hyperhive/swarm-secret-client/src/queue.rs

//! The queue agreements: where a queue credential lives in the store, and what
//! the object at that path holds.
//!
//! **Two kinds live here, and which is which is the thing to get right.**
//!
//! - [`agent_client_path`] / [`Credential`] are keyed per **hive**: one OIDC
//!   client minted per hive at deploy time, shared by every agent container on
//!   it. It says which hive a caller belongs to, never which agent.
//! - [`agent_queue_path`] / [`AgentCredential`] are keyed per **agent**: a
//!   secret the swarm mints for one agent, at swarm level, with no hive in the
//!   chain. It is what makes one agent distinguishable from its co-hived
//!   neighbours.
//!
//! They are separate objects rather than one with a nullable field because they
//! are minted by different principals on different events — the hive-scoped one
//! by authelia at deploy time, the per-agent one by `swarm-controller` when an
//! agent is created — and an object whose shape depends on who wrote it is a
//! reader that has to guess.
//!
//! The per-agent secret is deliberately **not** derived from the agent's mTLS
//! identity (the leaf at [`crate::mtls::identity_path`]). That leaf is for
//! reaching the store and nothing else; deriving a queue identity from it would
//! couple the two credentials' lifetimes, so that renewing one would mean
//! renewing the other.
//!
//! The token an agent presents that secret in at the queue is spelled by
//! `swarm_queue_client::agent_token`, which needs no store client.

use serde::{Deserialize, Serialize};

use crate::{
    Error,
    path::{Kind, principal_prefix},
};

/// The path holding the client secret that agent containers on `hive` present
/// to the swarm queue.
///
/// # Errors
/// [`Error::PathSegment`] when `hive` contains anything but `[A-Za-z0-9_-]`,
/// which is what keeps one hive's name from addressing another hive's secret.
pub fn agent_client_path(hive: &str) -> Result<String, Error> {
    let prefix = principal_prefix(Kind::Hive, hive)?;
    Ok(format!("{prefix}/queue/agent"))
}

/// The path holding the secret **one agent** presents to the swarm queue.
///
/// Under the agent's own principal prefix, and that is the whole reason for
/// this spelling rather than a new top-level one: every agent's ACL document
/// already grants read on `swarm/agents/<agent>/*`
/// ([`crate::policy::render_agent`]), so a credential here needs no new grant,
/// no policy re-render, and no rewrite of any existing agent's document.
///
/// # Errors
/// [`Error::PathSegment`] when `agent` contains anything but `[A-Za-z0-9_-]`,
/// which is what keeps one agent's name from addressing another agent's secret.
pub fn agent_queue_path(agent: &str) -> Result<String, Error> {
    let prefix = principal_prefix(Kind::Agent, agent)?;
    Ok(format!("{prefix}/queue"))
}

/// What [`agent_queue_path`] holds: the secret, and the agent it proves.
///
/// No hive: an agent's identity is not tied to one, and the subjects the
/// verifier grants are keyed on the agent alone. Objects written with a `hive`
/// field still decode, because unknown fields are ignored.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct AgentCredential {
    /// The secret itself. Named to match [`Credential::value`] and
    /// [`crate::matrix::Credential::value`] so a nix-side reader spells
    /// `bao kv get -field=value` for every kind.
    pub value: String,

    /// The agent this secret authenticates.
    pub agent: String,
}

/// What the path holds: the client secret, plus the client id it belongs to.
///
/// The id rides with the secret for the same reason the homeserver rides with
/// a matrix token — a credential has to be reconstructable from the store
/// alone. Deriving it on the reading side instead would mean spelling
/// `hive-<name>-agent` in a second place, and the authelia module's own option
/// says what a split spelling costs: every agent is denied as a timeout.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct Credential {
    /// The secret itself. Named to match [`crate::matrix::Credential::value`]
    /// so a nix-side reader spells `bao kv get -field=value` for either kind.
    pub value: String,

    /// The OIDC client id the secret authenticates.
    ///
    /// Required, unlike [`crate::matrix::Credential::homeserver`]: that field
    /// is optional because it was added to objects already in the store, and
    /// this path is new. An object without an id is not a usable credential,
    /// so failing to decode is the wanted behaviour — the alternative is a
    /// reader holding `None` whose only recourse is to spell the id itself.
    pub client_id: String,
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn a_hive_name_lands_under_its_own_principal_prefix() {
        assert_eq!(
            agent_client_path("alpha").expect("a plain name is legal"),
            "swarm/hives/alpha/queue/agent"
        );
    }

    #[test]
    fn a_traversal_in_the_hive_name_is_refused() {
        let e = agent_client_path("../beta").expect_err("a traversal is not");
        assert!(matches!(e, Error::PathSegment { kind: "hive", .. }), "{e}");
    }

