backup/chunk_store.rs: avoid allocation in read_chunk

src/backup/archive_index.rs

@@ -132,13 +132,15 @@ impl <'a> ArchiveIndexReader<'a> {
 
     pub fn dump_catar(&self, mut writer: Box<Write>) -> Result<(), Error> {
 
+        let mut buffer = Vec::with_capacity(1024*1024);
+
         for pos in 0..self.index_entries {
             let offset = unsafe { *(self.index.add(pos*40) as *const u64) };
             let digest = unsafe { std::slice::from_raw_parts(self.index.add(pos*40+8), 32) };
 
-            let chunk = self.store.read_chunk(digest)?;
-            println!("Dump {:08x} {}", offset, chunk.len());
-            writer.write_all(&chunk)?;
+            self.store.read_chunk(digest, &mut buffer)?;
+            println!("Dump {:08x} {}", offset, buffer.len(), );
+            writer.write_all(&buffer)?;
 
         }
 

src/backup/chunk_store.rs

@@ -165,7 +165,7 @@ impl ChunkStore {
         Ok(())
     }
 
-    pub fn read_chunk(&self, digest:&[u8]) -> Result<Vec<u8>, Error> {
+    pub fn read_chunk(&self, digest:&[u8], buffer: &mut Vec<u8>) -> Result<(), Error> {
 
         let mut chunk_path = self.chunk_dir.clone();
         let prefix = digest_to_prefix(&digest);
@@ -174,13 +174,22 @@ impl ChunkStore {
         chunk_path.push(&digest_str);
 
         let mut f = std::fs::File::open(&chunk_path)?;
-        let mut chunk = Vec::with_capacity(64*1024);
+
+        let stat = nix::sys::stat::fstat(f.as_raw_fd())?;
+        let size = stat.st_size as usize;
+
+        unsafe { buffer.set_len(buffer.capacity()); }
+        if buffer.len() < size {
+            let additional = size - buffer.len();
+            buffer.reserve(additional);
+        }
+        unsafe { buffer.set_len(size); }
 
         use std::io::Read;
 
-        f.read_to_end(&mut chunk)?;
+        f.read_exact(buffer.as_mut_slice())?;
 
-        Ok(chunk)
+        Ok(())
     }
 
     fn sweep_old_files(&self, handle: &mut nix::dir::Dir, status: &mut GarbageCollectionStatus) -> Result<(), Error> {