"Only" or "rules" keywords weirdly remove jobs from the CI pipeline - gitlab

Question

I am trying to use "rules" and "only" keywords to define my pipeline behaviors between merge requests, pushes into dev branch and pushes into master branch.
I noticed several weird behaviors in the Gitlab CI, let's see in my merge_requests pipelines.
With this gitlab-ci.yml file, without any rule, all the jobs are displayed and run.
image: "python:3.8"
stages:
- test_without_only_policy
- test_with_only_policy
test_without_only_policy:
stage: test_without_only_policy
when: manual
script:
- echo "Yay, I am in the pipeline"
test_with_only_policy:
stage: test_with_only_policy
script:
- echo "I am always in the pipeline"
Everything is working as expected, great 👍
With this gitlab-ci.yml file, without an "only" policy in the 2nd job, the 1st job without rules disappears.
image: "python:3.8"
stages:
- test_without_only_policy
- test_with_only_policy
test_without_only_policy:
stage: test_without_only_policy
when: manual
script:
- echo "No, I am not in the pipeline anymore"
test_with_only_policy:
stage: test_with_only_policy
only:
- merge_requests
script:
- echo "I am always in the pipeline"
Why did the 1st job without the rules or "only" policy disappear?
With this gitlab-ci.yml file, with a "rules" keyword in the 2nd job, the 1st job without rules disappears.
image: "python:3.8"
stages:
- test_without_only_policy
- test_with_only_policy
test_without_only_policy:
stage: test_without_only_policy
when: manual
script:
- echo "No, I am not in the pipeline anymore"
test_with_only_policy:
stage: test_with_only_policy
rules:
- if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
when: manual
script:
- echo "I am always in the pipeline"
Why did the 1st job without the rules or "only" policy disappear?
Thank you for your help, I don't understand why my job disappears when I add rules in other jobs.

Answer 1

According to the documentation for Merge Request Pipelines, if you have a pipeline where only some jobs use only/except/rules with merge_requests, only those jobs will be in the pipeline if it is a Merge Request pipeline. The other jobs will be left out.
Here's the example from the docs:
build:
stage: build
script: ./build
only:
- main
test:
stage: test
script: ./test
only:
- merge_requests
deploy:
stage: deploy
script: ./deploy
only:
- main
In this example, the only job that specifies it should run for Merge Request pipelines is the test job. For standard push pipelines, the build and deploy jobs will run, but when a new merge request is created, a change is pushed to a branch that is the source branch on an existing merge request, or you hit the Run Pipeline button on the Pipelines tab for a Merge Request, it will run a Merge Request pipeline.
Here's another example with a different scenario:
A:
stage: some_stage
only:
- branches
- tags
- merge_requests
script:
- script.sh
B:
stage: some_other_stage
only:
- branches
- tags
- merge_requests
script:
- script.sh
C:
stage: third_stage
only:
- merge_requests
script:
- script.sh
In this example, jobs A and B run for all pipeline types push, tags, merge_requests, etc., but job C only runs for merge_request pipelines.
That's why your test_without_only_policy job will never be in a pipeline where test_with_only_policy runs. test_with_only_policy specifically runs for Merge Request events, but test_without_only_policy does not.