Add guide for setting up cross-project testing (#710)

Add guide for setting up cross-project testing

This is based on https://cockpit-project.org/blog/tmt-cross-project-testing.html, focusing on the configuration.

Reviewed-by: Tomas Tomecek <tomas@tomecek.net>

docs/cross-project-testing.md

@@ -0,0 +1,99 @@
+---
+title: Setting up cross-project testing
+sidebar_position: 9
+---
+# Setting up cross-project testing
+Cross-project testing enables you to ensure the compatibility and reliability of interconnected repositories.
+And with a little bit of configuration on Packit and tmt parts, it can be easily automated within Packit!
+
+Big kudos to [Martin Pitt](https://github.com/martinpitt/) who experimented with these settings in 
+[`cockpit` repositories](https://github.com/cockpit-project/) and wrote an amazing blog post about it!
+If you are interested in all the details, make sure to read the 
+[full blog post](https://cockpit-project.org/blog/tmt-cross-project-testing.html). Alternatively, the following sections
+will describe the needed configuration for setting it up.
+
+After determining the dependency (or dependencies) you want to test, the configuration is relatively easy:
+1. For the dependent repository, Packit should be configured to build every commit of the `main` branch, or any other
+similar branch containing recent changes, into a dedicated Copr repository.
+2. For the dependency repository, you should add a separate Packit test job with a tmt test plan to run on all pull requests. 
+
+Let's break down these steps using examples from the `cockpit` setup (where `cockpit-podman` depends on the `cockpit` repo)!
+
+### Configuring Packit builds on commits of the dependent repository
+The goal here is to build the most recent code (usually in the `main` branch) of the repo in a dedicated Copr repository, 
+which can be easily done using Packit.
+Firstly a new dedicated [Copr](https://copr.fedorainfracloud.org) project needs to be created and configured as needed.
+For Packit to build in this project, the `Packit allowed forge projects` needs to be filled as well.
+
+Then you can configure a Packit `copr_build` job to do a build in the new Copr repo for each commit that lands in the desired 
+branch of the dependent repo (in this example `main` branch of `cockpit-podman`):
+```yaml
+- job: copr_build
+  trigger: commit
+  branch: "^main$"
+  owner: "@cockpit"
+  project: "main-builds"
+  preserve_project: True
+```
+
+You will need to approve the `packit` Copr user to be able to trigger builds in the Copr repo. 
+
+The additional details of the setup are described [here](/docs/configuration/upstream/copr_build#using-a-custom-copr-project).
+
+### Configuring the test plan 
+After that, a new test plan `plans/<your-project>.fmf` needs to be added to the dependency repository, which selects the
+tests you want to run from this repo. Unfortunately, it is impossible to auto-import all plans
+at the moment (but this may be useful since you can select a subset of tests that apply to the tested dependency).
+
+So this is mostly a copy of the dependent repo test plan, with one modification: they should not run by default, 
+but only in a `revdeps` tmt context. The test plan can then look like this (an example from `cockpit`):
+```yaml
+# reverse dependency test
+enabled: false
+
+adjust+:
+  when: revdeps == yes
+  enabled: true
+
+discover:
+    how: fmf
+    url: https://github.com/cockpit-project/cockpit-podman
+    ref: "main"
+execute:
+    how: tmt
+
+/podman-system:
+    summary: Run cockpit-podman system tests
+    discover+:
+        test: /test/browser/system
+
+# ... possibly more test plans here
+```
+For the other plans in this repository, you should mark it to not run in the `revdeps` scenario:
+```yaml
+adjust+:
+  when: revdeps == yes
+  enabled: false
+```
+
+### Configuring the Packit test job
+Finally, to plug everything together, a Packit test job needs to be configured that will run the `revdeps` context test plan 
+against the builds from `main` branch of the dependent repo by specifying [`tf_extra_params`](https://packit.dev/docs/configuration/upstream/tests#optional-parameters).
+The key here is to install the latest `main` build from the dependency: we are making sure we are not breaking `cockpit`.
+Make sure to also give it a meaningful `identifier` so that you can differentiate the statuses:
+
+```yaml
+  - job: tests
+    identifier: revdeps
+    trigger: pull_request
+    targets:
+      - fedora-latest-stable
+    tf_extra_params:
+      environments:
+        - artifacts:
+          - type: repository-file
+            id: https://copr.fedorainfracloud.org/coprs/g/cockpit/main-builds/repo/fedora-$releasever/group_cockpit-main-builds-fedora-$releasever.repo
+          tmt:
+            context:
+              revdeps: "yes"
+```

docs/fedora-releases-guide.md

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 ---
 title: How to do Fedora releases with Packit
-sidebar_position: 9
+sidebar_position: 8
 ---
 # How to do Fedora releases with Packit