backup/{dynamic, fixed}_index: improve error message for small index files

index files that were smaller than their respective header size,
would fail with

"failed to fill whole buffer"

instead now check explicitely for the size and fail with
"index too small (size)"

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

src/backup/dynamic_index.rs

@@ -95,6 +95,18 @@ impl DynamicIndexReader {
 
         let header_size = std::mem::size_of::<DynamicIndexHeader>();
 
+        let rawfd = file.as_raw_fd();
+        let stat = match nix::sys::stat::fstat(rawfd) {
+            Ok(stat) => stat,
+            Err(err) => bail!("fstat failed - {}", err),
+        };
+
+        let size = stat.st_size as usize;
+
+        if size < header_size {
+            bail!("index too small ({})", stat.st_size);
+        }
+
         let header: Box<DynamicIndexHeader> = unsafe { file.read_host_value_boxed()? };
 
         if header.magic != super::DYNAMIC_SIZED_CHUNK_INDEX_1_0 {
@@ -103,13 +115,7 @@ impl DynamicIndexReader {
 
         let ctime = proxmox::tools::time::epoch_i64();
 
-        let rawfd = file.as_raw_fd();
+        let index_size = stat.st_size as usize - header_size;
-
-        let stat = nix::sys::stat::fstat(rawfd)?;
-
-        let size = stat.st_size as usize;
-
-        let index_size = size - header_size;
         let index_count = index_size / 40;
         if index_count * 40 != index_size {
             bail!("got unexpected file size");

src/backup/fixed_index.rs

@@ -68,6 +68,19 @@ impl FixedIndexReader {
         file.seek(SeekFrom::Start(0))?;
 
         let header_size = std::mem::size_of::<FixedIndexHeader>();
+
+        let rawfd = file.as_raw_fd();
+        let stat = match nix::sys::stat::fstat(rawfd) {
+            Ok(stat) => stat,
+            Err(err) => bail!("fstat failed - {}", err),
+        };
+
+        let size = stat.st_size as usize;
+
+        if size < header_size {
+            bail!("index too small ({})", stat.st_size);
+        }
+
         let header: Box<FixedIndexHeader> = unsafe { file.read_host_value_boxed()? };
 
         if header.magic != super::FIXED_SIZED_CHUNK_INDEX_1_0 {
@@ -81,12 +94,6 @@ impl FixedIndexReader {
         let index_length = ((size + chunk_size - 1) / chunk_size) as usize;
         let index_size = index_length * 32;
 
-        let rawfd = file.as_raw_fd();
-
-        let stat = match nix::sys::stat::fstat(rawfd) {
-            Ok(stat) => stat,
-            Err(err) => bail!("fstat failed - {}", err),
-        };
 
         let expected_index_size = (stat.st_size as usize) - header_size;
         if index_size != expected_index_size {