autumn-plugin-grpc 0.1.0

//! The `[grpc]` section of `autumn.toml`.
//!
//! Each key has a safe default. The plugin merges the section in the same
//! order as Autumn core:
//!
//! 1. `[grpc]` in `autumn.toml`.
//! 2. `[profile.<name>.grpc]` in `autumn.toml`.
//! 3. `[grpc]` in `autumn-<profile>.toml`.
//! 4. `AUTUMN_GRPC__*` environment variables (also from `.env` files).
//!
//! ```toml
//! [grpc]
//! bind = "0.0.0.0:50051"
//! reflection = "auto"        # on in dev/test, off in other profiles
//! shutdown_grace_ms = 10000
//!
//! [grpc.tls]                 # needs the `tls` feature
//! cert_path = "certs/server.pem"
//! key_path = "certs/server.key"
//!
//! [grpc.clients.billing]     # needs the `client` feature
//! endpoint = "http://billing:50051"
//! timeout_ms = 10000
//! ```

use std::collections::BTreeMap;
use std::fmt;
use std::net::SocketAddr;
use std::path::{Path, PathBuf};
use std::time::Duration;

use autumn_web::config::Env;
use serde::{Deserialize, Deserializer, Serialize, Serializer};

/// The default gRPC port.
pub const DEFAULT_PORT: u16 = 50051;

/// The default config section.
pub const DEFAULT_SECTION: &str = "grpc";

/// A three-state switch. In TOML, write a boolean or `"auto"`.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
pub enum Toggle {
    /// The active profile decides. Each field gives the rule.
    #[default]
    Auto,
    /// Always on.
    On,
    /// Always off.
    Off,
}

impl Toggle {
    /// The value, with `auto` set to `auto_value`.
    #[must_use]
    pub const fn resolve(self, auto_value: bool) -> bool {
        match self {
            Self::Auto => auto_value,
            Self::On => true,
            Self::Off => false,
        }
    }
}

impl From<bool> for Toggle {
    fn from(value: bool) -> Self {
        if value { Self::On } else { Self::Off }
    }
}

impl fmt::Display for Toggle {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str(match self {
            Self::Auto => "auto",
            Self::On => "true",
            Self::Off => "false",
        })
    }
}

impl Serialize for Toggle {
    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
        match self {
            Self::Auto => serializer.serialize_str("auto"),
            Self::On => serializer.serialize_bool(true),
            Self::Off => serializer.serialize_bool(false),
        }
    }
}

impl<'de> Deserialize<'de> for Toggle {
    fn deserialize<D: Deserializer<'de>>(deserializer: D) -> Result<Self, D::Error> {
        #[derive(Deserialize)]
        #[serde(untagged)]
        enum Raw {
            Bool(bool),
            Text(String),
        }
        match Raw::deserialize(deserializer)? {
            Raw::Bool(value) => Ok(value.into()),
            Raw::Text(text) => match text.trim().to_ascii_lowercase().as_str() {
                "auto" => Ok(Self::Auto),
                "true" | "on" | "enabled" | "yes" => Ok(Self::On),
                "false" | "off" | "disabled" | "no" => Ok(Self::Off),
                other => Err(serde::de::Error::custom(format!(
                    "expected true, false or \"auto\", found \"{other}\""
                ))),
            },
        }
    }
}

/// Where the gRPC server listens.
#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash, Serialize, Deserialize)]
#[serde(rename_all = "lowercase")]
#[non_exhaustive]
pub enum Listener {
    /// A listener of its own, on `bind` (ADR 0002).
    #[default]
    Dedicated,
    /// Autumn's HTTP port. HTTP/2 requests with a gRPC content type go to
    /// the gRPC services. Needs the `multiplex` feature (ADR 0008).
    Shared,
}

impl Listener {
    /// The name in TOML.
    #[must_use]
    pub const fn as_str(self) -> &'static str {
        match self {
            Self::Dedicated => "dedicated",
            Self::Shared => "shared",
        }
    }
}

impl fmt::Display for Listener {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str(self.as_str())
    }
}

