async_db: add Reader::get_with_buf +test

lib/src/async_db.rs

@@ -1,4 +1,5 @@
 use std::marker::PhantomData;
+use std::vec;
 use std::{path::Path, sync::Arc};
 
 use async_compression::tokio::bufread::ZstdDecoder;
@@ -253,7 +254,8 @@ where
         0 == self.len()
     }
 
-    pub async fn get(&self, index: usize) -> Result<T, String> {
+    /// get item at index, reuse data buffer
+    pub async fn get_with_buf(&self, index: usize, data_buf: &mut Vec<u8>) -> Result<T, String> {
         if index >= self.len() {
             return Err("index out of range".into());
         }
@@ -286,15 +288,21 @@ where
 
         // read & unpack item data
         let mut decoder = ZstdDecoder::new(self.mmap.range_reader(data_pos, data_len).str_err()?);
-        let mut data = Vec::<u8>::new();
-        decoder.read_to_end(&mut data).await.str_err()?;
+        decoder.read_to_end(data_buf).await.str_err()?;
 
         // decode item
-        let item: (T, usize) = bincode::decode_from_slice(&data, BINCODE_CFG).str_err()?;
+        let item: (T, usize) = bincode::decode_from_slice(data_buf, BINCODE_CFG).str_err()?;
 
+        data_buf.clear();
         Ok(item.0)
     }
 
+    /// get item at index
+    pub async fn get(&self, index: usize) -> Result<T, String> {
+        let mut data_buf: Vec<u8> = vec![];
+        self.get_with_buf(index, &mut data_buf).await
+    }
+
     pub fn stream(&self) -> ReaderStream<'_, T> {
         ReaderStream::new(self)
     }
@@ -438,6 +446,35 @@ mod test {
         }
     }
 
+    #[tokio::test]
+    async fn test_write_read_get_with_buf() {
+        let dir = tempdir().expect("tempdir");
+        let tmpfile = dir.path().join("test.tmp");
+        let opts = WriterOpts {
+            data_buf_size: 10 * 1024 * 1024,
+            out_buf_size: 10 * 1024 * 1024,
+            ..Default::default()
+        };
+        let mut writer: Writer<TestData> = Writer::new(&tmpfile, opts).await.expect("new writer");
+
+        let items_iter = gen_data(5);
+        let items: Vec<TestData> = items_iter.collect();
+
+        let src = futures::stream::iter(items.clone());
+        pin_mut!(src);
+        writer.load(src).await.expect("load");
+        writer.finish().await.expect("finish write");
+
+        let reader: Reader<TestData> = Reader::new(&tmpfile, 2048).await.expect("new reader");
+        assert_eq!(items.len(), reader.len());
+
+        for (idx, item) in items.iter().enumerate() {
+            let mut data_buf: Vec<u8> = vec![];
+            let ritem = reader.get_with_buf(idx, &mut data_buf).await.expect("get");
+            assert_eq!(*item, ritem);
+        }
+    }
+
     #[tokio::test]
     async fn test_write_read_stream() {
         let dir = tempdir().expect("tempdir");