dotnet/runtime · error · RuntimeError

Couldn't determine baseline git hash

Error message

Couldn't determine baseline git hash

What it means

Raised by get_baseline_jit when `git merge-base <current_hash> <main_hash>` exits non-zero. The merge-base is needed to find a shared baseline commit between the current HEAD and origin/main.

Source

Thrown at src/coreclr/scripts/superpmi.py:4892

            # 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)
        else:
            baseline_hash = coreclr_args.base_git_hash

        if coreclr_args.base_git_hash is None:
            # 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: %s", " ".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")

View on GitHub (pinned to 290d5ab72c)

Solutions

  1. Deepen the clone: `git fetch --unshallow` or `git fetch --depth=<large>` then retry.
  2. Verify both hashes exist locally: `git cat-file -t <hash>`.
  3. Supply the baseline directly via `--base_git_hash <commit>` to skip merge-base computation.
  4. If on a divergent fork, rebase onto upstream main to establish shared history.

Example fix

// before
# shallow clone missing merge-base object -> raises [182]
// after
git fetch --unshallow origin && python superpmi.py ...
# or
python superpmi.py --base_git_hash <known-shared-commit> ...
Defensive patterns

Strategy: validation

Validate before calling

import subprocess
for h in (current_hash, main_hash):
    rc = subprocess.run(['git','cat-file','-t',h], cwd=runtime_repo_location).returncode
    if rc != 0:
        raise SystemExit(f'object {h} missing locally; deepen the clone')
rc = subprocess.run(['git','merge-base',current_hash,main_hash], cwd=runtime_repo_location).returncode
if rc != 0:
    raise SystemExit('no merge-base; disjoint history')

Type guard

def merge_base_exists(a: str, b: str, cwd: str) -> bool:
    import subprocess
    return subprocess.run(['git','merge-base',a,b], cwd=cwd,
                          stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL).returncode == 0

Try / catch

try:
    main(...)
except RuntimeError as e:
    if 'baseline git hash' in str(e):
        subprocess.run(['git','fetch','--unshallow','origin'], cwd=runtime_repo_location, check=False)
        main(...)

Prevention

When it happens

Trigger: current_hash and main_hash have no common ancestor (disjoint histories), one hash is unknown to the local object store (not fetched), or a hash is malformed.

Common situations: A shallow clone missing the merge-base object; an orphan/branch with no shared history; fetching was interrupted so the object for main_hash is absent; typo in an explicit --git_hash.

Related errors

AI-assisted analysis of dotnet/runtime@290d5ab72c (2026-08-06). Data as JSON: /api/errors/9b3c2c75e3149f8b. Report an issue: GitHub.