Rules do not sufficiently replace only/except

Rules are planned to be deprecated in Gitlab 13.0, but there is functionality missing from it.

stages:
  - a
  - b

job_a:
   stage: a
   only:
     - master
   when: manual

job_b:
   stage: b

Will results in a pipeline that runs job_b and leaves job_a to be run when decided. When converted to rules, the pipeline becomes blocked instead and job_b will not run.

stages:
  - a
  - b

job_a:
   stage: a
   rules:
     - if: $CI_COMMIT_BRANCH == "master"
       when: manual

job_b:
   stage: b

I know that allow_failure will not block, but this causes its own problems as they cannot prevent future jobs from running so if you kick off a manual job and its dependent job, the latter does not wait for the former.

Solution

Based on the following comment

The two usages are not the same;

job_a:
   stage: a
   only:
     - master
   when: manual

job_a:
   stage: a
   rules:
     - if: $CI_COMMIT_BRANCH == "master"
       when: manual

In the first usage, job_a is already a manual job. In the second one, we do this a manual job if the rules matches.

This would have been a correct mirror;

job_a:
   stage: a
   when: manual
   rules:
     - if: $CI_COMMIT_BRANCH == "master"