    #[test]
    fn two_hives_never_share_a_path() {
        assert_ne!(
            agent_client_path("alpha").expect("legal"),
            agent_client_path("beta").expect("legal")
        );
    }

    #[test]
    fn the_object_round_trips_through_the_store_representation() {
        let c = Credential {
            value: "s3cr3t".to_owned(),
            client_id: "hive-alpha-agent".to_owned(),
        };
        let json = serde_json::to_string(&c).expect("serialises");
        assert_eq!(
            serde_json::from_str::<Credential>(&json).expect("deserialises"),
            c
        );
    }

    #[test]
    fn the_field_names_the_nix_reader_asks_for_are_the_ones_written() {
        let json = serde_json::to_value(Credential {
            value: "s3cr3t".to_owned(),
            client_id: "hive-alpha-agent".to_owned(),
        })
        .expect("serialises");
        assert_eq!(json["value"], "s3cr3t");
        assert_eq!(json["client_id"], "hive-alpha-agent");
    }

    #[test]
    fn an_agent_name_lands_under_its_own_principal_prefix() {
        assert_eq!(
            agent_queue_path("atlas").expect("a plain name is legal"),
            "swarm/agents/atlas/queue"
        );
    }

    #[test]
    fn a_traversal_in_the_agent_name_is_refused() {
        let e = agent_queue_path("../beta").expect_err("a traversal is not");
        assert!(matches!(e, Error::PathSegment { kind: "agent", .. }), "{e}");
    }

    #[test]
    fn two_agents_never_share_a_path() {
        assert_ne!(
            agent_queue_path("atlas").expect("legal"),
            agent_queue_path("argus").expect("legal")
        );
    }

    /// The reason this path was chosen over a new top-level prefix: it is
    /// already inside the stanza every agent's own ACL document grants, so no
    /// policy has to change for the credential to be readable by the one agent
    /// it belongs to.
    #[test]
    fn the_agents_existing_policy_already_covers_its_queue_path() {
        let document = crate::policy::render_agent("atlas").expect("legal");
        let path = agent_queue_path("atlas").expect("legal");
        let stanza = path
            .rsplit_once('/')
            .map(|(prefix, _)| format!("secret/data/{prefix}/*"))
            .expect("the path has a parent");
        assert!(document.contains(&stanza), "{document}");
    }

    #[test]
    fn the_agent_object_round_trips_through_the_store_representation() {
        let c = AgentCredential {
            value: "s3cr3t".to_owned(),
            agent: "atlas".to_owned(),
        };
        let json = serde_json::to_string(&c).expect("serialises");
        assert_eq!(
            serde_json::from_str::<AgentCredential>(&json).expect("deserialises"),
            c
        );
    }

    #[test]
    fn the_agent_objects_field_names_are_the_ones_a_nix_reader_asks_for() {
        let json = serde_json::to_value(AgentCredential {
            value: "s3cr3t".to_owned(),
            agent: "atlas".to_owned(),
        })
        .expect("serialises");
        assert_eq!(json["value"], "s3cr3t");
        assert_eq!(json["agent"], "atlas");
        assert!(json.get("hive").is_none(), "{json}");
    }

    /// A stored object may carry a `hive` field. It is ignored, and the object
    /// decodes.
    #[test]
    fn a_stored_agent_object_that_still_names_a_hive_decodes() {
        let c: AgentCredential =
            serde_json::from_str(r#"{"value":"s3cr3t","agent":"atlas","hive":"alpha"}"#)
                .expect("a stored object carrying `hive` decodes");
        assert_eq!(
            c,
            AgentCredential {
                value: "s3cr3t".to_owned(),
                agent: "atlas".to_owned(),
            }
        );
    }

    /// The agent is required: it is what the verifier checks the presented
    /// name against.
    #[test]
    fn an_agent_object_missing_its_agent_does_not_decode() {
        assert!(serde_json::from_str::<AgentCredential>(r#"{"value":"s"}"#).is_err());
        assert!(
            serde_json::from_str::<AgentCredential>(r#"{"value":"s","hive":"alpha"}"#).is_err()
        );
    }

    /// decodes as the other — the property that keeps a reader from picking up
    /// a hive-shared credential where a per-agent one was meant.
    #[test]
    fn the_hive_credential_and_the_agent_credential_are_not_interchangeable() {
        let hive_json = serde_json::to_string(&Credential {
            value: "s".to_owned(),
            client_id: "hive-alpha-agent".to_owned(),
        })
        .expect("serialises");
        assert!(serde_json::from_str::<AgentCredential>(&hive_json).is_err());

        let agent_json = serde_json::to_string(&AgentCredential {
            value: "s".to_owned(),
            agent: "atlas".to_owned(),
        })
        .expect("serialises");
        assert!(serde_json::from_str::<Credential>(&agent_json).is_err());

        assert_ne!(
            agent_queue_path("atlas").expect("legal"),
            agent_client_path("atlas").expect("legal"),
        );
    }
}