chore: merge target branch before commit changes (#3406)

In this PR:
- Merge base branch to current branch before updating generation config.

Example run:
https://github.com/googleapis/sdk-platform-java/actions/runs/12148752357/job/33877902009

Author: JoeWang1127

.github/scripts/update_googleapis_commit.sh

@@ -54,18 +54,23 @@ fi
 current_branch="generate-libraries-${base_branch}"
 title="chore: update googleapis commit at $(date)"
 
-# try to find a open pull request associated with the branch
+git checkout "${base_branch}"
+# Try to find a open pull request associated with the branch
 pr_num=$(gh pr list -s open -H "${current_branch}" -q . --json number | jq ".[] | .number")
-# create a branch if there's no open pull request associated with the
+# Create a branch if there's no open pull request associated with the
 # branch; otherwise checkout the pull request.
 if [ -z "${pr_num}" ]; then
   git checkout -b "${current_branch}"
 else
   gh pr checkout "${pr_num}"
 fi
 
+# Only allow fast-forward merging; exit with non-zero result if there's merging
+# conflict.
+git merge -m "chore: merge ${base_branch} into ${current_branch}" "${base_branch}"
+
 mkdir tmp-googleapis
-# use partial clone because only commit history is needed.
+# Use partial clone because only commit history is needed.
 git clone --filter=blob:none https://github.com/googleapis/googleapis.git tmp-googleapis
 pushd tmp-googleapis
 git pull
@@ -79,9 +84,22 @@ changed_files=$(git diff --cached --name-only)
 if [[ "${changed_files}" == "" ]]; then
     echo "The latest googleapis commit is not changed."
     echo "Skip committing to the pull request."
+else
+    git commit -m "${title}"
+fi
+
+# There are potentially at most two commits: merge commit and change commit.
+# We want to exit the script if no commit happens (otherwise this will be an
+# infinite loop).
+# `git cherry` is a way to find whether the local branch has commits that are
+# not in the remote branch.
+# If we find any such commit, push them to remote branch.
+unpushed_commit=$(git cherry -v "origin/${current_branch}" | wc -l)
+if [[ "${unpushed_commit}" -eq 0 ]]; then
+    echo "No unpushed commits, exit"
     exit 0
 fi
-git commit -m "${title}"
+
 if [ -z "${pr_num}" ]; then
   git remote add remote_repo https://cloud-java-bot:"${GH_TOKEN}@github.com/${repo}.git"
   git fetch -q --unshallow remote_repo

.github/workflows/update_googleapis_commit.yaml

@@ -28,6 +28,7 @@ jobs:
     steps:
     - uses: actions/checkout@v4
       with:
+        fetch-depth: 0
         token: ${{ secrets.CLOUD_JAVA_BOT_TOKEN }}
     - name: Update googleapis commit to latest
       shell: bash

hermetic_build/library_generation/owlbot/templates/java_library/.github/scripts/update_generation_config.sh

@@ -95,6 +95,7 @@ fi
 current_branch="generate-libraries-${base_branch}"
 title="chore: Update generation configuration at $(date)"
 
+git checkout "${base_branch}"
 # Try to find a open pull request associated with the branch
 pr_num=$(gh pr list -s open -H "${current_branch}" -q . --json number | jq ".[] | .number")
 # Create a branch if there's no open pull request associated with the
@@ -105,6 +106,10 @@ else
   gh pr checkout "${pr_num}"
 fi
 
+# Only allow fast-forward merging; exit with non-zero result if there's merging
+# conflict.
+git merge -m "chore: merge ${base_branch} into ${current_branch}" "${base_branch}"
+
 mkdir tmp-googleapis
 # Use partial clone because only commit history is needed.
 git clone --filter=blob:none https://github.com/googleapis/googleapis.git tmp-googleapis
@@ -133,9 +138,22 @@ changed_files=$(git diff --cached --name-only)
 if [[ "${changed_files}" == "" ]]; then
     echo "The latest generation config is not changed."
     echo "Skip committing to the pull request."
+else
+    git commit -m "${title}"
+fi
+
+# There are potentially at most two commits: merge commit and change commit.
+# We want to exit the script if no commit happens (otherwise this will be an
+# infinite loop).
+# `git cherry` is a way to find whether the local branch has commits that are
+# not in the remote branch.
+# If we find any such commit, push them to remote branch.
+unpushed_commit=$(git cherry -v "origin/${current_branch}" | wc -l)
+if [[ "${unpushed_commit}" -eq 0 ]]; then
+    echo "No unpushed commits, exit"
     exit 0
 fi
-git commit -m "${title}"
+
 if [ -z "${pr_num}" ]; then
   git remote add remote_repo https://cloud-java-bot:"${GH_TOKEN}@github.com/${repo}.git"
   git fetch -q --unshallow remote_repo

hermetic_build/library_generation/owlbot/templates/java_library/.github/workflows/update_generation_config.yaml

@@ -28,6 +28,7 @@ jobs:
     steps:
     - uses: actions/checkout@v4
       with:
+        fetch-depth: 0
         token: ${{ secrets.CLOUD_JAVA_BOT_TOKEN }}
     - name: Update params in generation config to latest
       shell: bash