[FR] Pipeline-level sidecars

[lbernick]

Feature request

The ability to create sidecars that can be used by multiple Tasks in a Pipeline. (Note: this is different from #4235, which requests allowing a Task's sidecar to be defined in a Pipeline Task.) This is similar to the "daemon containers" feature in Argo Workflows.

My guess is we would implement this as separate pods that TaskRun pods can communicate with via the network (rather than shared volumes mounted onto each pod).

Use case

Use case #1: Docker sidecar

A user might want to run different Tasks that all use a docker daemon; for example, docker build followed by a docker exec python -m pytest. Today, these units of functionality tend to be squished into one Task (such as the "build and push" that occurs in most catalog builder tasks) or an image can be built and pushed in one task, and then downloaded from an artifact registry and used in a subsequent task.

Use case #2: integration tests

This sidecar or sidecars could set up test instances of an application and/or its DB, and testing tasks could seed it with test data for integration tests. I don't have a great example though for why someone might prefer to connect to the same test DB in multiple Tasks.

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[jerop]

here's a closed issue with the same feature request and related discussions: #2973

[lbernick]

I've opened tektoncd/community#943 with a problem statement for this feature.