/// Settings for one gRPC server.
///
/// The defaults limit resource use. For a setting that says "`0`: off",
/// the value `0` removes that limit or feature.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
#[serde(default, deny_unknown_fields)]
#[non_exhaustive]
#[allow(clippy::struct_excessive_bools)] // independent switches, as in TOML
pub struct GrpcConfig {
    /// Start the server. Default: `true`.
    pub enabled: bool,
    /// `dedicated`: a listener of its own, on `bind`. `shared`: Autumn's
    /// HTTP port (needs the `multiplex` feature). Default: `dedicated`.
    pub listener: Listener,
    /// Listen address, `IP:port`. Port `0` picks a free port. Empty: the
    /// profile decides. `dev` and `test` use `127.0.0.1:50051`. Other
    /// profiles use `0.0.0.0:50051`. Default: empty.
    pub bind: String,
    /// Serve `grpc.health.v1.Health`. Default: `true`.
    pub health: bool,
    /// Serve server reflection. `auto` is on in `dev` and `test` only.
    pub reflection: Toggle,
    /// Record `grpc_server_*` metrics. Default: `true`.
    pub metrics: bool,
    /// Maximum label sets per metric. The plugin counts new label sets
    /// over this limit as `other`. Default: `1000`.
    pub max_metric_series: usize,
    /// Time for in-flight calls to finish at shutdown. Must be more than
    /// `0`. Default: `10000`.
    pub shutdown_grace_ms: u64,
    /// Per-call timeout for unary calls. `0`: off. Default: `0`.
    pub timeout_ms: u64,
    /// Open connections. The listener waits when it reaches the limit.
    /// `0`: off. Default: `1000`.
    pub max_connections: usize,
    /// Concurrent calls per connection. A streaming call counts until its
    /// response starts. `0`: off. Default: `0`.
    pub concurrency_limit_per_connection: usize,
    /// HTTP/2 `SETTINGS_MAX_CONCURRENT_STREAMS`. Must be more than `0`.
    /// Default: `200`.
    pub max_concurrent_streams: u32,
    /// Server-side stream resets per connection before the server closes
    /// it (a protection against reset floods). Must be more than `0`.
    /// Default: `1024`.
    pub http2_max_local_error_reset_streams: usize,
    /// Set `TCP_NODELAY`. Default: `true`.
    pub tcp_nodelay: bool,
    /// TCP keepalive idle time. `0`: off. Default: `0`.
    pub tcp_keepalive_ms: u64,
    /// HTTP/2 PING interval. `0`: off. Default: `60000`.
    pub http2_keepalive_interval_ms: u64,
    /// Time to wait for a PING reply. `0`: tonic default (20 s).
    /// Default: `20000`.
    pub http2_keepalive_timeout_ms: u64,
    /// Close connections after this age. `0`: off. Default: `0`.
    pub max_connection_age_ms: u64,
    /// TLS settings. Empty paths mean plain text.
    pub tls: TlsConfig,
    /// Downstream services to call, by name (`[grpc.clients.<name>]`).
    /// Needs the `client` feature. The server does not use them.
    pub clients: BTreeMap<String, ClientConfig>,
}

/// Settings for one downstream gRPC service (`[grpc.clients.<name>]`).
/// Needs the `client` feature.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
#[serde(default, deny_unknown_fields)]
#[non_exhaustive]
pub struct ClientConfig {
    /// `http://host:port` or `https://host:port`, with no path. Empty is
    /// an error at boot, unless a test double serves the client.
    pub endpoint: String,
    /// Longest time for one call. The client sends the smallest of this
    /// value, the time left on the incoming request and the caller's
    /// timeout as `grpc-timeout`. `0`: off. Default: `10000`.
    pub timeout_ms: u64,
    /// Longest time to open a connection. Must be more than `0`.
    /// Default: `5000`.
    pub connect_timeout_ms: u64,
    /// TLS for an `https` endpoint. Needs the `tls` feature.
    pub tls: ClientTls,
}

/// Client TLS settings. Empty values mean "not set".
#[derive(Debug, Clone, Default, PartialEq, Eq, Serialize, Deserialize)]
#[serde(default, deny_unknown_fields)]
#[non_exhaustive]
pub struct ClientTls {
    /// PEM CA bundle that signs the server certificate. Necessary for
    /// `https`. A system bundle (for example
    /// `/etc/ssl/certs/ca-certificates.crt`) works for public services.
    pub ca_path: String,
    /// PEM client certificate chain, for mTLS. Needs `key_path`.
    pub cert_path: String,
    /// PEM client private key, for mTLS. Needs `cert_path`.
    pub key_path: String,
    /// Server name to verify. Empty: the endpoint host.
    pub domain_name: String,
}

impl Default for ClientConfig {
    fn default() -> Self {
        Self {
            endpoint: String::new(),
            timeout_ms: 10_000,
            connect_timeout_ms: 5_000,
            tls: ClientTls::default(),
        }
    }
}

impl ClientConfig {
    /// Per-call timeout. `None` when `timeout_ms` is `0`.
    #[must_use]
    pub const fn timeout(&self) -> Option<Duration> {
        millis(self.timeout_ms)
    }

