chunk_store: insert_chunk: write chunk again if it is empty on disk

and issue a warning. We can do this, because we know an empty chunk
cannot be valid, and we (assumedly) have a valid chunk in memory.

Having empty chunks on disk is currently possible when PBS crashes,
but the rename of the chunk was flushed to disk, when the actual data
was not.

If it's not empty but there is a size mismatch, return an error.

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

pbs-datastore/src/chunk_store.rs

@@ -458,17 +458,26 @@ impl ChunkStore {
 
         let lock = self.mutex.lock();
 
+        let raw_data = chunk.raw_data();
+        let encoded_size = raw_data.len() as u64;
+
         if let Ok(metadata) = std::fs::metadata(&chunk_path) {
-            if metadata.is_file() {
+            if !metadata.is_file() {
-                self.touch_chunk(digest)?;
-                return Ok((true, metadata.len()));
-            } else {
                 bail!(
                     "Got unexpected file type on store '{}' for chunk {}",
                     self.name,
                     digest_str
                 );
             }
+            let old_size = metadata.len();
+            if encoded_size == old_size {
+                self.touch_chunk(digest)?;
+                return Ok((true, old_size));
+            } else if old_size == 0 {
+                log::warn!("found empty chunk '{digest_str}' on disk, overwriting");
+            } else {
+                bail!("found chunk size mismatch for '{digest_str}': old {old_size} - new {encoded_size}");
+            }
         }
 
         let mut tmp_path = chunk_path.clone();
@@ -483,9 +492,6 @@ impl ChunkStore {
             )
         })?;
 
-        let raw_data = chunk.raw_data();
-        let encoded_size = raw_data.len() as u64;
-
         file.write_all(raw_data).map_err(|err| {
             format_err!(
                 "writing temporary chunk on store '{}' failed for {} - {}",