[pbs-devel] [PATCH v2 backup-qemu 5/5] read_image_at: iterate until buffer is filled

[Stefan Reiter <s.reiter@proxmox.com>]

QEMU will always assume EOF when less bytes than requested are returned
by a block drivers 'read' interface, so we need to fill the buffer up to
'size' if possible.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
---

v2 note: this was previously a QEMU patch, but honestly that's stupid so let's
do it in Rust instead.

 current-api.h  |  4 ++--
 src/lib.rs     |  4 ++--
 src/restore.rs | 20 ++++++++++++++++----
 3 files changed, 20 insertions(+), 8 deletions(-)

diff --git a/current-api.h b/current-api.h
index 15bb275..d77eff6 100644
--- a/current-api.h
+++ b/current-api.h
@@ -364,8 +364,8 @@ int proxmox_restore_read_image_at(ProxmoxRestoreHandle *handle,
  * Note: The data pointer needs to be valid until the async
  * opteration is finished.
  *
- * Note: It is not an error for a successful call to transfer fewer
- * bytes than requested.
+ * Note: The call will only ever transfer less than 'size' bytes if
+ * the end of the file has been reached.
  */
 void proxmox_restore_read_image_at_async(ProxmoxRestoreHandle *handle,
                                          uint8_t aid,
diff --git a/src/lib.rs b/src/lib.rs
index d4b9370..3346be8 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -934,8 +934,8 @@ pub extern "C" fn proxmox_restore_read_image_at(
 /// Note: The data pointer needs to be valid until the async
 /// opteration is finished.
 ///
-/// Note: It is not an error for a successful call to transfer fewer
-/// bytes than requested.
+/// Note: The call will only ever transfer less than 'size' bytes if
+/// the end of the file has been reached.
 #[no_mangle]
 #[allow(clippy::not_unsafe_ptr_arg_deref)]
 pub extern "C" fn proxmox_restore_read_image_at_async(
diff --git a/src/restore.rs b/src/restore.rs
index 2be0295..e43d040 100644
--- a/src/restore.rs
+++ b/src/restore.rs
@@ -262,10 +262,22 @@ impl RestoreTask {
         }
 
         let mut reader = reader.lock().await;
-        reader.seek(SeekFrom::Start(offset)).await?;
-        let buf: &mut [u8] = unsafe { std::slice::from_raw_parts_mut(data.0 as *mut u8, size as usize)};
-        let bytes = reader.read(buf).await?;
 
-        Ok(bytes.try_into()?)
+        let buf: &mut [u8] = unsafe { std::slice::from_raw_parts_mut(data.0 as *mut u8, size as usize)};
+        let mut read = 0;
+
+        while read < size {
+            reader.seek(SeekFrom::Start(offset + read)).await?;
+            let bytes = reader.read(&mut buf[read as usize..]).await?;
+
+            if bytes == 0 {
+                // EOF
+                break;
+            }
+
+            read += bytes as u64;
+        }
+
+        Ok(read.try_into()?)
     }
 }
-- 
2.20.1