    /// Connect timeout.
    #[must_use]
    pub const fn connect_timeout(&self) -> Duration {
        Duration::from_millis(self.connect_timeout_ms)
    }

    /// `true` for an `https` endpoint. Validation accepts only the
    /// lowercase schemes, so this test is safe.
    #[must_use]
    pub fn is_https(&self) -> bool {
        self.endpoint.trim().starts_with("https://")
    }

    /// Check one client. `name` is the key in `clients`.
    fn validate(&self, name: &str) -> Result<(), ConfigError> {
        let key = |field: &str| format!("clients.{name}.{field}");
        if self.connect_timeout_ms == 0 {
            return Err(ConfigError(format!(
                "`{}` must be more than 0",
                key("connect_timeout_ms")
            )));
        }
        validate_endpoint(&self.endpoint, &key("endpoint"))?;
        let tls = &self.tls;
        let has = |value: &str| !value.trim().is_empty();
        match (has(&tls.cert_path), has(&tls.key_path)) {
            (true, false) => {
                return Err(ConfigError(format!(
                    "`{}` is set, so `{}` is necessary",
                    key("tls.cert_path"),
                    key("tls.key_path")
                )));
            }
            (false, true) => {
                return Err(ConfigError(format!(
                    "`{}` is set, so `{}` is necessary",
                    key("tls.key_path"),
                    key("tls.cert_path")
                )));
            }
            _ => {}
        }
        let any_tls =
            has(&tls.ca_path) || has(&tls.cert_path) || has(&tls.key_path) || has(&tls.domain_name);
        if !self.is_https() {
            if any_tls {
                return Err(ConfigError(format!(
                    "`{}` needs an https:// endpoint",
                    key("tls")
                )));
            }
            return Ok(());
        }
        if !cfg!(feature = "tls") {
            return Err(ConfigError(format!(
                "`{}` uses https, which needs the `tls` feature of autumn-plugin-grpc",
                key("endpoint")
            )));
        }
        if !has(&tls.ca_path) {
            return Err(ConfigError(format!(
                "`{}` is necessary for an https endpoint",
                key("tls.ca_path")
            )));
        }
        Ok(())
    }
}

/// A client name is a TOML key and part of an env variable name.
pub fn validate_client_name(name: &str) -> Result<(), ConfigError> {
    let valid = !name.is_empty()
        && name.len() <= 64
        && name.starts_with(|c: char| c.is_ascii_lowercase())
        && name
            .bytes()
            .all(|b| b.is_ascii_lowercase() || b.is_ascii_digit() || b == b'_');
    if valid {
        Ok(())
    } else {
        Err(ConfigError(format!(
            "gRPC client name `{name}` must start with a-z and use only a-z, 0-9 and _ (at most 64)"
        )))
    }
}

/// An empty endpoint is "unset" here. The plugin checks it at boot.
fn validate_endpoint(endpoint: &str, key: &str) -> Result<(), ConfigError> {
    let endpoint = endpoint.trim();
    if endpoint.is_empty() {
        return Ok(());
    }
    let uri: http::Uri = endpoint.parse().map_err(|_| {
        ConfigError(format!(
            "`{key}` must be a URI like \"http://host:50051\", found \"{endpoint}\""
        ))
    })?;
    // Lowercase only: `is_https` must agree with the URI parser, which
    // accepts `HTTPS://` too.
    if !(endpoint.starts_with("http://") || endpoint.starts_with("https://")) {
        return Err(ConfigError(format!(
            "`{key}` must start with http:// or https://, found \"{endpoint}\""
        )));
    }
    if uri.host().is_none_or(str::is_empty) {
        return Err(ConfigError(format!(
            "`{key}` needs a host, found \"{endpoint}\""
        )));
    }
    let path = uri.path_and_query().map_or("", |p| p.as_str());
    if !matches!(path, "" | "/") {
        return Err(ConfigError(format!(
            "`{key}` must have no path or query, found \"{endpoint}\""
        )));
    }
    Ok(())
}

