Publishing Nightly Builds

Having users run “nightly” builds is a great way to get rapid feedback on current development activities. One key way to get this kind of feedback is to make it as easy as possible for people to install the nightly builds, and to keep updated. The F-Droid tools now let you automate this whole process for apps that are setup on GitLab CI.

The F-Droid nightly repos are built on top of the standard Android ~/.android/debug.keystore that the Android tools automatically generate. That signing key is used to sign the nightly builds as well as the repo index. This is meant to be easy to setup and manage, but it does not provide the same level of security as a standard F-Droid repo!

Since this is about smoothing the development process, it also makes sense to reuse a shared debug.keystore among your team. That way, all debug builds are interchangeable. This makes debugging certain problems a lot easier since you can keep the app state when installing your own debug builds, for example, to add logging.

Automatic Version Codes

With nightly builds, the normal logic for setting versionCode in git for each release no longer makes sense. In order for Android to treat each new build as a possible update, the versionCode must increase, but that does not make sense to commit to git each time. So there needs to be some automatic logic for setting versionCode:

  • timestamp in seconds (date +%s) will always increase
  • incrementing the last release’s versionCode (e.g. 104030034, 104030035, 104030036)
  • git commit count (git rev-list --first-parent --count HEAD) provides some hints of where in the code the build was made from

For many apps, some combo will make the most sense. For F-Droid, we have a variable DB_VERSION for the internal database. We only handle upgrading the database, never downgrading. So that needs to be the most important part of the versionCode. Any build with DB_VERSION = 77 should not be upgraded by any build with DB_VERSION = 75. Then the minor part is the commit count, so it looks like (7502543, 7502544, 7702601, etc). with DB_VERSION as the first two digits, then a 0-padded commit count as the last 5 digits.

Configuring GitLab CI

This assumes that the app is already setup with .gitlab-ci.yml and builds are running on GitLab CI. This also works for forks of the main project that has the .gitlab-ci-yml.

  1. Run fdroid nightly --show-secret-var in the local git repo that is being setup, e.g. fdroid/fdroidclient. It will print out the SSH Public Key. Using --show-secret-var will make it print out base 64 text to be pasted into secret variable called DEBUG_KEYSTORE. Careful! That text is the entire debug.keystore, so protect it accordingly!

  2. Create a new GitLab “project” by appending -nightly to the name. For example, becomes and becomes

  3. In that new project, add the SSH Public Key derived from your debug.keystore as a Deploy Key in your Repository Settings, e.g.

  4. In the CI/CD Settings of the project being built, paste that into GitLab CI Secret Variable called DEBUG_KEYSTORE, e.g.

  5. Then in that same page, in the Protected section, set up your master branch to limit the possibility of leaking the debug.keystore. If master cannot be force-pushed, then any attempts to get the stored secrets are permanently written to the git history. Without that protection, someone with push access could push a commit in a branch to print out the secrets, then remove that commit to hide their actions.

  6. Add a deploy stage to your .gitlab-ci.yml that produces only the build to publish to the nightly, then runs fdroid nightly. For example:

  stage: deploy
    - master
    - ./gradlew assembleDebug
    - pip3 install fdroidserver
    - export PATH=~/.local/bin:$PATH
    - fdroid nightly

For a real world example using gitlab-ci, see fdroidclient:

Configuring for GitHub and Travis CI

  1. Generate a new debug.keystore for each app, since GitHub only allows a deploy key to be used on a single repo. Keep that file safe, since it is the signing key for your nightly build.
    keytool -genkey -v -keystore im.zom.messenger-debug.keystore \
      -keyalg RSA -keysize 2048 -validity 10000 \
      -alias androiddebugkey -storepass android -keypass android \
      -dname "CN=Android Debug,O=Android,C=US"
  2. Set a GitHub Deploy Key for by going to The public SSH key is printed out by running:
    fdroid nightly --keystore im.zom.messenger-debug.keystore
  3. check Allow write access in the GitHub Deploy Key settings
  4. To print out the secret variable contents, use --show-secret-var. Careful! That text is the entire debug.keystore, so protect it accordingly! This prints out base64 text to be pasted into a Travis CI “Environment Variable” in Call the variable DEBUG_KEYSTORE and make sure “Display value in build log” is OFF.
    fdroid nightly --keystore im.zom.messenger-debug.keystore --show-secret-var

And here’s a simple example of how to add fdroidserver to your Travis CI setup to make it deploy the APK after a successful test:

sudo: required
language: android

      - sourceline: 'ppa:fdroid/fdroidserver'
      - fdroidserver
      - openssh-client

    - tools
    - platform-tools
    - build-tools-26.0.2
    - android-25
    - 'android-sdk-license-.+'

  - ./gradlew test

  - fdroid nightly -v

For a real world example, see Zom: