mcpmesh-node 0.7.0

//! The `run` backend: one child MCP server process per session.
//! Its stdio is pumped Value/semantic-faithfully to/from the QUIC-framed transport,
//! the resolved caller identity is injected as environment variables, the child is
//! killed when the session closes, and concurrent spawns are bounded per service.
//!
//! The platform PUMPS, it never INTERPRETS: frames move across as JSON `Value`s
//! (one codec everywhere; the child's stdout MCP-stdio framing IS our wire framing —
//! newline-delimited compact JSON), and nothing here inspects the child's MCP
//! method/result semantics. Fidelity is Value/semantic, not byte-for-byte: each frame
//! re-serializes through `serde_json::Value` (keys re-sorted, no arbitrary_precision),
//! the shared property with the mesh transport — see [`super`].
//!
//! Identity reaches a `run` child as ENV, not `_meta`: `MCPMESH_PEER_NAME`
//! (always, when resolved), `MCPMESH_PEER_USER` (the self-sovereign user_id — set
//! in roster mode, and in pairing mode once a device->user binding is verified),
//! and `MCPMESH_PEER_GROUPS` (comma-joined). It arrives PER-CALLER through `run`
//! (`Option<PeerIdentity>`), not as a construction field: `serve` builds each
//! backend once per service and reuses it across all callers, so the injected
//! identity cannot be baked in. `select_service` already stripped the
//! caller's reserved `mcpmesh/*` `_meta` keys upstream, so the forwarded
//! `initialize` is clean before it reaches the child.
use std::process::Stdio;

use anyhow::{Context, Result};
use mcpmesh_net::errors::synthesized_limited;
use mcpmesh_net::transport::NdjsonTransport;
use mcpmesh_net::{PeerIdentity, SessionBackend, SessionTransport};
use serde_json::Value;
use tokio::io::{AsyncRead, AsyncWrite};
use tokio::process::Command;
use tokio::sync::Semaphore;

use crate::audit::RequestAuditor;

/// The `retry_after_ms` hint on a concurrency-cap refusal. Unlike a token bucket there is
/// no exact refill instant — a held permit frees when a peer session ends — so a fixed ~1s nudge.
const CONCURRENCY_RETRY_MS: u64 = 1000;

/// The `run` backend for one registered service. `cmd` is an argv vector executed
/// directly — NEVER through a shell. `concurrency` is the per-service
/// spawn cap (default 4), shared across the sessions of one service. The
/// caller identity is NOT a field — it is threaded per-session through
/// [`SessionBackend::run`] (this backend is shared across callers).
pub struct SpawnBackend {
    pub cmd: Vec<String>,
    pub concurrency: std::sync::Arc<Semaphore>,
    /// This service's name (the registry key) — recorded as `service` in audit records.
    pub service: String,
    /// The audit sink. `AuditSink::disabled()` in tests / a non-audited build.
    pub audit: crate::audit::AuditSink,
    /// The per-authenticated-endpoint request limiter, shared across all backends.
    /// Consulted per proxied request line in `super::pump`. Keyed on `identity.endpoint`.
    pub limiter: std::sync::Arc<crate::limits::RateLimiter>,
}

#[async_trait::async_trait]
impl SessionBackend for SpawnBackend {
    /// Drive one mesh session against a freshly spawned child. The concrete
    /// `SessionTransport` is iroh-typed; the spawn + pump body lives in the
    /// transport-generic [`SpawnBackend::run_over`] so it is exercisable over an
    /// in-memory pipe in tests. Both paths run identical logic.
    async fn run(
        &self,
        identity: Option<PeerIdentity>,
        initialize: Value,
        transport: SessionTransport,
    ) -> anyhow::Result<()> {
        self.run_over(identity, initialize, transport).await
    }
}