/// TLS settings. Needs the `tls` crate feature.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
#[serde(default, deny_unknown_fields)]
#[non_exhaustive]
pub struct TlsConfig {
    /// PEM certificate chain of the server.
    pub cert_path: String,
    /// PEM private key of the server.
    pub key_path: String,
    /// PEM CA bundle. When set, clients must show a certificate (mTLS).
    pub client_ca_path: String,
    /// With `client_ca_path`, also accept clients without a certificate.
    /// Your interceptor must then check `request.peer_certs()`.
    pub client_auth_optional: bool,
    /// Time for a client to finish the TLS handshake. Must be more than
    /// `0`. Default: `10000`.
    pub handshake_timeout_ms: u64,
}

impl Default for TlsConfig {
    fn default() -> Self {
        Self {
            cert_path: String::new(),
            key_path: String::new(),
            client_ca_path: String::new(),
            client_auth_optional: false,
            handshake_timeout_ms: 10_000,
        }
    }
}

impl TlsConfig {
    /// `true` when a certificate is set.
    #[must_use]
    pub fn is_enabled(&self) -> bool {
        !self.cert_path.trim().is_empty()
    }

    /// TLS handshake timeout.
    #[must_use]
    pub const fn handshake_timeout(&self) -> Duration {
        Duration::from_millis(self.handshake_timeout_ms)
    }
}

impl Default for GrpcConfig {
    fn default() -> Self {
        Self {
            enabled: true,
            listener: Listener::Dedicated,
            bind: String::new(),
            health: true,
            reflection: Toggle::Auto,
            metrics: true,
            max_metric_series: 1000,
            shutdown_grace_ms: 10_000,
            timeout_ms: 0,
            max_connections: 1000,
            concurrency_limit_per_connection: 0,
            max_concurrent_streams: 200,
            http2_max_local_error_reset_streams: 1024,
            tcp_nodelay: true,
            tcp_keepalive_ms: 0,
            http2_keepalive_interval_ms: 60_000,
            http2_keepalive_timeout_ms: 20_000,
            max_connection_age_ms: 0,
            tls: TlsConfig::default(),
            clients: BTreeMap::new(),
        }
    }
}

/// A configuration problem. Boot stops. The plugin does not fall back to
/// defaults.
#[derive(Debug, Clone, PartialEq, Eq, thiserror::Error)]
#[error("invalid gRPC configuration: {0}")]
pub struct ConfigError(String);

impl ConfigError {
    /// The problem, without the prefix.
    #[must_use]
    pub fn message(&self) -> &str {
        &self.0
    }
}

/// A resolved configuration and the active profile.
#[derive(Debug, Clone)]
pub struct Resolved {
    pub(crate) config: GrpcConfig,
    pub(crate) profile: String,
}

impl Resolved {
    /// The merged, validated configuration.
    #[must_use]
    pub const fn config(&self) -> &GrpcConfig {
        &self.config
    }

    /// The canonical active profile, for example `dev`, `prod` or `test`.
    #[must_use]
    pub fn profile(&self) -> &str {
        &self.profile
    }

    /// Configuration from code. The plugin reads no files. The profile
    /// still comes from the environment.
    pub(crate) fn explicit(config: GrpcConfig) -> Self {
        let profile = autumn_web::dotenv::os_env_with_dotenv().map_or_else(
            |_| resolve_active_profile(&autumn_web::config::OsEnv).1,
            |env| resolve_active_profile(&env).1,
        );
        Self { config, profile }
    }

    /// `true` for the `dev` and `test` profiles.
    #[must_use]
    pub fn is_development(&self) -> bool {
        matches!(self.profile.as_str(), "dev" | "test")
    }
}

const fn millis(ms: u64) -> Option<Duration> {
    if ms == 0 {
        None
    } else {
        Some(Duration::from_millis(ms))
    }
}

impl GrpcConfig {
    /// The listen address. An empty `bind` gives `127.0.0.1:50051` in
    /// development and `0.0.0.0:50051` in other profiles.
    ///
    /// # Errors
    ///
    /// Returns [`ConfigError`] when `bind` is not `IP:port`.
    pub fn bind_addr(&self, development: bool) -> Result<SocketAddr, ConfigError> {
        let bind = self.bind.trim();
        if bind.is_empty() {
            let host = if development {
                [127, 0, 0, 1]
            } else {
                [0, 0, 0, 0]
            };
            return Ok(SocketAddr::from((host, DEFAULT_PORT)));
        }
        bind.parse().map_err(|_| {
            ConfigError(format!(
                "`bind` must be IP:port (for example \"0.0.0.0:50051\"), found \"{}\"",
                self.bind
            ))
        })
    }

    /// Per-call timeout.
    #[must_use]
    pub const fn timeout(&self) -> Option<Duration> {
        millis(self.timeout_ms)
    }

    /// Shutdown grace period.
    #[must_use]
    pub const fn shutdown_grace(&self) -> Duration {
        Duration::from_millis(self.shutdown_grace_ms)
    }

    /// TCP keepalive idle time.
    #[must_use]
    pub const fn tcp_keepalive(&self) -> Option<Duration> {
        millis(self.tcp_keepalive_ms)
    }

