block: Fix resize for block device backends

Block devices (LVM volumes, loop devices, RBD, etc.) cannot be resized
via ftruncate - they are resized externally. When vm.resize-disk is
called for a block device backend, verify the device size matches the
requested size instead of attempting ftruncate.

This enables the resize-disk API to work with block device backends by
validating the externally-resized device matches the expected size.

Signed-off-by: Vincent Thomas <vincent@v-thomas.com>

block/src/raw_async.rs

@@ -3,7 +3,8 @@
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 
 use std::fs::File;
-use std::io::Error;
+use std::io::{self, Error};
+use std::os::unix::fs::FileTypeExt;
 use std::os::unix::io::{AsRawFd, RawFd};
 
 use io_uring::{IoUring, opcode, types};
@@ -68,9 +69,30 @@ impl disk_file::SparseCapable for RawFileDisk {
 
 impl disk_file::Resizable for RawFileDisk {
     fn resize(&mut self, size: u64) -> BlockResult<()> {
-        self.file
+        let fd_metadata = self
-            .set_len(size)
+            .file
-            .map_err(|e| BlockError::new(BlockErrorKind::Io, DiskFileError::ResizeError(e)))
+            .metadata()
+            .map_err(|e| BlockError::new(BlockErrorKind::Io, DiskFileError::ResizeError(e)))?;
+
+        if fd_metadata.file_type().is_block_device() {
+            // Block devices cannot be resized via ftruncate - they are resized
+            // externally (LVM, losetup -c, etc.). Verify the size matches.
+            let (actual_size, _) = query_device_size(&self.file)
+                .map_err(|e| BlockError::new(BlockErrorKind::Io, DiskFileError::ResizeError(e)))?;
+            if actual_size != size {
+                return Err(BlockError::new(
+                    BlockErrorKind::Io,
+                    DiskFileError::ResizeError(io::Error::other(format!(
+                        "Block device size {actual_size} does not match requested size {size}"
+                    ))),
+                ));
+            }
+            Ok(())
+        } else {
+            self.file
+                .set_len(size)
+                .map_err(|e| BlockError::new(BlockErrorKind::Io, DiskFileError::ResizeError(e)))
+        }
     }
 }