block: Add unit tests for DISCARD zero flag

Add comprehensive tests for DISCARD and WRITE_ZEROES operations:

QCOW2 zero flag test validates the complete workflow: allocate
cluster, DISCARD it, verify reads return zeros, write new data,
verify cluster reallocated.

QcowSync tests verify punch_hole and write_zeroes with Arc<Mutex<>>
sharing, including tests for cache consistency with multiple async
I/O operations.

RawFileSync tests verify punch_hole and write_zeroes using
fallocate.

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

block/src/raw_sync.rs

@@ -199,3 +199,177 @@ impl AsyncIo for RawFileSync {
         Ok(())
     }
 }
+
+#[cfg(test)]
+mod unit_tests {
+    use std::io::{Read, Seek, SeekFrom, Write};
+
+    use vmm_sys_util::tempfile::TempFile;
+
+    use super::*;
+
+    #[test]
+    fn test_punch_hole() {
+        let temp_file = TempFile::new().unwrap();
+        let mut file = temp_file.into_file();
+
+        // Write 4MB of data
+        let data = vec![0xAA; 4 * 1024 * 1024];
+        file.write_all(&data).unwrap();
+        file.sync_all().unwrap();
+
+        // Create async IO instance
+        let mut async_io = RawFileSync::new(file.as_raw_fd());
+
+        // Punch hole in the middle (1MB at offset 1MB)
+        let offset = 1024 * 1024;
+        let length = 1024 * 1024;
+        async_io.punch_hole(offset, length, 1).unwrap();
+
+        // Check completion
+        let (user_data, result) = async_io.next_completed_request().unwrap();
+        assert_eq!(user_data, 1);
+        assert_eq!(result, 0);
+
+        // Verify the hole reads as zeros
+        file.seek(SeekFrom::Start(offset)).unwrap();
+        let mut read_buf = vec![0; length as usize];
+        file.read_exact(&mut read_buf).unwrap();
+        assert!(
+            read_buf.iter().all(|&b| b == 0),
+            "Punched hole should read as zeros"
+        );
+
+        // Verify data before hole is intact
+        file.seek(SeekFrom::Start(0)).unwrap();
+        let mut read_buf = vec![0; 1024];
+        file.read_exact(&mut read_buf).unwrap();
+        assert!(
+            read_buf.iter().all(|&b| b == 0xAA),
+            "Data before hole should be intact"
+        );
+
+        // Verify data after hole is intact
+        file.seek(SeekFrom::Start(offset + length)).unwrap();
+        let mut read_buf = vec![0; 1024];
+        file.read_exact(&mut read_buf).unwrap();
+        assert!(
+            read_buf.iter().all(|&b| b == 0xAA),
+            "Data after hole should be intact"
+        );
+    }
+
+    #[test]
+    fn test_write_zeroes() {
+        let temp_file = TempFile::new().unwrap();
+        let mut file = temp_file.into_file();
+
+        // Write 4MB of data
+        let data = vec![0xBB; 4 * 1024 * 1024];
+        file.write_all(&data).unwrap();
+        file.sync_all().unwrap();
+
+        // Create async IO instance
+        let mut async_io = RawFileSync::new(file.as_raw_fd());
+
+        // Write zeros in the middle (512KB at offset 2MB)
+        let offset = 2 * 1024 * 1024;
+        let length = 512 * 1024;
+        let write_zeroes_result = async_io.write_zeroes(offset, length, 2);
+
+        // FALLOC_FL_ZERO_RANGE might not be supported on all filesystems (e.g., tmpfs)
+        // If it fails with ENOTSUP, skip the test
+        if let Err(AsyncIoError::WriteZeroes(ref e)) = write_zeroes_result
+            && (e.raw_os_error() == Some(libc::EOPNOTSUPP)
+                || e.raw_os_error() == Some(libc::ENOTSUP))
+        {
+            eprintln!(
+                "Skipping test_write_zeroes: filesystem doesn't support FALLOC_FL_ZERO_RANGE"
+            );
+            return;
+        }
+        write_zeroes_result.unwrap();
+
+        // Check completion
+        let (user_data, result) = async_io.next_completed_request().unwrap();
+        assert_eq!(user_data, 2);
+        assert_eq!(result, 0);
+
+        // Verify the zeroed region reads as zeros
+        file.seek(SeekFrom::Start(offset)).unwrap();
+        let mut read_buf = vec![0; length as usize];
+        file.read_exact(&mut read_buf).unwrap();
+        assert!(
+            read_buf.iter().all(|&b| b == 0),
+            "Zeroed region should read as zeros"
+        );
+
+        // Verify data before zeroed region is intact
+        file.seek(SeekFrom::Start(offset - 1024)).unwrap();
+        let mut read_buf = vec![0; 1024];
+        file.read_exact(&mut read_buf).unwrap();
+        assert!(
+            read_buf.iter().all(|&b| b == 0xBB),
+            "Data before zeroed region should be intact"
+        );
+
+        // Verify data after zeroed region is intact
+        file.seek(SeekFrom::Start(offset + length)).unwrap();
+        let mut read_buf = vec![0; 1024];
+        file.read_exact(&mut read_buf).unwrap();
+        assert!(
+            read_buf.iter().all(|&b| b == 0xBB),
+            "Data after zeroed region should be intact"
+        );
+    }
+
+    #[test]
+    fn test_punch_hole_multiple_operations() {
+        let temp_file = TempFile::new().unwrap();
+        let mut file = temp_file.into_file();
+
+        // Write 8MB of data
+        let data = vec![0xCC; 8 * 1024 * 1024];
+        file.write_all(&data).unwrap();
+        file.sync_all().unwrap();
+
+        // Create async IO instance
+        let mut async_io = RawFileSync::new(file.as_raw_fd());
+
+        // Punch multiple holes
+        async_io.punch_hole(1024 * 1024, 512 * 1024, 10).unwrap();
+        async_io
+            .punch_hole(3 * 1024 * 1024, 512 * 1024, 11)
+            .unwrap();
+        async_io
+            .punch_hole(5 * 1024 * 1024, 512 * 1024, 12)
+            .unwrap();
+
+        // Check all completions
+        let (user_data, result) = async_io.next_completed_request().unwrap();
+        assert_eq!(user_data, 10);
+        assert_eq!(result, 0);
+
+        let (user_data, result) = async_io.next_completed_request().unwrap();
+        assert_eq!(user_data, 11);
+        assert_eq!(result, 0);
+
+        let (user_data, result) = async_io.next_completed_request().unwrap();
+        assert_eq!(user_data, 12);
+        assert_eq!(result, 0);
+
+        // Verify all holes read as zeros
+        file.seek(SeekFrom::Start(1024 * 1024)).unwrap();
+        let mut read_buf = vec![0; 512 * 1024];
+        file.read_exact(&mut read_buf).unwrap();
+        assert!(read_buf.iter().all(|&b| b == 0));
+
+        file.seek(SeekFrom::Start(3 * 1024 * 1024)).unwrap();
+        file.read_exact(&mut read_buf).unwrap();
+        assert!(read_buf.iter().all(|&b| b == 0));
+
+        file.seek(SeekFrom::Start(5 * 1024 * 1024)).unwrap();
+        file.read_exact(&mut read_buf).unwrap();
+        assert!(read_buf.iter().all(|&b| b == 0));
+    }
+}