    /// HTTP/2 PING interval.
    #[must_use]
    pub const fn http2_keepalive_interval(&self) -> Option<Duration> {
        millis(self.http2_keepalive_interval_ms)
    }

    /// HTTP/2 PING ack timeout.
    #[must_use]
    pub const fn http2_keepalive_timeout(&self) -> Option<Duration> {
        millis(self.http2_keepalive_timeout_ms)
    }

    /// Maximum connection age.
    #[must_use]
    pub const fn max_connection_age(&self) -> Option<Duration> {
        millis(self.max_connection_age_ms)
    }

    /// Parse `[section]` from a whole `autumn.toml` text. No profiles and
    /// no environment.
    ///
    /// # Errors
    ///
    /// Returns [`ConfigError`] for bad TOML, unknown keys, wrong types or
    /// values that [`validate`](Self::validate) rejects.
    pub fn from_toml_str(text: &str, section: &str) -> Result<Self, ConfigError> {
        let document: toml::Table = toml::from_str(text).map_err(|e| ConfigError(e.to_string()))?;
        let config = Self::from_section(document.get(section))?;
        config.validate()?;
        Ok(config)
    }

    fn from_section(section: Option<&toml::Value>) -> Result<Self, ConfigError> {
        section.map_or_else(
            || Ok(Self::default()),
            |value| {
                value
                    .clone()
                    .try_into()
                    .map_err(|e: toml::de::Error| ConfigError(e.to_string()))
            },
        )
    }

    /// Resolve `[section]` from the app's files and the process environment.
    ///
    /// # Errors
    ///
    /// Returns [`ConfigError`] when a file cannot be read or parsed, or when
    /// the merged section is not valid.
    pub fn resolve(section: &str) -> Result<Resolved, ConfigError> {
        let env = autumn_web::dotenv::os_env_with_dotenv()
            .map_err(|e| ConfigError(format!("cannot read .env files: {e}")))?;
        Self::resolve_with_env(section, &env)
    }

    /// Like [`resolve`](Self::resolve), with env leaves for the client
    /// names from code.
    pub(crate) fn resolve_with_clients(
        section: &str,
        clients: &[&str],
    ) -> Result<Resolved, ConfigError> {
        let env = autumn_web::dotenv::os_env_with_dotenv()
            .map_err(|e| ConfigError(format!("cannot read .env files: {e}")))?;
        Self::resolve_with_env_and_clients(section, &env, clients)
    }

    /// Like [`resolve`](Self::resolve), but reads only `env`.
    ///
    /// # Errors
    ///
    /// See [`resolve`](Self::resolve).
    pub fn resolve_with_env(section: &str, env: &dyn Env) -> Result<Resolved, ConfigError> {
        Self::resolve_with_env_and_clients(section, env, &[])
    }

    /// Like [`resolve_with_env`](Self::resolve_with_env). Env overrides
    /// apply to each client in the files and to each name in `clients`
    /// (the plugin passes the names from [`client`](crate::GrpcPlugin)).
    ///
    /// # Errors
    ///
    /// See [`resolve`](Self::resolve).
    pub fn resolve_with_env_and_clients(
        section: &str,
        env: &dyn Env,
        clients: &[&str],
    ) -> Result<Resolved, ConfigError> {
        let (selected, canonical) = resolve_active_profile(env);
        let mut merged = toml::Value::Table(toml::map::Map::new());

        if let Some(base) = read_optional_toml(&find_config_file("autumn.toml", env))? {
            deep_merge(&mut merged, base.clone());
            for name in profile_inline_lookup_names(&canonical) {
                if let Some(profile) = profile_section(&base, name) {
                    deep_merge(&mut merged, profile);
                }
            }
        }
        for name in autumn_web::config::profile_override_file_lookup_names(&canonical, &selected) {
            let path = find_config_file(&format!("autumn-{name}.toml"), env);
            if let Some(overlay) = read_optional_toml(&path)? {
                deep_merge(&mut merged, overlay);
                break;
            }
        }

        let mut section_value = merged
            .get(section)
            .cloned()
            .unwrap_or_else(|| toml::Value::Table(toml::map::Map::new()));
        if !section_value.is_table() {
            return Err(ConfigError(format!("`{section}` must be a table")));
        }
        let mut config = Self::from_section(Some(&section_value))?;
        apply_env_overrides(section, &mut section_value, &mut config, env, clients)?;
        config.validate()?;
        Ok(Resolved {
            config,
            profile: canonical,
        })
    }

