tests: never reuse an adapter sandbox's name within a test process

tests/runtime-adapter/README.md

@@ -2564,6 +2564,11 @@ greeting and answer of the VM tests' listener client on Linux and Windows.
 On Linux it also checks that a self-connection leaves the guest's publication
 port bindable and that a connection reset before inspection is retried.
 
+`tests/sandbox_names.rs` checks, in the host-only configuration, that a test
+process never gives a VM test's sandbox the name of one that it dropped, since
+Windows refuses to create a directory under the name of one that another test
+is still removing.
+
 `tests/staged_images.rs` checks, in the host-only configuration, on Linux and
 Windows:
 

tests/runtime-adapter/tests/common/mod.rs

@@ -21,10 +21,11 @@ use std::error::Error;
 use std::ffi::{OsStr, OsString};
 use std::fs::{self, File};
 use std::io::{self, Read, Write};
+use std::iter;
 use std::net::{Shutdown, SocketAddr, TcpStream};
 use std::path::{Path, PathBuf};
 use std::process::{Child, Command, Stdio};
-use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
 use std::thread;
 use std::time::{Duration, Instant};
 
@@ -45,6 +46,8 @@ pub(crate) const SCRATCH_TEMPLATE: &str = "ubuntu-smoke-scratch.ext4";
 const COMMAND_TIMEOUT: Duration = Duration::from_secs(180);
 /// How much of the end of the VMM's log a failure shows.
 const LOG_TAIL: usize = 8192;
+/// The number of the next sandbox of the test process.
+static NEXT_SANDBOX: AtomicUsize = AtomicUsize::new(0);
 
 /// The `KEY=VALUE` lines that a command of the host program printed.
 pub(crate) type Printed = BTreeMap<String, String>;
@@ -150,11 +153,12 @@ impl Drop for Started {
 impl Sandbox {
     /// Creates the directory of the test `name`, in the system's temporary directory, so that the
     /// path of a Linux control socket in it stays short. A directory that an earlier test process
-    /// kept stays as it is.
+    /// kept stays as it is. Each sandbox of the process takes a name of its own, since Windows
+    /// refuses to create a directory under the name of one that another test is still removing.
     pub(crate) fn new(name: &str) -> Self {
         let pid = std::process::id();
-        let root = (0..)
-            .map(|attempt| env::temp_dir().join(format!("ra-{pid}-{attempt}")))
+        let root = iter::repeat_with(|| NEXT_SANDBOX.fetch_add(1, Ordering::Relaxed))
+            .map(|number| env::temp_dir().join(format!("ra-{pid}-{number}")))
             .find(|root| match fs::create_dir(root) {
                 Ok(()) => true,
                 Err(error) if error.kind() == io::ErrorKind::AlreadyExists => false,

tests/runtime-adapter/tests/sandbox_names.rs

@@ -0,0 +1,23 @@
+//! Regressions for the names that the VM tests give their sandboxes.
+#![cfg(feature = "host")]
+
+mod common;
+
+use common::Sandbox;
+
+/// Windows refuses to create a directory under the name of one that another test of the process is
+/// still removing, so a sandbox never takes the name of one that the process dropped, even for a
+/// test of the same name.
+#[test]
+fn a_process_never_reuses_the_name_of_a_dropped_sandbox() {
+    let dropped = Sandbox::new("names");
+    let name = dropped.root.clone();
+    drop(dropped);
+    assert!(
+        !name.exists(),
+        "the dropped sandbox kept {}",
+        name.display()
+    );
+    let next = Sandbox::new("names");
+    assert_ne!(next.root, name);
+}