Add real stdio MCP process wrapper
Add a minimal runtime stdio MCP launcher that spawns configured server processes with piped stdin/stdout, applies transport env, and exposes async write/read/terminate/wait helpers for future JSON-RPC integration. The wrapper stays intentionally small: it does not yet implement protocol framing or connection lifecycle management, but it is real process orchestration rather than placeholder scaffolding. Tests use a temporary executable script to prove env propagation and bidirectional stdio round-tripping. Constraint: Keep the slice minimal and testable while using the real tokio process surface Constraint: Runtime verification must pass cleanly under fmt, clippy, and tests Rejected: Add full JSON-RPC framing and session orchestration in the same commit | too much scope for a clean launcher slice Rejected: Fake the process wrapper behind mocks only | would not validate spawning, env injection, or stdio wiring Confidence: high Scope-risk: narrow Reversibility: clean Directive: Layer future MCP protocol framing on top of McpStdioProcess rather than bypassing it with ad hoc process management Tested: cargo fmt --all; cargo clippy -p runtime --all-targets -- -D warnings; cargo test -p runtime Not-tested: live third-party MCP servers; long-running process supervision; stderr capture policy
This commit is contained in:
Yeachan-Heo
211
rust/crates/runtime/src/mcp_stdio.rs
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211
rust/crates/runtime/src/mcp_stdio.rs
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use std::collections::BTreeMap;
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use std::io;
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use std::process::Stdio;
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use tokio::io::{AsyncReadExt, AsyncWriteExt};
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use tokio::process::{Child, ChildStdin, ChildStdout, Command};
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use crate::mcp_client::{McpClientBootstrap, McpClientTransport, McpStdioTransport};
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#[derive(Debug)]
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#[allow(dead_code)]
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pub struct McpStdioProcess {
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child: Child,
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stdin: ChildStdin,
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stdout: ChildStdout,
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}
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#[allow(dead_code)]
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impl McpStdioProcess {
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pub fn spawn(transport: &McpStdioTransport) -> io::Result<Self> {
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let mut command = Command::new(&transport.command);
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command
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.args(&transport.args)
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.stdin(Stdio::piped())
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.stdout(Stdio::piped())
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.stderr(Stdio::inherit());
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apply_env(&mut command, &transport.env);
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let mut child = command.spawn()?;
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let stdin = child
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.stdin
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.take()
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.ok_or_else(|| io::Error::other("stdio MCP process missing stdin pipe"))?;
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let stdout = child
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.stdout
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.take()
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.ok_or_else(|| io::Error::other("stdio MCP process missing stdout pipe"))?;
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Ok(Self {
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child,
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stdin,
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stdout,
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})
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}
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pub async fn write_all(&mut self, bytes: &[u8]) -> io::Result<()> {
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self.stdin.write_all(bytes).await
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}
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pub async fn flush(&mut self) -> io::Result<()> {
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self.stdin.flush().await
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}
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pub async fn read_available(&mut self) -> io::Result<Vec<u8>> {
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let mut buffer = vec![0_u8; 4096];
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let read = self.stdout.read(&mut buffer).await?;
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buffer.truncate(read);
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Ok(buffer)
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}
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pub async fn terminate(&mut self) -> io::Result<()> {
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self.child.kill().await
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}
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pub async fn wait(&mut self) -> io::Result<std::process::ExitStatus> {
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self.child.wait().await
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}
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}
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#[allow(dead_code)]
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pub fn spawn_mcp_stdio_process(bootstrap: &McpClientBootstrap) -> io::Result<McpStdioProcess> {
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match &bootstrap.transport {
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McpClientTransport::Stdio(transport) => McpStdioProcess::spawn(transport),
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other => Err(io::Error::new(
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io::ErrorKind::InvalidInput,
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format!(
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"MCP bootstrap transport for {} is not stdio: {other:?}",
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bootstrap.server_name
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),
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)),
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}
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}
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#[allow(dead_code)]
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fn apply_env(command: &mut Command, env: &BTreeMap<String, String>) {
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for (key, value) in env {
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command.env(key, value);
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}
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}
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#[cfg(test)]
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mod tests {
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use std::collections::BTreeMap;
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use std::fs;
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use std::io::ErrorKind;
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use std::os::unix::fs::PermissionsExt;
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use std::path::{Path, PathBuf};
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use std::time::{SystemTime, UNIX_EPOCH};
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use tokio::runtime::Builder;
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use crate::config::{
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ConfigSource, McpServerConfig, McpStdioServerConfig, ScopedMcpServerConfig,
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};
