//! This agent's swarm-queue credentials, resolved once at boot.
//!
//! Three of the four coordinates arrive as environment variables the meta
//! flake renders into the harness unit. The fourth — the OIDC client id —
//! arrives as a *file*, delivered beside the secret as one systemd
//! credential pair (`nix/agent-modules/queue.nix`). That is the whole reason
//! this module exists rather than a bare [`QueueConfig::from_env`] call: the
//! id rides with the secret so a reader never has to spell `hive-<name>-agent`
//! a second time, and `from_env` wants it as a value.
//!
//! Reading the file and assigning the variable would put the same rule back
//! in `from_env`'s hands, but `std::env::set_var` is unsound in a process that
//! has already spawned threads, and this one has. So the rule is restated
//! here over the inputs this consumer actually has, and [`decide`] is the
//! single place it lives.

use std::path::Path;
use std::sync::OnceLock;

use swarm_queue_client::QueueConfig;

/// Variable prefix for this agent's coordinates. Distinct from `HIVE_C0RE`'s
/// on purpose: an agent authenticates as its own client, not as its hive.
const ENV_PREFIX: &str = "HIVE_AGENT";

/// Resolved once at boot, so the publisher that connects to the queue reads
/// one answer rather than re-deriving it per call.
static CONFIG: OnceLock<Option<QueueConfig>> = OnceLock::new();

/// The four variables the harness unit sets, before the client-id file is
/// read. Collected into a struct so [`decide`] is pure over them and the
/// process env is touched in exactly one place.
struct QueueEnv {
    nats_url: Option<String>,
    token_endpoint: Option<String>,
    client_id_file: Option<String>,
    client_secret_file: Option<String>,
    /// Independent of the all-or-none group below, exactly as in
    /// `QueueConfig::from_env`: a swarm behind a publicly-trusted
    /// certificate needs no extra anchor. Nothing in this tree sets it for
    /// an agent — a container already trusts the swarm root, which
    /// `hive_c0re::meta` embeds at build time — so it is here for a
    /// deployment that needs a different one, not for ours.
    ca_file: Option<String>,
}

impl QueueEnv {
    fn from_env() -> Self {
        let var = |suffix: &str| std::env::var(format!("{ENV_PREFIX}_{suffix}")).ok();
        Self {
            nats_url: var("NATS_URL"),
            token_endpoint: var("OIDC_TOKEN_ENDPOINT"),
            client_id_file: var("OIDC_CLIENT_ID_FILE"),
            client_secret_file: var("OIDC_CLIENT_SECRET_FILE"),
            ca_file: var("OIDC_CA_FILE"),
        }
    }
}

/// What the environment plus the client-id file add up to.
enum Resolution {
    /// Everything is here; the agent can reach the queue.
    Configured(Box<QueueConfig>),
    /// No queue for this agent, and that is a legal state — carries why.
    Absent(&'static str),
    /// Some of the environment, not all of it. A deployment bug rather than
    /// an absent integration, so it is reported and then survived.
    Partial,
}

/// Read the client id out of the file the credential landed at.
///
/// `None` for a missing or empty file, which is the ordinary state of a hive
/// whose secret store has nothing published yet — the credential simply is
/// not there, and nspawn forwards nothing. Trailing newline stripped: the
/// reader unit writes one and an id with a newline in it authenticates as
/// nobody.
fn read_client_id(path: &Path) -> Option<String> {
    match std::fs::read_to_string(path) {
        Ok(raw) => {
            let id = raw.trim();
            (!id.is_empty()).then(|| id.to_owned())
        }
        Err(e) if e.kind() == std::io::ErrorKind::NotFound => None,
        Err(e) => {
            tracing::warn!(path = %path.display(), error = %e, "reading the queue client id failed");
            None
        }
    }
}

/// Decide what this agent's queue configuration is, given the environment and
/// whatever the client-id file held.
///
/// All four variables or none, for the reason `QueueConfig::from_env` gives:
/// a half-set environment produces a process that comes up fine and never
/// connects. The client id is graded separately from the four because its
/// file has a legal absence the variables do not — the unit names the path a
/// credential *would* arrive at whether or not one has been published yet.
fn decide(env: &QueueEnv, client_id: Option<String>) -> Resolution {
    match (
        env.nats_url.as_ref(),
        env.token_endpoint.as_ref(),
        env.client_id_file.as_ref(),
        env.client_secret_file.as_ref(),
    ) {
        (None, None, None, None) => Resolution::Absent("this hive has no swarm queue configured"),
        (Some(url), Some(token_endpoint), Some(_), Some(secret)) => match client_id {
            Some(client_id) => Resolution::Configured(Box::new(QueueConfig {
                url: url.clone(),
                token_endpoint: token_endpoint.clone(),
                client_id,
                client_secret_file: secret.into(),
                ca_file: env.ca_file.as_ref().map(Into::into),
            })),
            None => {
                Resolution::Absent("the queue credential has not been published to this hive yet")
            }
        },
        _ => Resolution::Partial,
    }
}

