fix: dynamic QEMU path resolution (#198)

Prior to this commit, it is assumed that the QEMU binary resides within
the canonical Linux path. However, certain systems like NixOS may place
QEMU in a local user folder instead of the global `/usr/bin` path,
causing an error when attempting to locate the QEMU binary.

This change incorporates a `which` command, which searches through the
current `PATH` environment variable and identifies the location of the
QEMU binary.

It also creates a new CLI argument `qemu_path`.

crates/node/Cargo.toml

@@ -27,6 +27,7 @@ thiserror = "1"
 tokio = { version = "1", features = ["full", "io-util", "tracing"] }
 tracing = "0.1"
 uuid = { version = "1", features = [ "v4", "fast-rng", "macro-diagnostics", "serde" ] }
+which = "6.0"
 
 bytes = { version = "1.5", optional = true }
 clap = { version = "4", features = ["derive", "env", "string"], optional = true }

crates/node/src/cli.rs

@@ -141,6 +141,34 @@ pub struct Config {
         env = "GEVULOT_METRICS_LISTEN_ADDR"
     )]
     pub http_metrics_listen_addr: Option<SocketAddr>,
+
+    #[arg(
+        long,
+        long_help = "QEMU path to system-x86_64 runner",
+        env = "GEVULOT_QEMU_PATH",
+        default_value = "qemu-system-x86_64"
+    )]
+    pub qemu_path: Option<PathBuf>,
+}
+
+impl Config {
+    /// Resolves the provided QEMU path.
+    ///
+    /// # Panics
+    ///
+    /// The absence of a valid QEMU runnner is an unrecoverable error.
+    pub fn resolve_qemu_path(&self) -> PathBuf {
+        self.qemu_path
+            .as_ref()
+            .map(which::which)
+            .transpose()
+            .ok()
+            .flatten()
+            .filter(|p| p.is_file())
+            .expect(
+                "a valid QEMU binary must be provided. Check `--qemu_path` or `GEVULOT_QEMU_PATH`.",
+            )
+    }
 }
 
 #[derive(Debug, Args)]

crates/node/src/rpc_server/mod.rs

@@ -495,6 +495,7 @@ mod tests {
             http_download_port: 0,
             http_healthcheck_listen_addr: "127.0.0.1:8888".parse().unwrap(),
             http_metrics_listen_addr: None,
+            qemu_path: None,
         });
 
         let db = Arc::new(Database::new(&cfg.db_url).await.unwrap());

crates/node/src/vmm/qemu.rs

@@ -13,7 +13,7 @@ use std::{
     any::Any,
     collections::{BTreeSet, HashMap},
     fs::File,
-    path::Path,
+    path::{Path, PathBuf},
     process::{Child, Command, Stdio},
     sync::{
         atomic::{AtomicU32, Ordering},
@@ -71,15 +71,19 @@ pub struct Qemu {
     next_cid: AtomicU32,
     cid_allocations: BTreeSet<u32>,
     vm_registry: HashMap<u32, QEMUVMHandle>,
+    qemu_path: PathBuf,
 }
 
 impl Qemu {
     pub fn new(config: Arc<Config>) -> Self {
+        let qemu_path = config.resolve_qemu_path();
+
         Qemu {
             config,
             next_cid: AtomicU32::new(4),
             cid_allocations: Default::default(),
             vm_registry: HashMap::new(),
+            qemu_path,
         }
     }
 
@@ -189,7 +193,9 @@ impl Provider for Qemu {
 
         // Update the child process field.
         let qemu_vm_handle = &mut self.vm_registry.get_mut(&cid).unwrap();
-        let mut cmd = Command::new("/usr/bin/qemu-system-x86_64");
+
+        // Resolve the QEMU system command
+        let mut cmd = Command::new(&self.qemu_path);
 
         //define the VirtFs local path and create all necessary folder
         let workspace_path = TaskVmFile::get_workspace_path(&self.config.data_directory, tx_hash);

flake.nix

@@ -20,6 +20,7 @@
               buildInputs =
               [
                 p.openssl
+                p.ops
                 p.pkg-config
                 p.protobuf
               ];