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use crate::mcp_client::McpClientBootstrap;
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use super::{spawn_mcp_stdio_process, McpStdioProcess};
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fn temp_dir() -> PathBuf {
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let nanos = SystemTime::now()
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.duration_since(UNIX_EPOCH)
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.expect("time should be after epoch")
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.as_nanos();
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std::env::temp_dir().join(format!("runtime-mcp-stdio-{nanos}"))
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}
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fn write_echo_script() -> PathBuf {
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let root = temp_dir();
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fs::create_dir_all(&root).expect("temp dir");
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let script_path = root.join("echo-mcp.sh");
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fs::write(
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&script_path,
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"#!/bin/sh\nprintf 'READY:%s\\n' \"$MCP_TEST_TOKEN\"\nIFS= read -r line\nprintf 'ECHO:%s\\n' \"$line\"\n",
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)
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.expect("write script");
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let mut permissions = fs::metadata(&script_path).expect("metadata").permissions();
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permissions.set_mode(0o755);
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fs::set_permissions(&script_path, permissions).expect("chmod");
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script_path
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}
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fn sample_bootstrap(script_path: &Path) -> McpClientBootstrap {
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let config = ScopedMcpServerConfig {
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scope: ConfigSource::Local,
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config: McpServerConfig::Stdio(McpStdioServerConfig {
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command: script_path.to_string_lossy().into_owned(),
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args: Vec::new(),
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env: BTreeMap::from([("MCP_TEST_TOKEN".to_string(), "secret-value".to_string())]),
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}),
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};
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McpClientBootstrap::from_scoped_config("stdio server", &config)
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}
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#[test]
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fn spawns_stdio_process_and_round_trips_io() {
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let runtime = Builder::new_current_thread()
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.enable_all()
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.build()
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.expect("runtime");
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runtime.block_on(async {
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let script_path = write_echo_script();
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let bootstrap = sample_bootstrap(&script_path);
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let mut process = spawn_mcp_stdio_process(&bootstrap).expect("spawn stdio process");
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let ready = process.read_available().await.expect("read ready");
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assert_eq!(String::from_utf8_lossy(&ready), "READY:secret-value\n");
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process
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.write_all(b"ping from client\n")
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.await
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.expect("write input");
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process.flush().await.expect("flush");
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let echoed = process.read_available().await.expect("read echo");
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assert_eq!(String::from_utf8_lossy(&echoed), "ECHO:ping from client\n");
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let status = process.wait().await.expect("wait for exit");
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assert!(status.success());
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fs::remove_file(&script_path).expect("cleanup script");
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fs::remove_dir_all(script_path.parent().expect("script parent")).expect("cleanup dir");
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});
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}
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#[test]
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fn rejects_non_stdio_bootstrap() {
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let config = ScopedMcpServerConfig {
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scope: ConfigSource::Local,
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config: McpServerConfig::Sdk(crate::config::McpSdkServerConfig {
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name: "sdk-server".to_string(),
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}),
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};
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let bootstrap = McpClientBootstrap::from_scoped_config("sdk server", &config);
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let error = spawn_mcp_stdio_process(&bootstrap).expect_err("non-stdio should fail");
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assert_eq!(error.kind(), ErrorKind::InvalidInput);
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}
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#[test]
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fn direct_spawn_uses_transport_env() {
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let runtime = Builder::new_current_thread()
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.enable_all()
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.build()
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.expect("runtime");
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runtime.block_on(async {
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let script_path = write_echo_script();
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let transport = crate::mcp_client::McpStdioTransport {
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command: script_path.to_string_lossy().into_owned(),
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args: Vec::new(),
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env: BTreeMap::from([("MCP_TEST_TOKEN".to_string(), "direct-secret".to_string())]),
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};
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let mut process = McpStdioProcess::spawn(&transport).expect("spawn transport directly");
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let ready = process.read_available().await.expect("read ready");
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assert_eq!(String::from_utf8_lossy(&ready), "READY:direct-secret\n");
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process.terminate().await.expect("terminate child");
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let _ = process.wait().await.expect("wait after kill");
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fs::remove_file(&script_path).expect("cleanup script");
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fs::remove_dir_all(script_path.parent().expect("script parent")).expect("cleanup dir");
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});
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}
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}
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