/// Resolve this agent's queue configuration and record it for later use.
///
/// Never fails: a harness that cannot reach the queue still serves its
/// operator, its web UI and its turn loop, so every outcome here is a log
/// line and not an exit.
pub fn init() {
    let env = QueueEnv::from_env();
    let client_id = env
        .client_id_file
        .as_deref()
        .map(Path::new)
        .and_then(read_client_id);
    let resolved = match decide(&env, client_id) {
        Resolution::Configured(cfg) => {
            // url + client id only. The secret is a path in this struct and
            // stays one: neither it nor its contents belong in a log.
            tracing::info!(url = %cfg.url, client_id = %cfg.client_id, "swarm queue configured");
            Some(*cfg)
        }
        Resolution::Absent(why) => {
            tracing::info!(why, "no swarm queue for this agent");
            None
        }
        Resolution::Partial => {
            tracing::error!(
                prefix = ENV_PREFIX,
                "swarm queue half-configured: {ENV_PREFIX}_NATS_URL, \
                 {ENV_PREFIX}_OIDC_TOKEN_ENDPOINT, {ENV_PREFIX}_OIDC_CLIENT_ID_FILE and \
                 {ENV_PREFIX}_OIDC_CLIENT_SECRET_FILE are set together or not at all — \
                 this agent will not connect"
            );
            None
        }
    };
    let _ = CONFIG.set(resolved);
}

#[cfg(test)]
mod tests {
    use super::{QueueEnv, Resolution, decide, read_client_id};

    fn env(parts: [Option<&str>; 4]) -> QueueEnv {
        let [nats_url, token_endpoint, client_id_file, client_secret_file] = parts;
        QueueEnv {
            nats_url: nats_url.map(str::to_owned),
            token_endpoint: token_endpoint.map(str::to_owned),
            client_id_file: client_id_file.map(str::to_owned),
            client_secret_file: client_secret_file.map(str::to_owned),
            ca_file: None,
        }
    }

    fn full() -> QueueEnv {
        env([
            Some("nats://10.42.0.1:4222"),
            Some("https://auth.t.local/api/oidc/token"),
            Some("/run/credentials/hive-agent.service/hive-queue-agent-client-id"),
            Some("/run/credentials/hive-agent.service/hive-queue-agent-secret"),
        ])
    }

    /// The reader unit writes the id with a trailing newline; an id carrying
    /// one is a client authelia has never heard of.
    #[test]
    fn a_client_id_file_is_read_without_its_newline() {
        let dir = tempfile::tempdir().expect("tempdir");
        let path = dir.path().join("client_id");
        std::fs::write(&path, "hive-h1-agent\n").expect("write");
        assert_eq!(read_client_id(&path).as_deref(), Some("hive-h1-agent"));
    }

    /// Both shapes of "no credential here": never delivered, or delivered
    /// empty. Neither is an id, and treating an empty string as one would
    /// authenticate as the anonymous client rather than failing.
    #[test]
    fn a_missing_or_empty_client_id_file_reads_as_no_id() {
        let dir = tempfile::tempdir().expect("tempdir");
        let missing = dir.path().join("client_id");
        assert_eq!(read_client_id(&missing), None);
        std::fs::write(&missing, "\n").expect("write");
        assert_eq!(read_client_id(&missing), None);
    }

    #[test]
    fn a_complete_environment_with_a_published_id_configures_the_queue() {
        let e = full();
        let Resolution::Configured(cfg) = decide(&e, Some("hive-h1-agent".to_owned())) else {
            panic!("expected a configured queue");
        };
        assert_eq!(cfg.url, "nats://10.42.0.1:4222");
        assert_eq!(cfg.client_id, "hive-h1-agent");
        assert!(
            cfg.client_secret_file.ends_with("hive-queue-agent-secret"),
            "the secret stays a path: {}",
            cfg.client_secret_file.display()
        );
    }

    /// A hive with no swarm queue at all. Silence here is correct, and it has
    /// to be distinguishable from the half-set case below — that distinction
    /// is the only thing that makes the error branch worth logging.
    #[test]
    fn an_empty_environment_is_no_queue_rather_than_an_error() {
        let e = env([None, None, None, None]);
        assert!(matches!(decide(&e, None), Resolution::Absent(_)));
    }

    /// The credential's own absence. The unit names the path unconditionally
    /// once the hive has a queue, so this is the state of every agent on a
    /// swarm whose publisher has not run — legal, and not the error branch.
    #[test]
    fn a_complete_environment_with_no_published_id_is_no_queue() {
        let e = full();
        assert!(matches!(decide(&e, None), Resolution::Absent(_)));
    }

    /// Each single-variable omission, because the failure a partial set
    /// produces is a harness that looks healthy and publishes nothing.
    #[test]
    fn any_missing_variable_is_a_partial_configuration() {
        for drop in 0..4 {
            let mut parts = [
                Some("nats://10.42.0.1:4222"),
                Some("https://auth.t.local/api/oidc/token"),
                Some("/run/credentials/hive-agent.service/hive-queue-agent-client-id"),
                Some("/run/credentials/hive-agent.service/hive-queue-agent-secret"),
            ];
            parts[drop] = None;
            let e = env(parts);
            assert!(
                matches!(
                    decide(&e, Some("hive-h1-agent".to_owned())),
                    Resolution::Partial
                ),
                "dropping variable {drop} must report a partial configuration"
            );
        }
    }
}