feat: rewrite envd guest agent in Rust (envd-rs)

Complete Rust rewrite of the Go envd guest daemon that runs as PID 1
inside Firecracker microVMs. Feature-complete across all 8 phases:

- Health, metrics, and env var endpoints
- Crypto (SHA-256/512, HMAC), auth (secure token, signing), init/snapshot
- Connect RPC via connectrpc + buffa (process + filesystem services)
- File transfer (GET/POST /files) with gzip, multipart, chown, ENOSPC
- Port subsystem (/proc/net/tcp scanner, socat forwarder)
- Cgroup2 manager with noop fallback
- Snapshot/restore lifecycle (conntracker, port subsystem stop/restart)
- SIGTERM graceful shutdown, --cmd initial process spawn
- MMDS metadata polling for Firecracker mode

42 source files, ~4200 LOC, 4.1MB stripped release binary.
Makefile updated: build-envd now targets Rust (musl static),
build-envd-go preserved for Go builds.

envd-rs/src/host/mmds.rs

@@ -0,0 +1,113 @@
+use std::sync::Arc;
+use std::time::Duration;
+
+use dashmap::DashMap;
+use serde::Deserialize;
+use tokio_util::sync::CancellationToken;
+
+use crate::config::{MMDS_ADDRESS, MMDS_POLL_INTERVAL, MMDS_TOKEN_EXPIRATION_SECS, WRENN_RUN_DIR};
+
+#[derive(Debug, Clone, Deserialize)]
+pub struct MMDSOpts {
+    #[serde(rename = "instanceID")]
+    pub sandbox_id: String,
+    #[serde(rename = "envID")]
+    pub template_id: String,
+    #[serde(rename = "address")]
+    pub logs_collector_address: String,
+    #[serde(rename = "accessTokenHash", default)]
+    pub access_token_hash: String,
+}
+
+async fn get_mmds_token(client: &reqwest::Client) -> Result<String, String> {
+    let resp = client
+        .put(format!("http://{MMDS_ADDRESS}/latest/api/token"))
+        .header(
+            "X-metadata-token-ttl-seconds",
+            MMDS_TOKEN_EXPIRATION_SECS.to_string(),
+        )
+        .send()
+        .await
+        .map_err(|e| format!("mmds token request failed: {e}"))?;
+
+    let token = resp.text().await.map_err(|e| format!("mmds token read: {e}"))?;
+    if token.is_empty() {
+        return Err("mmds token is an empty string".into());
+    }
+    Ok(token)
+}
+
+async fn get_mmds_opts(client: &reqwest::Client, token: &str) -> Result<MMDSOpts, String> {
+    let resp = client
+        .get(format!("http://{MMDS_ADDRESS}"))
+        .header("X-metadata-token", token)
+        .header("Accept", "application/json")
+        .send()
+        .await
+        .map_err(|e| format!("mmds opts request failed: {e}"))?;
+
+    resp.json::<MMDSOpts>()
+        .await
+        .map_err(|e| format!("mmds opts parse: {e}"))
+}
+
+pub async fn get_access_token_hash() -> Result<String, String> {
+    let client = reqwest::Client::builder()
+        .timeout(Duration::from_secs(10))
+        .no_proxy()
+        .build()
+        .map_err(|e| format!("http client: {e}"))?;
+
+    let token = get_mmds_token(&client).await?;
+    let opts = get_mmds_opts(&client, &token).await?;
+    Ok(opts.access_token_hash)
+}
+
+/// Polls MMDS every 50ms until metadata is available.
+/// Stores sandbox_id and template_id in env_vars and writes to /run/wrenn/ files.
+pub async fn poll_for_opts(
+    env_vars: Arc<DashMap<String, String>>,
+    cancel: CancellationToken,
+) -> Option<MMDSOpts> {
+    let client = reqwest::Client::builder()
+        .no_proxy()
+        .build()
+        .ok()?;
+
+    let mut interval = tokio::time::interval(MMDS_POLL_INTERVAL);
+
+    loop {
+        tokio::select! {
+            _ = cancel.cancelled() => {
+                tracing::warn!("context cancelled while waiting for mmds opts");
+                return None;
+            }
+            _ = interval.tick() => {
+                let token = match get_mmds_token(&client).await {
+                    Ok(t) => t,
+                    Err(e) => {
+                        tracing::debug!(error = %e, "mmds token poll");
+                        continue;
+                    }
+                };
+
+                let opts = match get_mmds_opts(&client, &token).await {
+                    Ok(o) => o,
+                    Err(e) => {
+                        tracing::debug!(error = %e, "mmds opts poll");
+                        continue;
+                    }
+                };
+
+                env_vars.insert("WRENN_SANDBOX_ID".into(), opts.sandbox_id.clone());
+                env_vars.insert("WRENN_TEMPLATE_ID".into(), opts.template_id.clone());
+
+                let run_dir = std::path::Path::new(WRENN_RUN_DIR);
+                let _ = std::fs::write(run_dir.join(".WRENN_SANDBOX_ID"), &opts.sandbox_id);
+                let _ = std::fs::write(run_dir.join(".WRENN_TEMPLATE_ID"), &opts.template_id);
+
+                return Some(opts);
+            }
+        }
+    }
+}