t8y2/dbx · error

a cancellation token is always available

Error message

a cancellation token is always available

What it means

In the single-driver install path, the code builds active_cancellation from either an owned token (created from operation_id via begin_install_cancellation) or a caller-supplied cancellation token. The expect("a cancellation token is always available") enforces the invariant that at least one of operation_id/cancellation is present; if both are None the program panics instead of proceeding.

Source

Thrown at crates/dbx-core/src/agent_service.rs:1137

        }
        None => &[],
    };
    let _driver_guard = lock_or_cancel(&driver_lock, command_tokens).await?;
    // Use the command-scoped token when one was registered before any awaitable
    // setup (blocker check, lock wait, registry fetch) so a cancel fired during
    // that window is observed here instead of being lost. Otherwise register a
    // token owned by this call keyed by a fresh operation id so two concurrent
    // installs of the same driver cannot replace each other's token.
    let owned_operation_id: Option<String> =
        if cancellation.is_some() { None } else { Some(uuid::Uuid::new_v4().to_string()) };
    let owned_cancellation: Option<Arc<AgentInstallCancellation>> = match owned_operation_id.as_deref() {
        Some(operation_id) => Some(am.begin_install_cancellation(&install_cancellation_key(operation_id)).await),
        None => None,
    };
    let active_cancellation: &AgentInstallCancellation = owned_cancellation
        .as_deref()
        .or_else(|| cancellation.map(|token| token.as_ref()))
        .expect("a cancellation token is always available");
    if active_cancellation.is_cancelled() {
        if let (Some(operation_id), Some(token)) = (owned_operation_id.as_deref(), owned_cancellation.as_ref()) {
            am.finish_install_cancellation(&install_cancellation_key(operation_id), token).await;
        }
        return Err(AGENT_DOWNLOAD_CANCELED_ERROR.to_string());
    }

    let result = install_agent_driver_with_batch_unlocked(
        am,
        db_type,
        source,
        progress,
        current,
        total_drivers,
        &[active_cancellation],
    )
    .await;

View on GitHub (pinned to c0390bff16)

Solutions

1. Guarantee every caller supplies an operation id or a cancellation token before entering the install path.
2. Create a token when missing: call begin_install_cancellation with a fresh operation id instead of expecting.
3. Convert the expect into an explicit error return if the path can legitimately run without cancellation.
4. Add an integration test covering the None/None input combination.

Example fix

// before
let active_cancellation = owned_cancellation.as_deref()
    .or_else(|| cancellation.map(|t| t.as_ref()))
    .expect("a cancellation token is always available");
// after
let Some(active_cancellation) = owned_cancellation.as_deref()
    .or_else(|| cancellation.map(|t| t.as_ref())) else {
    return Err("no cancellation token provided".to_string());
};

// validate inputs before the single-driver install path
assert!(operation_id.is_some() || cancellation.is_some(),
    "install requires an operation id or a cancellation token");

fn has_install_cancellation(op_id: Option<&OperationId>, token: Option<&AgentInstallCancellation>) -> bool {
    op_id.is_some() || token.is_some()
}

let active = owned_cancellation.as_deref()
    .or_else(|| cancellation.map(|t| t.as_ref()))
    .ok_or_else(|| "no cancellation token provided for install".to_string())?;

When it happens

Trigger: Invoking the install path with neither an operation id nor a borrowed cancellation token — e.g. a new caller passes None for both, or refactoring changed the fallback order so cancellation is consumed before the expect.

Common situations: Call sites added after the invariant was introduced that forget to thread a cancellation token; internal callers that previously always created an operation id start passing None; test harnesses constructing the context manually with all-None fields.

Related errors

• a batch cancellation token is always available
• driver token registered
• checked one driver
• checked one native platform
• root count checked above

AI-assisted analysis of t8y2/dbx@c0390bff16 (2026-09-05). Data as JSON: /api/errors/094e01870a21af19. Report an issue: GitHub.