[resolver] Select a non-main branch option (#5075)

Co-authored-by: Graham Neubig <neubig@gmail.com>

.github/workflows/openhands-resolver.yml

@@ -11,6 +11,11 @@ on:
         required: false
         type: string
         default: "@openhands-agent"
+      target_branch:
+        required: false
+        type: string
+        default: "main"
+        description: 'Target branch to pull and create PR against'
     secrets:
       LLM_MODEL:
         required: true
@@ -135,6 +140,9 @@ jobs:
           echo "MAX_ITERATIONS=${{ inputs.max_iterations || 50 }}" >> $GITHUB_ENV
           echo "SANDBOX_ENV_GITHUB_TOKEN=${{ secrets.GITHUB_TOKEN }}" >> $GITHUB_ENV
 
+          # Set branch variables
+          echo "TARGET_BRANCH=${{ inputs.target_branch }}" >> $GITHUB_ENV
+
       - name: Comment on issue with start message
         uses: actions/github-script@v7
         with:
@@ -175,7 +183,8 @@ jobs:
             --issue-number ${{ env.ISSUE_NUMBER }} \
             --issue-type ${{ env.ISSUE_TYPE }} \
             --max-iterations ${{ env.MAX_ITERATIONS }} \
-            --comment-id ${{ env.COMMENT_ID }}
+            --comment-id ${{ env.COMMENT_ID }} \
+            --target-branch ${{ env.TARGET_BRANCH }}
 
       - name: Check resolution result
         id: check_result

openhands/resolver/resolve_issue.py

@@ -315,6 +315,7 @@ async def resolve_issue(
     repo_instruction: str | None,
     issue_number: int,
     comment_id: int | None,
+    target_branch: str | None = None,
     reset_logger: bool = False,
 ) -> None:
     """Resolve a single github issue.
@@ -333,6 +334,7 @@ async def resolve_issue(
         repo_instruction: Repository instruction to use.
         issue_number: Issue number to resolve.
         comment_id: Optional ID of a specific comment to focus on.
+        target_branch: Optional target branch to create PR against (for PRs).
         reset_logger: Whether to reset the logger for multiprocessing.
     """
     issue_handler = issue_handler_factory(issue_type, owner, repo, token)
@@ -422,15 +424,32 @@ async def resolve_issue(
     try:
         # checkout to pr branch if needed
         if issue_type == 'pr':
+            branch_to_use = target_branch if target_branch else issue.head_branch
             logger.info(
-                f'Checking out to PR branch {issue.head_branch} for issue {issue.number}'
+                f'Checking out to PR branch {target_branch} for issue {issue.number}'
             )
 
+            if not branch_to_use:
+                raise ValueError('Branch name cannot be None')
+
+            # Fetch the branch first to ensure it exists locally
+            fetch_cmd = ['git', 'fetch', 'origin', branch_to_use]
             subprocess.check_output(
-                ['git', 'checkout', f'{issue.head_branch}'],
+                fetch_cmd,
                 cwd=repo_dir,
             )
 
+            # Checkout the branch
+            checkout_cmd = ['git', 'checkout', branch_to_use]
+            subprocess.check_output(
+                checkout_cmd,
+                cwd=repo_dir,
+            )
+
+            # Update issue's base_branch if using custom target branch
+            if target_branch:
+                issue.base_branch = target_branch
+
             base_commit = (
                 subprocess.check_output(['git', 'rev-parse', 'HEAD'], cwd=repo_dir)
                 .decode('utf-8')
@@ -553,6 +572,12 @@ def main():
         choices=['issue', 'pr'],
         help='Type of issue to resolve, either open issue or pr comments.',
     )
+    parser.add_argument(
+        '--target-branch',
+        type=str,
+        default=None,
+        help="Target branch to pull and create PR against (for PRs). If not specified, uses the PR's base branch.",
+    )
 
     my_args = parser.parse_args()
 
@@ -613,6 +638,7 @@ def main():
             repo_instruction=repo_instruction,
             issue_number=my_args.issue_number,
             comment_id=my_args.comment_id,
+            target_branch=my_args.target_branch,
         )
     )