test_infra: Make guest ID generation multiprocess safe

When using nextest for running tests each test is run in its own process
so the old solution of using a static variable for the guest ID (used to
determine the network segment) no longer works.

Instead use a text file on the filesystem protected with an exclusive
lock. The test process will read from it and then write back the next ID
that can be used. It wraps around at the limit of u8 and skips ID 0.

This function intentionally panics rather than propagate errors as it
should only be called for testing purposes and there the panic handler
will give a useful backtrace and cleanup.

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

test_infra/src/lib.rs

@@ -7,14 +7,14 @@
 
 use std::ffi::OsStr;
 use std::fmt::Display;
-use std::io::{Read, Write};
+use std::fs::OpenOptions;
+use std::io::{Read, Seek, SeekFrom, Write};
 use std::net::{TcpListener, TcpStream};
 use std::os::unix::fs::PermissionsExt;
 use std::os::unix::io::{AsRawFd, FromRawFd};
 use std::path::Path;
 use std::process::{Child, Command, ExitStatus, Output, Stdio};
 use std::str::FromStr;
-use std::sync::{LazyLock, Mutex};
 use std::time::Duration;
 use std::{env, fmt, fs, io, thread};
 
@@ -849,14 +849,45 @@ pub fn kill_child(child: &mut Child) {
 
 pub const PIPE_SIZE: i32 = 32 << 20;
 
-static NEXT_VM_ID: LazyLock<Mutex<u8>> = LazyLock::new(|| Mutex::new(1));
-
 pub struct Guest {
     pub tmp_dir: TempDir,
     pub disk_config: Box<dyn DiskConfig>,
     pub network: GuestNetworkConfig,
 }
 
+// Return the next id that can be used for this guest. This is stored in a
+// file in the filesystem and is protected by a filesystem lock allowing
+// multiple test processes to safely access it with the process blocking
+// until the lock is released.
+fn next_guest_id() -> u8 {
+    let mut id_file_path = dirs::home_dir().unwrap();
+    id_file_path.push("workloads");
+    id_file_path.push("id.counter");
+
+    let mut id_file = OpenOptions::new()
+        .create(true)
+        .truncate(false)
+        .write(true)
+        .read(true)
+        .open(id_file_path)
+        .unwrap();
+
+    id_file.lock().unwrap();
+
+    // Use a string in the file for (human) readability
+    let mut buf = String::default();
+    id_file.read_to_string(&mut buf).unwrap();
+    let id = buf.trim().parse::<u8>().unwrap_or(1);
+    let next_id = u8::max(1, id.overflowing_add(1).0);
+    id_file.set_len(0).unwrap();
+    id_file.seek(SeekFrom::Start(0)).unwrap();
+    id_file.write_all(next_id.to_string().as_bytes()).unwrap();
+
+    id_file.unlock().unwrap();
+
+    id
+}
+
 // Safe to implement as we know we have no interior mutability
 impl std::panic::RefUnwindSafe for Guest {}
 
@@ -890,11 +921,7 @@ impl Guest {
     }
 
     pub fn new(disk_config: Box<dyn DiskConfig>) -> Self {
-        let mut guard = NEXT_VM_ID.lock().unwrap();
-        let id = *guard;
-        *guard = id + 1;
-
-        Self::new_from_ip_range(disk_config, "192.168", id)
+        Self::new_from_ip_range(disk_config, "192.168", next_guest_id())
     }
 
     pub fn default_net_string(&self) -> String {