hyperhive/hive-c0re/src/workers/credential.rs

//! Delivering an agent's external-account credential from the swarm's secret
//! store into that agent's own state dir.
//!
//! The controller publishes a [`CredentialNotice`] naming an agent and an
//! account; this reads the value out of the store and writes it where the
//! agent's matrix daemon already watches for it. Nothing here activates
//! anything: `nix/agent-modules/matrix.nix` has a `systemd.paths` unit
//! globbing `/agents/*/state/matrix-token*` that re-fires the daemon when a
//! token appears, so arrival is the whole trigger.
//!
//! 🔑 The notice carries no secret — see [`swarm_queue_client::credential_subject`]
//! for why that is a requirement rather than a preference. The value is read
//! from the store under this hive's own identity.

use std::os::unix::fs::PermissionsExt as _;
use std::path::PathBuf;

use anyhow::{Context, Result};
use hive_types::Ident;
use swarm_queue_client::CredentialNotice;
use swarm_secret_client::{SecretStore, path};

use crate::paths::agent_state_dir;

/// The basename every matrix token must start with.
///
/// `nix/agent-modules/matrix.nix` asserts the same prefix on every configured
/// `tokenFile`, because its path-watcher globs for it. A name written here
/// that did not match would land a file the daemon never notices — no error,
/// just a credential that silently never arrives.
const TOKEN_PREFIX: &str = "matrix-token";

/// Where `agent`'s credential for `account` is written.
///
/// Takes an [`Ident`] rather than a `&str` because the name arrives off the
/// queue: `agent_state_dir` addresses a directory, and an unvalidated name
/// there is a path-traversal argument. The compiler refusing the `&str` is the
/// check — nothing here has to remember to perform one.
#[must_use]
pub fn token_path(agent: &Ident, account: &str) -> PathBuf {
    agent_state_dir(agent).join(format!("{TOKEN_PREFIX}-{account}"))
}

/// Read the credential `notice` names and write it into the agent's state dir.
///
/// `cert_role` is the role on the store's `cert` auth mount whose policy scopes
/// what this hive may read.
///
/// # Errors
/// The store refusing, being unreachable, or holding nothing at that path; a
/// name that is not a single path segment; or the write failing.
pub async fn deliver(notice: &CredentialNotice, cert_role: &str) -> Result<()> {
    // Parsed before anything is read, so a malformed name costs a decode and
    // not a round trip to the store.
    let agent = Ident::parse(&notice.agent)
        .map_err(|e| anyhow::anyhow!("agent name {:?} off the queue: {e}", notice.agent))?;
    let secret_path = path::matrix_account(&notice.agent, &notice.account)
        .context("building the credential's path in the store")?;

    let store = SecretStore::from_env(cert_role)
        .await
        .context("connecting to the swarm secret store")?;
    let value = store
        .read(&secret_path)
        .await
        .with_context(|| format!("reading {secret_path} from the store"))?;

    write_token(&token_path(&agent, &notice.account), &value)
}

/// Write `value` to `dest` at `0600`, atomically.
///
/// Atomic because the daemon's path-watcher fires on the file *appearing*: a
/// token written in place would be visible while still partial, and the daemon
/// would read a truncated credential exactly once, at the moment it is hardest
/// to reproduce. The temp name is dot-prefixed so it cannot match the
/// `matrix-token*` glob on its way past.
fn write_token(dest: &PathBuf, value: &str) -> Result<()> {
    let dir = dest
        .parent()
        .context("a token path always has a parent state dir")?;
    let name = dest
        .file_name()
        .context("a token path always has a file name")?;
    let mut tmp = dir.join(".");
    tmp.as_mut_os_string().push(name);
    tmp.as_mut_os_string().push(".partial");

    std::fs::write(&tmp, value).with_context(|| format!("writing {}", tmp.display()))?;
    // Before the rename, so the file is never briefly readable by others under
    // its final name.
    std::fs::set_permissions(&tmp, std::fs::Permissions::from_mode(0o600))
        .with_context(|| format!("restricting {}", tmp.display()))?;
    std::fs::rename(&tmp, dest).with_context(|| format!("publishing {}", dest.display()))
}

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

    #[test]
    fn the_written_name_matches_the_glob_the_daemon_watches() {
        // The other end of this agreement is an assertion in
        // `nix/agent-modules/matrix.nix` and a `systemd.paths` glob — neither
        // reachable from a Rust test, so the prefix is pinned here.
        let p = token_path(&Ident::parse("dmatrix").expect("a legal agent name"), "ccc");
        let name = p.file_name().unwrap().to_str().unwrap();
        assert!(name.starts_with("matrix-token"), "got {name}");
        assert_eq!(name, "matrix-token-ccc");
    }

    #[test]
    fn the_partial_file_cannot_match_that_glob() {
        // A temp name starting with `matrix-token` would be picked up
        // mid-write; the dot prefix is what stops it.
        let p = token_path(&Ident::parse("dmatrix").expect("a legal agent name"), "ccc");
        let name = p.file_name().unwrap().to_str().unwrap();
        let tmp = format!(".{name}.partial");
        assert!(!tmp.starts_with("matrix-token"), "got {tmp}");
    }

    #[test]
    fn a_name_that_could_address_another_agent_is_refused() {
        // `path::matrix_account` owns this rule; asserted here because this is
        // the module that feeds it names off the wire.
        assert!(path::matrix_account("../argus", "ccc").is_err());
        assert!(path::matrix_account("dmatrix", "../../etc/x").is_err());
        assert!(path::matrix_account("dmatrix", "ccc").is_ok());
    }

    #[test]
    fn an_agent_name_off_the_queue_must_pass_the_ident_parser_too() {
        // Two independent refusals, not one restated: `path::matrix_account`
        // guards the address in the *store*, `Ident` guards the address on
        // *disk*. `token_path` cannot even be called without the second,
        // which is why it takes an `Ident` rather than validating internally.
        for bad in ["../argus", "dmatrix/../argus", "Dmatrix", "d matrix", ""] {
            assert!(Ident::parse(bad).is_err(), "{bad:?} must be refused");
        }
        // The control: without it, a parser that rejected everything would
        // satisfy the loop above.
        assert!(Ident::parse("dmatrix").is_ok());
    }
}