The use case:

• Imagine I want to have an ephemeral Jenkins (Docker/K8S/etc).
• I want it to be preconfigured with multibranch pipelines and/or Github/Bitbucket organization folders.
• I want it to come online and when it does, it will scan for unknown repositories and branches and cause a build storm.

• The Problem as I see it: 
  1. There are many plugins out there who try to deal with this, but none of them work fully.
     • Build Strategy - https://github.com/jenkinsci/basic-branch-build-strategies-plugin/blob/master/src/main/java/jenkins/branch/buildstrategies/basic/SkipInitialBuildOnFirstBranchIndexing.java
       • Doesnt fully work because the next time indexing runs - it builds.  So it delays the build storm.
  2. There are active PRs/Comments working to try to solve it different ways.
     • https://issues.jenkins.io/browse/JENKINS-38552?focusedCommentId=289692&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-289692
     • https://github.com/jenkinsci/branch-api-plugin/pull/244
       • Solves some of it - but still assumes every webhook is delivered
       • Their discussion - https://github.com/jenkinsci/branch-api-plugin/pull/186 

I think the requirements are relatively simple and so is the implementation:

• Here is where all branches come in the front door
• It will always hit `observeNew` first.
  • We can use a BuildStrategy to not run any build that has a commit older than some EPOCH time.
• The next time a webhook/index comes in it hits `observeExisting`
  • Most times I notice it hits
    • if (changesDetected(revision, project, scmLastBuiltRevision)) {
  • changesDetected only evaluates last built revision. Problem 1
  • And it always hits lastSeenRevisionOrDefault which saves last seen revision
  • _factory.setLastSeenRevisionHash(project, scmLastBuiltRevision);
• So we should be able to do this in a 2 step process.
  1. Add build strategy that refuses to build if commit ID is not after a starting time (easily programmed on jenkins master startup)
  2. Have the branch-api-plugin have an option to save lastseenrevision as lastbuilt revision.

#1 - Is a little harder since we are not handed any kind of probe so we have to hit Github right now (API) (More unnecessary API calls).  But it does stop the build.  In #2 I added SCMEvent as an extension of BuildBranchStrategy that allows us to see time of commit to help make less API calls to github.

#2 - A literal property/config item - copy/boolean for when lastSeenRevisionOrDefault is called - also call  setRevisionHash on the same factory.  So if we have seen it (assume built).  (Is there anything wrong with this design?). Feature Flag?  Another option is to impact how `changesDetected` works or make it extendable.

• #1 - 
• #2 - https://github.com/jenkinsci/branch-api-plugin/pull/291

I will start on #1