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/tests/config.rs

@@ -0,0 +1,118 @@
+//! Tests for `brokerd`'s configuration. Do not edit: these define the required behaviour.
+
+use brokerd::config::{Approvals, Config, ConfigError, Sockets};
+use std::path::{Path, PathBuf};
+
+fn fixture(name: &str) -> PathBuf {
+    Path::new(env!("CARGO_MANIFEST_DIR"))
+        .join("tests/fixtures/config")
+        .join(name)
+}
+
+/// What `home` must default to in this process. The test does not set the variable: changing the
+/// environment of a running test binary would race with the other tests.
+fn default_home() -> PathBuf {
+    std::env::var_os("BOXMAKER_HOME")
+        .map(PathBuf::from)
+        .unwrap_or_else(|| PathBuf::from("/var/lib/boxmaker"))
+}
+
+#[test]
+fn an_empty_file_gets_every_default() {
+    let c = Config::load(&fixture("empty.toml")).unwrap();
+    assert_eq!(c.paths.home, default_home());
+    assert_eq!(c.paths.grants, PathBuf::from("/etc/boxmaker/grants"));
+    assert_eq!(c.sockets, Sockets::default());
+    assert_eq!(c.approvals, Approvals { ttl_ms: 900_000 });
+    assert_eq!(Approvals::default(), Approvals { ttl_ms: 900_000 });
+    assert_eq!(c, Config::parse("").unwrap());
+    assert_eq!(c, Config::default());
+}
+
+#[test]
+fn sockets_and_directories_default_to_places_under_home() {
+    let c = Config::load(&fixture("home_only.toml")).unwrap();
+    assert_eq!(c.paths.home, PathBuf::from("/srv/boxmaker"));
+    assert_eq!(
+        c.broker_socket(),
+        PathBuf::from("/srv/boxmaker/run/loop-broker/broker.sock")
+    );
+    assert_eq!(
+        c.admin_socket(),
+        PathBuf::from("/srv/boxmaker/run/owner-broker/admin.sock")
+    );
+    assert_eq!(c.audit_dir(), PathBuf::from("/srv/boxmaker/audit"));
+    assert_eq!(
+        c.state_dir(),
+        PathBuf::from("/srv/boxmaker/broker/sessions")
+    );
+    // The grants are not under home: the owner writes them, brokerd only reads them.
+    assert_eq!(c.paths.grants, PathBuf::from("/etc/boxmaker/grants"));
+}
+
+#[test]
+fn every_key_can_be_set() {
+    let c = Config::load(&fixture("full.toml")).unwrap();
+    assert_eq!(c.paths.home, PathBuf::from("/srv/boxmaker"));
+    assert_eq!(c.paths.grants, PathBuf::from("/srv/boxmaker-grants"));
+    assert_eq!(c.broker_socket(), PathBuf::from("/run/bx/broker.sock"));
+    assert_eq!(c.admin_socket(), PathBuf::from("/run/bx/admin.sock"));
+    assert_eq!(c.approvals.ttl_ms, 60_000);
+    // The two directories always follow home.
+    assert_eq!(c.audit_dir(), PathBuf::from("/srv/boxmaker/audit"));
+    assert_eq!(
+        c.state_dir(),
+        PathBuf::from("/srv/boxmaker/broker/sessions")
+    );
+}
+
+#[test]
+fn one_socket_set_leaves_the_other_at_its_default() {
+    let c =
+        Config::parse("[paths]\nhome = \"/h\"\n[sockets]\nadmin = \"/x/admin.sock\"\n").unwrap();
+    assert_eq!(c.admin_socket(), PathBuf::from("/x/admin.sock"));
+    assert_eq!(
+        c.broker_socket(),
+        PathBuf::from("/h/run/loop-broker/broker.sock")
+    );
+}
+
+#[test]
+fn unknown_keys_and_tables_are_errors() {
+    for name in ["unknown_key.toml", "unknown_table.toml", "wrong_type.toml"] {
+        match Config::load(&fixture(name)) {
+            Err(ConfigError::Parse(path, _)) => assert_eq!(path, fixture(name)),
+            other => panic!("{name}: expected a parse error, got {other:?}"),
+        }
+    }
+    // In every table, not only the one the fixture shows.
+    for text in [
+        "[paths]\nhome = \"/h\"\nhouse = \"/h\"\n",
+        "[sockets]\nbroker = \"/b.sock\"\nloop = \"/l.sock\"\n",
+        "[approvals]\nttl_ms = 1\nttl_s = 1\n",
+        "top = 1\n",
+        "[approvals]\nttl_ms = -5\n",
+    ] {
+        assert!(Config::parse(text).is_err(), "accepted: {text}");
+    }
+}
+
+#[test]
+fn a_missing_file_is_a_read_error_that_names_the_file() {
+    let path = fixture("does-not-exist.toml");
+    match Config::load(&path) {
+        Err(ConfigError::Read(p, _)) => assert_eq!(p, path),
+        other => panic!("expected a read error, got {other:?}"),
+    }
+    let text = Config::load(&path).unwrap_err().to_string();
+    assert!(text.contains("does-not-exist.toml"), "{text}");
+}
+
+#[test]
+fn a_parse_error_names_the_file_and_the_key() {
+    let text = Config::load(&fixture("unknown_key.toml"))
+        .unwrap_err()
+        .to_string();
+    assert!(text.contains("unknown_key.toml"), "{text}");
+    assert!(text.contains("ttl"), "{text}");
+}