How to dynamically pick a git branch to use in Jenkins build

Question

I'm trying to create a new project configuration for Jenkins build server. To simplify what I'm trying to do, I will use only two components to describe the problem.

ComponentA

• Change in this component triggers the build of this project on CI server.
• CI server has statically configured branch to monitor for changes and build. Eg. master or develop branch.
• This component contains a configuration file with required version of ComponentB it depends on.

ComponentB

• Changes to this component don't trigger build of this project on CI server (there will be another project to cover development of ComponentB).
• Individual versions of component are tagged
• ComponentA has required version of ComponentB in its configuration file
• CI server does not know what branch (tag) to checkout until configuration file of ComponentA is somehow parsed.

What is the right way to achieve this on Jenkins? I was trying to find out how to add this dynamic behavior of parsing the config file and making Git Plugin to check out branch based on expected version of ComponentB but so far I have no clue.

In the next step I may even want to have wildcards (like 5.3.*) in configuration file so I will have to find a the newest ComponentB's tag matching the wildcard.

EDIT

Now I see that I simplified my problem too much and due to the simplification, the main limitation is no longer present.

The main limitation is that Component A and B must be built together. It is not possible to build them separately as they form one executable / library and the build script needs source files from both components.

If you ask why such a strange configuration, let's give Component A and B some description:

• ComponentA: Native platform specific code
• ComponentB: Native platform independent code

There may be many Component As - one for each platform, but just single Component B. Merging particular A with B produces full source code for single platform but not every platform may be updated to the latest version of B so it needs to have control over which version of B should be used for built.

Answer 1

One option to achieve what you want is to use the following setup:

Create two Jenkins jobs:

• "Component A" (automatically triggered on SCM changes)
• "Component B" ("manually" triggered)

Step #1

Define the branch build parameter for "Component B":

Use this parameter as the "Git Plugin" branch specifier:

Now you should be able to manually trigger "Component B" build, by specifying a proper branch (tag) parameter to it, e.g. tags/5.3.0.

Step #2

Add a new "Execute Shell" build step to your "Component A" build, which will extract the "Component B" version from the configuration file in the workspace, and prepare b.properties file with the "Component B" build parameters.

Install a Parameterized Trigger Jenkins plugin, and add a new "Trigger/call builds on other projects" build step to "Component A" job:

Using your b.properties file as the source of build parameters.

Now each time "Component A" is re-build, a new "Component B" build will get triggered, with the target branch/tag as a build parameter.

Adding wildcard support

If you want to support wildcard versions, you can use git ls-remote command to find the latest tag, like that:

#B=$(obtain B version from the config file in a usual way)   

LATEST=$(\
    git ls-remote --tags YOUR_REPOSITORY_URL "$B"\
    |cut -d / -f3|sort -r --version-sort|head -1\
)
 
cat <<EOF > b.properties
    branch=tags/$LATEST
EOF

This will list all the tags, matching "B" version pattern, in the remote "Component B" repository, and save the latest version number in the LATEST variable.

Add this to your "Execute Shell" step of the "Component A" job, and it should be able to handle version numbers patterns like: 5.3.*

The catch is that the shell script will run as the Jenkins daemon user, so it must have proper credentials configured, to access the remote Git repository (e.g. via the ssh pubkey).

Alternatively you may want to look into the Credentials Binding Plugin, to reuse the Git credentials stored in Jenkins itself.

Using Jenkins 2.0 style pipeline

You can also solve the task at hand by using a Jenkins 2.0-style Pipeline, which will allow you to checkout the code for components A and B, into a single workspace, and then apply some common build step to them.

Your pipeline could look something like this:

node {

   //Settings
   def credentialsId = '8fd28e34-b04e-4bc5-874a-87f4c0e05a03'    
   def repositoryA = 'ssh://[email protected]/projects/a.git'
   def repositoryB = 'ssh://[email protected]/projects/b.git'

   stage('Checkout component A') {
      git credentialsId: credentialsId , 
      url: repositoryA , branch : "master"
   }
   
   stage("Resolve and checkout component B") {
      def deps = readProperties file: 'meta.properties'
      echo "Resolved B version = ${deps['b']}"
      
      dir("module/b") {
           //Clone/Fetch Component B 
           checkout scm:[
                $class: 'GitSCM', 
                userRemoteConfigs: [[url: repositoryB, credentialsId: credentialsId]], 
                branches: [[name: 'refs/tags/*']]
           ], 
           changelog: false, poll: false

           //Checkout the tag, matching deps['b'] pattern     
           sshagent([credentialsId]) {
                sh "git checkout \$(git tag -l \"${deps['b']}\" |sort -r --version-sort|head -1)"
           }
      }
   }
   
   stage("Build A+B") {
        //Apply a common build step
   }

}

Here we use the "readProperties" command, which is part of the Pipeline Utility Steps Plugin to extract the "Component B" version pattern from meta.properties. There are also readYaml, readJSON commands available.

Next we fetch/clone the "Component B", with the changelog: false, poll: false flags, to prevent it from being registered for the SCM poll, into the "module/b" folder of the current workspace.

Then invoke a shell command to select the tag, based on the version pattern, we have obtained above, and checkout it (5.3.* style wildcards should also work).

The sh invocation, is wrapped in the sshagent, to make it reuse the appropriate credentials from the Jenkins credential store.

Answer 2

Using the Credentials Binding Plugin worked very well for me (also mentioned by @zeppelin)

Steps:

In the Global credentials section:

1. Add Credentials of the type: "Username with password". This should be the username and password for component B repository git server using HTTPS protocol (the SSH option is not good for this purpose)

In your Jenkins job configuration:

2. Put component A in the regular Source Code Management under the Git section all required fields (Repositories, Branches, etc.).
   • It will clearer and cleaner to put the repository in a sub-directory: under Additional Behaviours choose Check out to a sub-directory and write: component_a
3. Make sure also to check in Build Triggers the Build when a change is pushed to GitHub

4. In the Build Environment section tick the Use secret text(s) or file(s)

   • put in Variable some name: MY_CRED
   • in Credentials choose the Specific credentials you created in step 1.

   

5. Now using the MY_CRED in the Execute shell code you will have access to the component B repository:

   DIR="component_b"
   if [ "$(ls -A $DIR/.git)" ]; then
       cd $DIR
       git fetch
   else
       git clone https://[email protected]/proj/component_b.git $DIR 
       cd $DIR
   fi
   git show
   

   

   • Note: you will NOT see the user and password in the logs, so it should be safe. you would see: git clone 'https://****@github.com/proj/component_b.git' component_b

6. Do all your parsing of config from component A to get the desired tag: TAG=$(cat ./component_a/config.cfg | grep ... | sed ...)

7. Checkout the desired Tag: cd component_b; git checkout -f $TAG
   • Note: the -f force tag.
8. Now run the code and test as desired...