    /// Reject values that fail at runtime.
    ///
    /// # Errors
    ///
    /// Returns [`ConfigError`] that names the bad key.
    pub fn validate(&self) -> Result<(), ConfigError> {
        self.bind_addr(true)?;
        for (key, value) in [
            ("shutdown_grace_ms", self.shutdown_grace_ms),
            ("max_metric_series", self.max_metric_series as u64),
            (
                "max_concurrent_streams",
                u64::from(self.max_concurrent_streams),
            ),
            (
                "http2_max_local_error_reset_streams",
                self.http2_max_local_error_reset_streams as u64,
            ),
            ("tls.handshake_timeout_ms", self.tls.handshake_timeout_ms),
        ] {
            if value == 0 {
                return Err(ConfigError(format!("`{key}` must be more than 0")));
            }
        }
        let tls = &self.tls;
        let has = |value: &str| !value.trim().is_empty();
        match (has(&tls.cert_path), has(&tls.key_path)) {
            (true, false) => {
                return Err(ConfigError(
                    "`tls.cert_path` is set, so `tls.key_path` is necessary".to_owned(),
                ));
            }
            (false, true) => {
                return Err(ConfigError(
                    "`tls.key_path` is set, so `tls.cert_path` is necessary".to_owned(),
                ));
            }
            _ => {}
        }
        if has(&tls.client_ca_path) && !has(&tls.cert_path) {
            return Err(ConfigError(
                "`tls.client_ca_path` needs `tls.cert_path` and `tls.key_path`".to_owned(),
            ));
        }
        if tls.client_auth_optional && !has(&tls.client_ca_path) {
            return Err(ConfigError(
                "`tls.client_auth_optional` needs `tls.client_ca_path`".to_owned(),
            ));
        }
        if !self.clients.is_empty() && !cfg!(feature = "client") {
            return Err(ConfigError(
                "`clients` needs the `client` feature of autumn-plugin-grpc".to_owned(),
            ));
        }
        for (name, client) in &self.clients {
            validate_client_name(name)?;
            client.validate(name)?;
        }
        if self.listener == Listener::Shared {
            if has(&tls.cert_path) || has(&tls.client_ca_path) {
                return Err(ConfigError(
                    "`tls` has no effect with `listener = \"shared\"`; set TLS in `[server.tls]`"
                        .to_owned(),
                ));
            }
            if !cfg!(feature = "multiplex") {
                return Err(ConfigError(
                    "`listener = \"shared\"` needs the `multiplex` feature of autumn-plugin-grpc"
                        .to_owned(),
                ));
            }
        }
        Ok(())
    }

    /// Settings of the dedicated listener that are not at their defaults.
    /// They have no effect with `listener = "shared"`.
    #[must_use]
    pub(crate) fn dedicated_only_settings(&self) -> Vec<&'static str> {
        let base = Self::default();
        [
            ("bind", self.bind.trim() != base.bind),
            (
                "max_connections",
                self.max_connections != base.max_connections,
            ),
            (
                "concurrency_limit_per_connection",
                self.concurrency_limit_per_connection != base.concurrency_limit_per_connection,
            ),
            (
                "max_concurrent_streams",
                self.max_concurrent_streams != base.max_concurrent_streams,
            ),
            (
                "http2_max_local_error_reset_streams",
                self.http2_max_local_error_reset_streams
                    != base.http2_max_local_error_reset_streams,
            ),
            ("tcp_nodelay", self.tcp_nodelay != base.tcp_nodelay),
            (
                "tcp_keepalive_ms",
                self.tcp_keepalive_ms != base.tcp_keepalive_ms,
            ),
            (
                "http2_keepalive_interval_ms",
                self.http2_keepalive_interval_ms != base.http2_keepalive_interval_ms,
            ),
            (
                "http2_keepalive_timeout_ms",
                self.http2_keepalive_timeout_ms != base.http2_keepalive_timeout_ms,
            ),
            (
                "max_connection_age_ms",
                self.max_connection_age_ms != base.max_connection_age_ms,
            ),
            (
                "tls.handshake_timeout_ms",
                self.tls.handshake_timeout_ms != base.tls.handshake_timeout_ms,
            ),
        ]
        .into_iter()
        .filter_map(|(key, changed)| changed.then_some(key))
        .collect()
    }
}

/// The env prefix for a section: `grpc` → `AUTUMN_GRPC__`.
fn env_prefix(section: &str) -> String {
    format!("AUTUMN_{}__", section.to_ascii_uppercase())
}

