test_infra: Prepare QCOW2 overlay for Windows guests

WindowsDiskConfig now creates a qcow2 overlay backed by the raw
Windows image during prepare_files(). The overlay is placed under
~/workloads alongside the raw image. Writes go into the overlay
so the backing raw image stays unmodified, matching the CoW
semantics already provided by the dm snapshot for raw tests.

Drop removes the qcow2 file.

The DiskConfig trait gains a qcow2_disk() default method returning
None. WindowsDiskConfig overrides it to expose the overlay path.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

test_infra/src/lib.rs

@@ -226,6 +226,9 @@ pub trait DiskConfig {
     fn prepare_files(&mut self, tmp_dir: &TempDir, network: &GuestNetworkConfig);
     fn prepare_cloudinit(&self, tmp_dir: &TempDir, network: &GuestNetworkConfig) -> String;
     fn disk(&self, disk_type: DiskType) -> Option<String>;
+    fn qcow2_disk(&self) -> Option<String> {
+        None
+    }
 }
 
 #[derive(Clone)]
@@ -248,6 +251,7 @@ impl UbuntuDiskConfig {
 pub struct WindowsDiskConfig {
     image_name: String,
     osdisk_path: String,
+    osdisk_qcow2_path: String,
     loopback_device: String,
     windows_snapshot_cow: String,
     windows_snapshot: String,
@@ -258,6 +262,7 @@ impl WindowsDiskConfig {
         WindowsDiskConfig {
             image_name,
             osdisk_path: String::new(),
+            osdisk_qcow2_path: String::new(),
             loopback_device: String::new(),
             windows_snapshot_cow: String::new(),
             windows_snapshot: String::new(),
@@ -286,6 +291,10 @@ impl Drop for WindowsDiskConfig {
             .args(["-d", self.loopback_device.as_str()])
             .output()
             .expect("Expect removing loopback device to succeed");
+
+        if !self.osdisk_qcow2_path.is_empty() {
+            let _ = fs::remove_file(&self.osdisk_qcow2_path);
+        }
     }
 }
 
@@ -451,7 +460,7 @@ impl DiskConfig for WindowsDiskConfig {
         let mut osdisk_path = workload_path;
         osdisk_path.push(&self.image_name);
 
-        let osdisk_blk_size = fs::metadata(osdisk_path)
+        let osdisk_blk_size = fs::metadata(&osdisk_path)
             .expect("Expect retrieving Windows image metadata")
             .len()
             >> 9;
@@ -530,6 +539,27 @@ impl DiskConfig for WindowsDiskConfig {
         self.osdisk_path = format!("/dev/mapper/{windows_snapshot}");
         self.windows_snapshot_cow = windows_snapshot_cow;
         self.windows_snapshot = windows_snapshot;
+
+        // Create a qcow2 overlay backed by the raw image.
+        let mut workload_path = dirs::home_dir().unwrap();
+        workload_path.push("workloads");
+        let qcow2_name = format!("windows-qcow2-{}.qcow2", random_extension.to_str().unwrap());
+        let qcow2_path = workload_path.join(&qcow2_name);
+        let output = Command::new("qemu-img")
+            .args([
+                "create",
+                "-f",
+                "qcow2",
+                "-b",
+                osdisk_path.to_str().unwrap(),
+                "-F",
+                "raw",
+                qcow2_path.to_str().unwrap(),
+            ])
+            .output()
+            .expect("Expect creating qcow2 overlay to succeed");
+        assert!(output.status.success(), "qemu-img create failed");
+        self.osdisk_qcow2_path = qcow2_path.to_str().unwrap().to_string();
     }
 
     fn disk(&self, disk_type: DiskType) -> Option<String> {
@@ -538,6 +568,10 @@ impl DiskConfig for WindowsDiskConfig {
             DiskType::CloudInit => None,
         }
     }
+
+    fn qcow2_disk(&self) -> Option<String> {
+        Some(self.osdisk_qcow2_path.clone())
+    }
 }
 
 pub fn rate_limited_copy<P: AsRef<Path>, Q: AsRef<Path>>(from: P, to: Q) -> io::Result<u64> {