Purpose

When creating a Pull Request, the Diff view in Bitbucket Server compares the changes between branches with the ultimate goal of providing an overview of which changes will be merged when a merge is performed.

In other words, the git diff starts at the common ancestor.

This also applies to the following use cases:

• Comparing branches
• Comparing tags

This is achieved by running the Git merge-base command.

Solution

Let's check the behavior with a step-by-step example.

In this example there are three branches involved:

• target_branch
• source_branch_A
• source_branch_B

Step #1 - Create the branches

These three branches are created in the following order:

• target_branch
• source_branch_A
• source_branch_B

The following commands have been used for the creation of these branches:

# Define the source branch for the checkout
git checkout master
 
# target_branch: a branch with master as source
git checkout -b target_branch
git push origin target_branch
 
# source_branch_A: a branch with target_branch as source
git checkout -b source_branch_A
git push origin source_branch_A

# source_branch_B: a branch with target_branch as source
git checkout target_branch # To reset the source branch to target_branch
git checkout -b source_branch_B
git push origin source_branch_B

The git log command returns the following

git log --oneline --abbrev-commit --all --graph --decorate
* bfa489b (HEAD -> source_branch_B, origin/master, origin/HEAD, target_branch, source_branch_A, master) final

The branches will be presented as follows in the Bitbucket Server Commits page:

Step #2 - Change the target_branch

Apply a change to the target branch, commit it and push the change to the remote repository.

git checkout target_branch
# change the file
git add file1.txt
git commit -m "change on target_branch"
git push origin target_branch

When checking the Diffs tab in a pull request, no changes are shown between the two branches.

The screenshot shows source_branch_B but source_branch_A will present the same behaviour.

This is the same, empty, result of the following Git command:

git diff $(git merge-base source_branch_B target_branch) source_branch_B

Note that when comparing the two branches with 2 . instead of one, the change applied to the target_branch is shown:

git diff source_branch_B..target_branchdiff --git a/file1.txt b/file1.txt
index 6168ae0..f99b4b2 100644
--- a/file1.txt
+++ b/file1.txt
@@ -1,3 +1,6 @@
+A change on the target_branch

Step #3 - Change the source_branch_A

Apply a change to the source_branch_A branch, commit it, and push the change to the remote repository.

git checkout source_branch_A
# change the file
git add file1.txt
git commit -m "change on source_branch_A"
git push origin source_branch_A

When checking the Diffs tab in a pull request, the changes applied to the source_branch_A are shown.

This is the same result of the following Git command:

git diff $(git merge-base source_branch_A target_branch) source_branch_A
 
diff --git a/file1.txt b/file1.txt
index 6168ae0..43bb5b6 100644
--- a/file1.txt
+++ b/file1.txt
@@ -25,3 +25,5 @@ master2
+
+A change on source_branch_A

2019-03-18 14:14:03,250 DEBUG [http-nio-25160-exec-2] bitadmin @7FMTJ5x854x1106x0 15ddvyz 0:0:0:0:0:0:0:1 "GET /rest/api/latest/projects/~BITADMIN/repos/mine/pull-requests/1/changes HTTP/1.1" c.a.s.i.s.g.p.DefaultPullRequestRefHelper ~BITADMIN/mine[2]:1@5: Resolving effective diff (7e955114c9c616fc8cb0c9b676fa08052c3b03bb -> d805b33ef5b64a874f082890225d4860e5279a5e)

cat /bitbucket-home/atlassian-bitbucket-5.16.0/shared/data/repositories/2/stash-refs/pull-requests/1/merge
7d86796957509036b0520a1d1881e4594235ee08

git show 7d86796957509036b0520a1d1881e4594235ee08
commit 7d86796957509036b0520a1d1881e4594235ee08
Merge: d805b33 7e95511
Author: bitadmin <bitadmin@bit.com>
Date:   Mon Mar 18 12:33:50 2019 +1100

    Automatic merge

/usr/local/bin/git diff-tree -C -r --format=%H <hash_1> <hash_2> --
/usr/local/bin/git diff -C --color=never -U10 --dst-prefix=dst:// --src-prefix=src:// <hash_1> <hash_2> – <files>

/usr/local/bin/git diff-tree -C -r --format=%H <hash_1> <hash_2> --
/usr/local/bin/git diff -C --color=never -U10 --dst-prefix=dst:// --src-prefix=src:// <merge commit hash> <hash_2> – <files>

FAQ

Q: Does this apply only to a branch comparison or a diff in a pull request?

A: No, this also applies to a tag comparison. The following example shows that no Diff are displayed, even if a new change has been pushed to the target branch (as in Step #2 above):

Q: If I change the order of the compare and I see a difference, is it wrong?

A: No, that's expected and it depends on the Git changes applied.