dotnet/runtime · error · RuntimeError

Couldn't determine baseline git hash

Error message

Couldn't determine baseline git hash

What it means

Thrown by process_git_hash_arg() when 'git merge-base current_hash main_hash' returns a non-zero exit code. The merge-base identifies the common ancestor between the current commit and the newest remote main commit, which serves as the starting point for searching JIT changes in the rolling build store.

Source

Thrown at src/coreclr/scripts/jitrollingbuild.py:206

        # First find the newest hash for any branch matching */main.
        command = [ "git", "branch", "-r", "--sort=-committerdate", "-v", "--list", "*/main" ]
        logging.debug("Invoking: %s", " ".join(command))
        proc = subprocess.Popen(command, stdout=subprocess.PIPE)
        stdout_git_main_branch, _ = proc.communicate()
        return_code = proc.returncode
        if return_code != 0:
            raise RuntimeError("Couldn't determine newest 'main' git hash")

        main_hash = stdout_git_main_branch.decode('utf-8').strip().split()[1]

        # Get the merge-base between the newest main and our current rev
        command = [ "git", "merge-base", current_hash, main_hash ]
        logging.debug("Invoking: %s", " ".join(command))
        proc = subprocess.Popen(command, stdout=subprocess.PIPE)
        stdout_git_merge_base, _ = proc.communicate()
        return_code = proc.returncode
        if return_code != 0:
            raise RuntimeError("Couldn't determine baseline git hash")

        baseline_hash = stdout_git_merge_base.decode('utf-8').strip()
        logging.info("Baseline hash: %s", baseline_hash)

        # Enumerate the last 20 changes, starting with the baseline, that included JIT and JIT-EE GUID changes.
        command = [ "git", "log", "--pretty=format:%H", baseline_hash, "-20", "--", "src/coreclr/jit/*", "src/coreclr/inc/jiteeversionguid.h" ]
        logging.debug("Invoking: {}".format(" ".join(command)))
        proc = subprocess.Popen(command, stdout=subprocess.PIPE)
        stdout_change_list, _ = proc.communicate()
        return_code = proc.returncode
        change_list_hashes = []
        if return_code == 0:
            change_list_hashes = stdout_change_list.decode('utf-8').strip().splitlines()
        else:
            raise RuntimeError("Couldn't determine list of JIT changes starting with baseline hash")

        if len(change_list_hashes) == 0:
            raise RuntimeError("No JIT changes found starting with baseline hash")

View on GitHub (pinned to 290d5ab72c)

Solutions

  1. Run 'git merge-base HEAD origin/main' manually to see the exact error.
  2. If using a shallow clone, deepen it with 'git fetch --unshallow' to ensure the common ancestor is present.
  3. Pass -git_hash explicitly to bypass auto-detection and avoid the merge-base computation.
  4. Verify that main_hash (from the previous git branch command) was correctly identified as a valid commit.

Example fix

# before: shallow clone missing merge-base ancestor
python jitrollingbuild.py download

# after: deepen the clone
git fetch --unshallow
python jitrollingbuild.py download
Defensive patterns

Strategy: validation

Validate before calling

# Verify merge-base is computable before running auto-detection
import subprocess
result = subprocess.run(['git', 'merge-base', 'HEAD', 'origin/main'], capture_output=True, cwd=repo_path)
if result.returncode != 0:
    print('Cannot compute merge-base. Try deepening a shallow clone or pass -git_hash.')

Type guard

def can_compute_merge_base(repo_path: str, ref_a: str, ref_b: str) -> bool:
    result = subprocess.run(['git', 'merge-base', ref_a, ref_b], capture_output=True, cwd=repo_path)
    return result.returncode == 0

Try / catch

try:
    process_git_hash_arg(coreclr_args)
except RuntimeError as e:
    if 'baseline git hash' in str(e):
        logging.error('Merge-base failed. If using shallow clone, run: git fetch --unshallow')
        sys.exit(1)
    raise

Prevention

When it happens

Trigger: Called after successfully determining current_hash and main_hash. The script runs 'git merge-base <current_hash> <main_hash>'. If git returns non-zero, the RuntimeError is raised at line 206.

Common situations: The current_hash and main_hash have no common ancestor (e.g., the repository was rebased onto an unrelated history, or the commits reference different object stores in a shallow/partial clone). A shallow clone truncated the common ancestor commit. The main_hash was parsed incorrectly from the branch listing output.

Related errors

AI-assisted analysis of dotnet/runtime@290d5ab72c (2026-08-06). Data as JSON: /api/errors/39e59661d996f72d. Report an issue: GitHub.