After upgrading to Bitbucket Server 7.0.1, we get this kind of error for every pull request in multibranch projects in which Jenkins has been configured to build the result of merging the pull request:

 ERROR: Could not do lightweight checkout, falling back to heavyweight
 java.io.FileNotFoundException: URL: /rest/api/1.0/projects/REDACTED/repos/redacted/browse/Jenkinsfile?at=pull-requests%2F771%2Fmerge&start=0&limit=500
 	at com.cloudbees.jenkins.plugins.bitbucket.server.client.BitbucketServerAPIClient.getRequest(BitbucketServerAPIClient.java:831)
 	at com.cloudbees.jenkins.plugins.bitbucket.server.client.BitbucketServerAPIClient.getFileContent(BitbucketServerAPIClient.java:1123)
 	at com.cloudbees.jenkins.plugins.bitbucket.filesystem.BitbucketSCMFile.content(BitbucketSCMFile.java:98)
 	at jenkins.scm.api.SCMFile.contentAsString(SCMFile.java:335)
 	at org.jenkinsci.plugins.workflow.multibranch.SCMBinder.create(SCMBinder.java:107)
 	at org.jenkinsci.plugins.workflow.job.WorkflowRun.run(WorkflowRun.java:303)
 	at hudson.model.ResourceController.execute(ResourceController.java:97)
 	at hudson.model.Executor.run(Executor.java:427)

git ls-remote origin shows that Bitbucket Server now publishes "refs/pull-requests/771/from" but not "refs/pull-requests/771/merge", even though the pull request has no merge conflicts (the target branch even has no new commits). This might be related to "the switch from a 3-way diff to a 2-way diff in pull requests" in Bitbucket Server 7.0.0.

Jenkins then does the heavyweight checkout and successfully merges the pull request, so the project ultimately builds OK, but the heavyweight checkout consumes a lot of time and disk space.

There might not be any simple way to improve this in the bitbucket-branch-source plugin. If Bitbucket Server has deliberately ceased publishing "refs/pull-requests/*/merge", perhaps the plugin could do a three-way merge on Jenkinsfile only, rather than on the entire tree.

• One way might be to use the REST API to get Jenkinsfile from the target branch (/rest/api/1.0/projects/{projectKey}/repos/{repositorySlug}/browse/{path:.*}) and any Jenkinsfile changes of the pull request as a patch, and then apply the patch to Jenkinsfile at the Jenkins master. Unfortunately, it does not seem easy to get a patch that can be applied:
  • /rest/api/1.0/projects/{projectKey}/repos/{repositorySlug}/pull-requests/{pullRequestId}.patch gives the standard diff format but does not support limiting to a single file.
  • /rest/api/1.0/projects/{projectKey}/repos/{repositorySlug}/pull-requests/{pullRequestId}/diff/{path:.*} supports limiting to a single file, but the output is JSON and would thus be more difficult to apply.
• Alternatively, if Bitbucket Server has a REST API for querying the commit ID of a merge base of a pull request, then the plugin could first use that, then get Jenkinsfile from the three commits, and finally do the three-way merge. However, there does not seem to be such an API.