block: qcow: Test freed clusters stay on the free list

After churning a single L2 region with an fsync per write, scan the file
for refcount==0 clusters and assert every one is on the runtime free
list. The reuse bug left relocated refcount block clusters free on disk
but off the list, so this fails without the fix, reporting the exact
count of stranded clusters.

Assisted-by: Claude:Opus-4.8
Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

block/src/formats/qcow/engine_sync.rs

@@ -516,6 +516,37 @@ mod unit_tests {
         }
     }
 
+    // Every refcount==0 cluster in the file must be on the runtime free list.
+    // The bug left relocated refcount-block clusters free on disk yet absent
+    // from the list, so the allocator never reused them.
+    #[test]
+    fn freed_clusters_are_tracked_in_free_list() {
+        const CL: u64 = 65536;
+        let virtual_size = 512 * 1024 * 1024;
+        let (temp, disk) = create_disk_with_data(virtual_size, &[], 0, false, false);
+        let n: u64 = 400;
+
+        for i in 0..n {
+            let pattern = vec![(i as u8).wrapping_add(1); CL as usize];
+            async_write(&disk, i * CL, &pattern);
+            async_fsync(&disk);
+        }
+
+        let file_clusters = temp.as_file().metadata().unwrap().len() / CL;
+        let mut free_on_disk = 0u64;
+        for c in 0..file_clusters {
+            if disk.metadata().cluster_refcount(c * CL).unwrap() == 0 {
+                free_on_disk += 1;
+            }
+        }
+        let tracked = disk.metadata().free_list_len() as u64;
+        assert_eq!(
+            free_on_disk, tracked,
+            "{free_on_disk} free clusters on disk but {tracked} tracked; \
+             relocated clusters are stranded off the free list",
+        );
+    }
+
     #[test]
     fn test_qcow_sync_rejects_out_of_bounds_allocated_l2_entry_on_read() {
         let data = vec![0x5a; 4096];

block/src/formats/qcow/metadata.rs

@@ -357,6 +357,12 @@ impl QcowMetadata {
         self.inner.read().unwrap().header.clone()
     }
 
+    #[cfg(test)]
+    pub fn free_list_len(&self) -> usize {
+        let inner = self.inner.read().unwrap();
+        inner.avail_clusters.len() + inner.unref_clusters.len()
+    }
+
     #[cfg(test)]
     pub fn cluster_refcount(&self, address: u64) -> io::Result<u64> {
         let mut inner = self.inner.write().unwrap();