(pipeline).not_to have_job("a_job")
+end
+```
+
+For this statement to pass, `have_job("a_job")` has to return `false` so that `not_to` can negate it.
+The problem is that `have_job("a_job")` waits up to ten seconds for `"a job"` to appear before
+returning `false`. Under the expected condition this test will take ten seconds longer than it needs to.
+
+Instead, we could force no wait:
+
+```ruby
+# Not as bad but potentially flaky
+Page::Project::Pipeline::Show.perform do |pipeline|
+  expect(pipeline).not_to have_job("a_job", wait: 0)
+end
+```
+
+The problem is that if `"a_job"` is present and we're waiting for it to disappear, this statement
+will fail.