/// Apply `AUTUMN_<SECTION>__<PATH>` overrides for each known leaf key.
///
/// Values are TOML literals (`true`, `10`, `"x"`) or bare strings. An
/// override with the wrong type is an error: a bad value must not leave a
/// file value (for example `reflection = true`) in place.
///
/// A map has no leaves in the defaults. Client leaves come from the names
/// in the section and in `clients`.
fn apply_env_overrides(
    section_name: &str,
    section: &mut toml::Value,
    config: &mut GrpcConfig,
    env: &dyn Env,
    clients: &[&str],
) -> Result<(), ConfigError> {
    let defaults =
        toml::Value::try_from(GrpcConfig::default()).map_err(|e| ConfigError(e.to_string()))?;
    let prefix = env_prefix(section_name);
    let mut leaves = Vec::new();
    collect_leaves(&defaults, &mut Vec::new(), &mut leaves);
    let client_defaults =
        toml::Value::try_from(ClientConfig::default()).map_err(|e| ConfigError(e.to_string()))?;
    let mut names: Vec<String> = section
        .get("clients")
        .and_then(toml::Value::as_table)
        .map(|table| table.keys().cloned().collect())
        .unwrap_or_default();
    names.extend(clients.iter().map(|name| (*name).to_owned()));
    names.sort();
    names.dedup();
    for name in names {
        let mut prefix = vec!["clients".to_owned(), name];
        collect_leaves(&client_defaults, &mut prefix, &mut leaves);
    }
    for path in leaves {
        let key = format!(
            "{prefix}{}",
            path.iter()
                .map(|segment| segment.to_ascii_uppercase())
                .collect::<Vec<_>>()
                .join("__")
        );
        let Some(raw) = env_trimmed(env, &key) else {
            continue;
        };
        let mut candidate = section.clone();
        set_path(&mut candidate, &path, parse_env_value(&raw));
        *config = GrpcConfig::from_section(Some(&candidate))
            .map_err(|error| ConfigError(format!("{key}: {}", error.message())))?;
        *section = candidate;
    }
    Ok(())
}

fn parse_env_value(raw: &str) -> toml::Value {
    toml::from_str::<toml::Table>(&format!("v = {raw}"))
        .ok()
        .and_then(|mut table| table.remove("v"))
        .unwrap_or_else(|| toml::Value::String(raw.to_owned()))
}

fn collect_leaves(value: &toml::Value, prefix: &mut Vec<String>, out: &mut Vec<Vec<String>>) {
    if let toml::Value::Table(table) = value {
        for (key, child) in table {
            prefix.push(key.clone());
            collect_leaves(child, prefix, out);
            prefix.pop();
        }
    } else {
        out.push(prefix.clone());
    }
}

fn set_path(root: &mut toml::Value, path: &[String], value: toml::Value) {
    let Some((last, parents)) = path.split_last() else {
        return;
    };
    let mut cursor = root;
    for segment in parents {
        let Some(table) = cursor.as_table_mut() else {
            return;
        };
        cursor = table
            .entry(segment.clone())
            .or_insert_with(|| toml::Value::Table(toml::map::Map::new()));
    }
    if let Some(table) = cursor.as_table_mut() {
        table.insert(last.clone(), value);
    }
}

/// A non-blank env value. A blank value counts as unset, as in core.
fn env_trimmed(env: &dyn Env, key: &str) -> Option<String> {
    env.var(key)
        .ok()
        .map(|value| value.trim().to_owned())
        .filter(|value| !value.is_empty())
}

/// `(selected spelling, canonical name)` of the active profile.
///
/// Same order as core: `AUTUMN_ENV`, `AUTUMN_PROFILE`, `--profile`,
/// `AUTUMN_IS_DEBUG=0` (→ `prod`), then `dev`.
fn resolve_active_profile(env: &dyn Env) -> (String, String) {
    let selected = resolve_profile_input(env);
    let canonical =
        autumn_web::config::normalize_profile_name(&selected).unwrap_or_else(|| "dev".to_owned());
    (selected, canonical)
}

fn resolve_profile_input(env: &dyn Env) -> String {
    if let Some(value) = env_trimmed(env, "AUTUMN_ENV") {
        return value;
    }
    if let Some(value) = env_trimmed(env, "AUTUMN_PROFILE") {
        return value;
    }
    let args: Vec<String> = std::env::args().collect();
    for (index, arg) in args.iter().enumerate() {
        if arg == "--profile"
            && let Some(profile) = args.get(index.saturating_add(1))
            && !profile.trim().is_empty()
        {
            return profile.trim().to_owned();
        }
        if let Some(profile) = arg.strip_prefix("--profile=")
            && !profile.trim().is_empty()
        {
            return profile.trim().to_owned();
        }
    }
    if env_trimmed(env, "AUTUMN_IS_DEBUG").as_deref() == Some("0") {
        return "prod".to_owned();
    }
    "dev".to_owned()
}