impl SpawnBackend {
    /// The transport-generic core of [`SessionBackend::run`]: acquire a spawn
    /// permit, launch the child with the identity env, then pump frames both ways
    /// until either side ends. Generic over the transport's byte substrate so the
    /// real path (iroh streams) and the test path (`tokio::io::duplex`) share one
    /// implementation — the concrete `SessionTransport` alias would otherwise
    /// forbid a hermetic in-memory test.
    ///
    /// The concurrency permit is held for the whole session (`_permit` lives until
    /// this future returns). On acquisition failure the backend fully HANDLES the
    /// session: it synthesizes a `-32053` refusal frame on the transport (id echoed
    /// from the `initialize`), flushes it via `shutdown()`, and returns `Ok(())` — a
    /// clean, load-triggered refusal, NOT a session error (mirrors the -32054
    /// `ServiceDecision::Refuse` path, which also returns `Ok` so `serve`'s session
    /// task does not `warn!("session ended with error")` for a normal refusal). The
    /// caller sees a well-formed rate-limit answer, not a hang. `retry_after_ms` is
    /// a token-bucket concept, omitted here.
    pub async fn run_over<R, W>(
        &self,
        identity: Option<PeerIdentity>,
        initialize: Value,
        mut transport: NdjsonTransport<R, W>,
    ) -> Result<()>
    where
        R: AsyncRead + Send + Unpin,
        W: AsyncWrite + Send + Unpin,
    {
        let _permit = match self.concurrency.clone().try_acquire_owned() {
            Ok(permit) => permit,
            Err(_) => {
                // Cap reached: answer the caller with a well-formed -32053{retry_after_ms} and close
                // cleanly (FAIL-SAFE deny — bounds a spawn-bomb). Best-effort writes; returning Ok
                // keeps this off `serve`'s session-error log.
                let id = initialize.get("id").cloned().unwrap_or(Value::Null);
                let _ = transport
                    .send_value(synthesized_limited(id, CONCURRENCY_RETRY_MS))
                    .await;
                let _ = transport.shutdown().await;
                return Ok(());
            }
        };

        let (program, args) = self.cmd.split_first().context("run backend cmd is empty")?;
        let mut command = Command::new(program);
        command
            .args(args)
            .stdin(Stdio::piped())
            .stdout(Stdio::piped())
            .stderr(Stdio::inherit()) // child diagnostics flow to the daemon's stderr
            .kill_on_drop(true); // session close (this future returning) kills the child
        if let Some(id) = &identity {
            command.env("MCPMESH_PEER_NAME", &id.name);
            if let Some(user) = &id.user_id {
                command.env("MCPMESH_PEER_USER", user);
            }
            command.env("MCPMESH_PEER_GROUPS", id.groups.join(","));
        }

        let mut child = command
            .spawn()
            .with_context(|| format!("spawn run backend child `{program}`"))?;
        let child_stdin = child.stdin.take().expect("stdin piped above");
        let child_stdout = child.stdout.take().expect("stdout piped above");

        // Session lifecycle audit: attribute to the gate-resolved identity (roster
        // user_id if present, else nickname). The cap-refusal path above returns before this, so a
        // refused session is not recorded as opened.
        let peer = identity
            .as_ref()
            .map(|id| id.user_id.clone().unwrap_or_else(|| id.name.clone()));
        // Session lifecycle via the RAII guard: it emits `session_open` now and, on drop (every exit
        // path — EOF, error, panic), emits `session_close` and removes the live-table row. Held for
        // the whole session scope, so it MUST outlive the pump below.
        let _session = self
            .audit
            .session(peer.clone().unwrap_or_default(), self.service.clone());
        let auditor = RequestAuditor::new(self.audit.clone(), peer.clone(), self.service.clone());

        // Pump the two directions concurrently until either side EOFs/closes (shared
        // with the socket backend — one codec, one pump, no full-duplex deadlock); the
        // child then drops here (kill_on_drop fires) and the permit is released on
        // EVERY exit path.
        let outcome = super::pump(
            initialize,
            &mut transport,
            child_stdout,
            child_stdin,
            auditor,
            crate::limits::RateGate::new(
                self.limiter.clone(),
                identity.as_ref().map(|i| i.endpoint),
            ),
        )
        .await;
        // `_session` drops here (or on any early return above), emitting `session_close`.
        outcome
    }
}