Salsa CI new feature to build reverse dependencies

[Otto Kekäläinen <otto@debian.org>]

Hi Go team,

Salsa CI now supports doing automatic builds of reverse dependencies.
Try this out next time you are updating a library that has a lot of
consumers to check that the new version does not break the old
versions, or if they do, you can repeat the build while updating the
old versions until you get a state where everything is compatible
again.

Docs at
https://salsa.debian.org/salsa-ci-team/pipeline#build-reverse-dependencies
copied here for convenience:




## Build reverse dependencies

Build reverse dependencies jobs allow maintainers to automatically rebuild
packages that build depend on their package.

This is extremely valuable in many contexts, for example during transitions,
library updates, or any change where you need confidence that reverse
dependencies still build successfully. Running build reverse-dependencies jobs
helps catch breakages early and provides stronger QA before uploads reach the
archive.

### Enable build reverse dependencies checking

Build reverse dependencies jobs are **disabled**, by default. It is recommended
to run these jobs **on demand** only, for specific pipelines. There are a
couple of ways to do it:

* From the Salsa web UI:

  1. Open `https://salsa.debian.org/<project>/-/pipelines/new`
  2. Scroll down to **Variables**
  3. Find the entry for `SALSA_CI_DISABLE_BUILD_REVERSE_DEPENDENCIES`
  4. Change its value from `1` (default, disabled) to `0` (on-demand enabled)

* From the command line when `git push`ing:

  ```bash
  git push -o ci.variable="SALSA_CI_DISABLE_BUILD_REVERSE_DEPENDENCIES=0"
  ```

This way, build reverse dependencies jobs are enabled only when you choose to
run them, avoiding unnecessary load on the Salsa infrastructure.

:warning: **Please, avoid enabling build reverse dependencies jobs directly in
`debian/salsa-ci.yml`:**

Hard-coding the variable that enables this check will make build reverse
dependencies jobs run on **every push**. Salsa admins have explicitly warned
that this is considered **abuse of shared infrastructure**, see for example
[this thread on
debian-project@](https://lists.debian.org/debian-project/2025/08/msg00021.html).

### Maximum of 100 reverse dependencies per pipeline

Some packages (for example core libraries or popular node modules) have
hundreds or even thousands of reverse dependencies. Rebuilding all of them in a
single pipeline would overwhelm the shared CI infrastructure.

To prevent this, Salsa CI limits build reverse dependencies to at most 100 per
pipeline. This safeguard ensures that no single package consumes a
disproportionate share of the available resources. The limit may be adjusted in
the future when Salsa gains additional hardware and runner capacity.