facebook/flow · critical

Failed to query mergebase: {}

Error message

Failed to query mergebase: {}

What it means

The same Watchman mergebase check, but the VCS was found and the query itself failed: ErrorStatus::Errored(msg) carries the VCS command's failure, and the panic reproduces that message verbatim. Typical payloads are an unknown revision/ref, a corrupt repository, or an auth/hook failure in the daemon's non-interactive environment.

Source

Thrown at rust_port/crates/flow_watchman/src/lib.rs:1298

    }
}

async fn check_for_changed_mergebase(env: &mut Env) -> Option<bool> {
    match env.mergebase.clone() {
        None => {
            flow_hh_logger::debug!(
                "Unable to check for changed mergebase: unknown previous mergebase"
            );
            None
        }
        Some(old_mergebase) => {
            let new_mergebase = match get_mergebase(env).await {
                Ok(mergebase) => mergebase,
                Err(vcs_utils::ErrorStatus::NotInstalled { .. }) => {
                    panic!("Failed to query mergebase: unable to find vcs");
                }
                Err(vcs_utils::ErrorStatus::Errored(msg)) => {
                    panic!("Failed to query mergebase: {}", msg);
                }
            };
            match new_mergebase {
                Some(mergebase) => {
                    let changed_mergebase = mergebase != old_mergebase;
                    if changed_mergebase {
                        flow_hh_logger::info!(
                            "Watchman reports mergebase changed from {:?} to {:?}",
                            old_mergebase,
                            mergebase
                        );
                        env.mergebase = Some(mergebase);
                    } else {
                        flow_hh_logger::debug!(
                            "Watchman reports mergebase is unchanged at {:?}",
                            mergebase
                        );
                    }

View on GitHub (pinned to f88ac94bcf)

Solutions

1. Reproduce manually: git -C <root> merge-base HEAD <mergebase_with> — its stderr is the text inside the panic; fix what it names.
2. Fix the ref: git fetch the remote, point mergebase_with at an existing branch, or create the missing ref.
3. If the repo is corrupt, run git fsck and re-clone if needed.
4. Restart the watcher/server after the fix; the panic already killed the watch loop.

Example fix

# before: mergebase_with points at a deleted branch
# panic: Failed to query mergebase: unknown revision 'refs/heads/old-base'

# after: fetch and point at a ref that exists
git fetch origin
git -C . merge-base HEAD origin/main   # verify, then set mergebase_with=origin/main

# before enabling mergebase tracking, prove the ref resolves
VCS="git"   # or sapling/hg to match your setup
$VCS -C "$WATCH_ROOT" merge-base HEAD "$MERGEBASE_WITH" >/dev/null 2>&1 || {
  echo "mergebase_with ref '$MERGEBASE_WITH' does not resolve at $WATCH_ROOT" >&2
  exit 1
}

# 0 only when the configured base ref resolves locally
mergebase_resolves() {
  git -C "$1" merge-base HEAD "$2" >/dev/null 2>&1
}

When it happens

Trigger: The configured mergebase_with ref does not resolve in the local repo (deleted branch, typo, shallow/trimmed CI checkout, never-fetched upstream ref), the repository is corrupt (interrupted repack/gc), or a VCS hook/prompt fails when the daemon runs `merge-base` — the panic message contains the underlying error.

Common situations: See trigger scenarios.

Related errors

• Failed to query mergebase: unable to find vcs
• $FLOW_STACK_SIZE must be a number, got {s}
• workers required for init
• workers required for recheck
• max_workers should be positive

AI-assisted analysis of facebook/flow@f88ac94bcf (2026-08-20). Data as JSON: /api/errors/1681843b78adc0e9. Report an issue: GitHub.