Add brokerd's configuration

Implemented crates/brokerd/src/config.rs: typed Paths, Sockets, Approvals and
Config with serde(deny_unknown_fields, default) on every struct, hand-written
ConfigError (Read/Parse) with Display and std::error::Error, and the parse/load/
broker_socket/admin_socket/audit_dir/state_dir methods. Added serde, serde_json
and toml to crates/brokerd/Cargo.toml, registered pub mod config; in lib.rs,
added brokerd to the serde and serde_json Used-by cells in docs/dependencies.md,
and copied the given test and six fixtures byte-identical. 7 config tests pass;
make gate prints gate: ok.

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

crates/brokerd/src/config.rs

@@ -0,0 +1,107 @@
+//! `brokerd` configuration: read `brokerd.toml` into a typed `Config`.
+//!
+//! This is our own format, so unknown keys are errors in every table: a
+//! misspelt key that silently fell back to its default would be a setting the
+//! owner believes is set and is not.
+
+use serde::Deserialize;
+use std::path::{Path, PathBuf};
+
+#[derive(Debug, Clone, PartialEq, Eq, Deserialize)]
+#[serde(deny_unknown_fields, default)]
+pub struct Paths {
+    pub home: PathBuf,
+    pub grants: PathBuf,
+}
+
+impl Default for Paths {
+    fn default() -> Self {
+        Self {
+            home: std::env::var_os("BOXMAKER_HOME")
+                .map(PathBuf::from)
+                .unwrap_or_else(|| PathBuf::from("/var/lib/boxmaker")),
+            grants: PathBuf::from("/etc/boxmaker/grants"),
+        }
+    }
+}
+
+/// An empty path means "the default under `home`".
+#[derive(Debug, Clone, PartialEq, Eq, Deserialize, Default)]
+#[serde(deny_unknown_fields, default)]
+pub struct Sockets {
+    pub broker: PathBuf,
+    pub admin: PathBuf,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Deserialize)]
+#[serde(deny_unknown_fields, default)]
+pub struct Approvals {
+    pub ttl_ms: u64,
+}
+
+impl Default for Approvals {
+    fn default() -> Self {
+        Self { ttl_ms: 900_000 }
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Deserialize, Default)]
+#[serde(deny_unknown_fields)]
+pub struct Config {
+    #[serde(default)]
+    pub paths: Paths,
+    #[serde(default)]
+    pub sockets: Sockets,
+    #[serde(default)]
+    pub approvals: Approvals,
+}
+
+#[derive(Debug)]
+pub enum ConfigError {
+    Read(PathBuf, std::io::Error),
+    Parse(PathBuf, toml::de::Error),
+}
+
+impl std::fmt::Display for ConfigError {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        match self {
+            ConfigError::Read(path, err) => write!(f, "{}: {err}", path.display()),
+            ConfigError::Parse(path, err) => write!(f, "{}: {err}", path.display()),
+        }
+    }
+}
+
+impl std::error::Error for ConfigError {}
+
+impl Config {
+    pub fn parse(text: &str) -> Result<Config, toml::de::Error> {
+        toml::from_str(text)
+    }
+    pub fn load(path: &Path) -> Result<Config, ConfigError> {
+        let text =
+            std::fs::read_to_string(path).map_err(|e| ConfigError::Read(path.to_path_buf(), e))?;
+        let config: Config =
+            toml::from_str(&text).map_err(|e| ConfigError::Parse(path.to_path_buf(), e))?;
+        Ok(config)
+    }
+    pub fn broker_socket(&self) -> PathBuf {
+        if self.sockets.broker.as_os_str().is_empty() {
+            self.paths.home.join("run/loop-broker/broker.sock")
+        } else {
+            self.sockets.broker.clone()
+        }
+    }
+    pub fn admin_socket(&self) -> PathBuf {
+        if self.sockets.admin.as_os_str().is_empty() {
+            self.paths.home.join("run/owner-broker/admin.sock")
+        } else {
+            self.sockets.admin.clone()
+        }
+    }
+    pub fn audit_dir(&self) -> PathBuf {
+        self.paths.home.join("audit")
+    }
+    pub fn state_dir(&self) -> PathBuf {
+        self.paths.home.join("broker/sessions")
+    }
+}

crates/brokerd/src/lib.rs

@@ -1,4 +1,5 @@
 //! The broker: the only role that holds authority.
 
+pub mod config;
 pub mod policy;
 pub mod runner;