hyperhive/hive-c0re/src/agent_config/mod.rs

//! Per-agent configuration registries: resource limits, topology (JSON files
//! under `/var/lib/hyperhive/meta/`), the permissions the swarm grants
//! (outside the meta repo) and the shared wire-protocol size limits. Each
//! submodule is re-exported at the crate root, so `crate::topology::…` etc.
//! keep working unchanged.
//!
//! Note the two similarly-named modules: [`limits`] caps inline
//! *message body* sizes on the sockets, while [`resource_limits`] holds
//! per-agent CPU/memory caps for the container drop-in.

pub mod limits;
pub mod permissions;
pub mod resource_limits;
pub mod topology;

use std::io::{self, Write as _};
use std::path::Path;

use serde::Serialize;
use serde::de::DeserializeOwned;

/// Read one of these JSON registries. A missing file is `T::default()`
/// (the empty map or set); any other read failure, or content that
/// doesn't parse, is an error, so a read-modify-write can't write back a
/// registry that has lost every other agent's entry.
fn read_map<T: DeserializeOwned + Default>(path: &Path) -> io::Result<T> {
    let raw = match std::fs::read_to_string(path) {
        Ok(raw) => raw,
        Err(e) if e.kind() == io::ErrorKind::NotFound => return Ok(T::default()),
        Err(e) => {
            return Err(io::Error::new(
                e.kind(),
                format!("read {}: {e}", path.display()),
            ));
        }
    };
    serde_json::from_str(&raw).map_err(|e| {
        io::Error::new(
            io::ErrorKind::InvalidData,
            format!("parse {}: {e}", path.display()),
        )
    })
}

/// Serialise `value` as pretty JSON and replace `path` with it atomically:
/// a temp file in the same directory (`rename(2)` is only atomic within a
/// filesystem), fsynced, renamed over `path`, then the directory fsynced
/// so the rename itself survives a crash. A reader sees the old file or
/// the new one, never a truncated one.
fn write_map<T: Serialize + ?Sized>(path: &Path, value: &T) -> io::Result<()> {
    let text = serde_json::to_string_pretty(value)
        .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))?;
    let (Some(dir), Some(name)) = (path.parent(), path.file_name()) else {
        return Err(io::Error::new(
            io::ErrorKind::InvalidInput,
            format!("{} has no parent directory or file name", path.display()),
        ));
    };
    std::fs::create_dir_all(dir)?;
    let tmp = dir.join(format!(".{}.tmp", name.to_string_lossy()));
    let mut file = std::fs::File::create(&tmp)?;
    file.write_all(format!("{text}\n").as_bytes())?;
    file.sync_all()?;
    drop(file);
    std::fs::rename(&tmp, path)?;
    std::fs::File::open(dir)?.sync_all()
}

#[cfg(test)]
mod tests {
    use std::collections::BTreeMap;

    use super::*;

    type Map = BTreeMap<String, Vec<String>>;

    #[test]
    fn write_map_round_trips_and_leaves_no_temp_file() {
        let dir = tempfile::tempdir().expect("tempdir");
        let path = dir.path().join("map.json");
        let map = BTreeMap::from([("alice".to_owned(), vec!["messaging".to_owned()])]);
        write_map(&path, &map).expect("first write");
        let replaced = BTreeMap::from([("bob".to_owned(), vec!["inbox".to_owned()])]);
        write_map(&path, &replaced).expect("replacing write");
        assert_eq!(read_map::<Map>(&path).expect("read"), replaced);
        let entries: Vec<_> = std::fs::read_dir(dir.path())
            .expect("read_dir")
            .map(|e| e.expect("entry").file_name())
            .collect();
        assert_eq!(entries, vec![std::ffi::OsString::from("map.json")]);
    }

    #[test]
    fn read_map_of_a_missing_file_is_empty() {
        let dir = tempfile::tempdir().expect("tempdir");
        let map =
            read_map::<Map>(&dir.path().join("absent.json")).expect("missing file is not an error");
        assert!(map.is_empty());
    }

    #[test]
    fn read_map_of_a_truncated_file_is_an_error() {
        let dir = tempfile::tempdir().expect("tempdir");
        let path = dir.path().join("map.json");
        std::fs::write(&path, "{\n  \"alice\": [\"messag").expect("seed");
        let err = read_map::<Map>(&path).expect_err("truncated JSON must not read as a map");
        assert_eq!(err.kind(), io::ErrorKind::InvalidData);
    }
}