Summary

Unable to view the Diff and Commits tab in a Pull request because merge between a branch in forked repository and branch in Main repository gives error Couldn't find remote ref <ref>

Environment

Bitbucket version: 7.7 to 7.11

Diagnosis

atlassian-bitbucket.log shows these errors related to branch in question:

DEBUG [http-nio-7990-exec-7] <username> @1Q1D2K7x626x1237485x0 jdsoxe 10.0.16.128,127.0.0.1 "GET /rest/api/latest/projects/<PROJECT-KEY>/repos/<repo-slug>/pull-requests/<ID>/changes HTTP/1.1" c.a.s.i.r.e.ServiceExceptionMapper Mapping ServiceException to REST response 500
com.atlassian.bitbucket.scm.CommandFailedException: '/usr/local/bin/git fetch /var/atlassian/application-data/stash/shared/data/repositories/1820 <branch-ref>:' exited with code 128 saying: fatal: Couldn't find remote ref <branch-ref>
fatal: The remote end hung up unexpectedly
at com.atlassian.bitbucket.scm.DefaultCommandExitHandler.onError(DefaultCommandExitHandler.java:47)
at java.lang.Thread.run(Thread.java:748)
... 287 frames trimmed
Caused by: com.atlassian.utils.process.ProcessException: Non-zero exit code: 128
at com.atlassian.bitbucket.internal.process.nu.NioNuProcessHandler.callExitHandler(NioNuProcessHandler.java:277)
... 78 common frames omitted

If you run the Git command on the Git terminal, you'll get the same error:

git fetch /var/atlassian/application-data/stash/shared/data/repositories/1820 <branch-ref>
fatal: Couldn't find remote ref <branch-ref>
fatal: The remote end hung up unexpectedly

Running the command locally where Bitbucket is running also gives the same error:

/usr/local/bin/git fetch /var/atlassian/application-data/stash/shared/data/repositories/1820 refs/heads/bug/ARGUS-23284
fatal: Couldn't find remote ref refs/heads/bug/ARGUS-23284
fatal: The remote end hung up unexpectedly

Cause

This can happen when are using a fork-merge like workflow for your development project, so all your developers always have an up-to-date master in their own fork and they create their own branch and do pull requests to the upstream master. So basically the developers fork the main repository, do development on their own local branch in their own fork and then do pull requests to their own master in the main project.

These errors occur generally when you try to merge changes from a forked repository to the main repository and check the box for "Deleting the source branch", while merging. The missing ref would be the branch you're merging from.

Following are the steps to follow where you can see these errors:

1. First create a PR between branch bug/ARGUS-23284 from the fork repo, to the master branch in the main repo:
   
2. Merge the PR, the option for Delete source after merging is checked
   
3. Try to access the Diff tab of the PR, and you see the below error:
   

Solution

This is caused by this bug: 

• BSERV-12896-Merging PR from fork to main repo gives 500 error

A potential workaround is to upgrade Bitbucket to the 7.12 or above.