/// Find a config file as core does: `AUTUMN_MANIFEST_DIR` first, then the
/// working directory.
fn find_config_file(filename: &str, env: &dyn Env) -> PathBuf {
    if let Some(dir) = env_trimmed(env, "AUTUMN_MANIFEST_DIR") {
        let candidate = PathBuf::from(dir).join(filename);
        if candidate.exists() {
            return candidate;
        }
    }
    PathBuf::from(filename)
}

fn read_optional_toml(path: &Path) -> Result<Option<toml::Value>, ConfigError> {
    match std::fs::read_to_string(path) {
        Ok(contents) => {
            let table = toml::from_str::<toml::Table>(&contents)
                .map_err(|e| ConfigError(format!("cannot parse {}: {e}", path.display())))?;
            Ok(Some(toml::Value::Table(table)))
        }
        Err(error) if error.kind() == std::io::ErrorKind::NotFound => Ok(None),
        Err(error) => Err(ConfigError(format!(
            "cannot read {}: {error}",
            path.display()
        ))),
    }
}

fn profile_inline_lookup_names(canonical: &str) -> Vec<&str> {
    match canonical {
        "prod" => vec!["production", "prod"],
        "dev" => vec!["development", "dev"],
        other => vec![other],
    }
}

fn profile_section(base: &toml::Value, profile: &str) -> Option<toml::Value> {
    base.get("profile")
        .and_then(toml::Value::as_table)
        .and_then(|profiles| profiles.get(profile))
        .and_then(toml::Value::as_table)
        .map(|table| toml::Value::Table(table.clone()))
}

/// Merge `overlay` into `base`. Tables merge; other values replace.
fn deep_merge(base: &mut toml::Value, overlay: toml::Value) {
    deep_merge_at(base, overlay, 0);
}

fn deep_merge_at(base: &mut toml::Value, overlay: toml::Value, depth: usize) {
    const MAX_MERGE_DEPTH: usize = 16;
    if depth > MAX_MERGE_DEPTH {
        return;
    }
    let toml::Value::Table(overlay_table) = overlay else {
        return;
    };
    let Some(base_table) = base.as_table_mut() else {
        return;
    };
    for (key, overlay_value) in overlay_table {
        let recurse =
            overlay_value.is_table() && base_table.get(&key).is_some_and(toml::Value::is_table);
        if recurse {
            if let Some(base_value) = base_table.get_mut(&key) {
                deep_merge_at(base_value, overlay_value, depth.saturating_add(1));
            }
        } else {
            base_table.insert(key, overlay_value);
        }
    }
}

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

    type Setter = fn(&mut GrpcConfig);

    #[test]
    fn dedicated_only_settings_names_each_changed_key() {
        assert_eq!(
            GrpcConfig::default().dedicated_only_settings(),
            Vec::<&str>::new()
        );
        let setters: [(&str, Setter); 11] = [
            ("bind", |c| c.bind = "127.0.0.1:0".to_owned()),
            ("max_connections", |c| c.max_connections = 1),
            ("concurrency_limit_per_connection", |c| {
                c.concurrency_limit_per_connection = 1;
            }),
            ("max_concurrent_streams", |c| c.max_concurrent_streams = 1),
            ("http2_max_local_error_reset_streams", |c| {
                c.http2_max_local_error_reset_streams = 1;
            }),
            ("tcp_nodelay", |c| c.tcp_nodelay = false),
            ("tcp_keepalive_ms", |c| c.tcp_keepalive_ms = 1),
            ("http2_keepalive_interval_ms", |c| {
                c.http2_keepalive_interval_ms = 1;
            }),
            ("http2_keepalive_timeout_ms", |c| {
                c.http2_keepalive_timeout_ms = 1;
            }),
            ("max_connection_age_ms", |c| c.max_connection_age_ms = 1),
            ("tls.handshake_timeout_ms", |c| {
                c.tls.handshake_timeout_ms = 1;
            }),
        ];
        for (key, set) in setters {
            let mut config = GrpcConfig::default();
            set(&mut config);
            assert_eq!(config.dedicated_only_settings(), [key]);
        }
        let mut config = GrpcConfig::default();
        " ".clone_into(&mut config.bind);
        config.timeout_ms = 5; // applies in both modes
        assert!(config.dedicated_only_settings().is_empty(), "blank bind");
    }
}