Ensure buffers are flushed to avoid decompression stream underread

src/stream/read/mod.rs

@@ -93,7 +93,9 @@ impl<'a, R: BufRead> Decoder<'a, R> {
     ///
     /// Calling `finish()` is not *required* after reading a stream -
     /// just use it if you need to get the `Read` back.
-    pub fn finish(self) -> R {
+    pub fn finish(mut self) -> R {
+        // Ensure the input buffers have been flushed by reading to a zero-length buffer.
+        let _ = self.reader.read(&mut [0; 0]);
         self.reader.into_inner()
     }
 

tests/issue_251.rs

@@ -0,0 +1,38 @@
+use std::io::Read;
+
+#[test]
+fn test_issue_251() {
+    // This is 64 compressed zero bytes.
+    let compressed_data = include_bytes!("../assets/zeros64.zst");
+    let decompressed_size = 64;
+
+    // Construct a decompressor using `with_buffer`.  This should be ok as
+    // `Cursor` is `BufRead`.
+    let reader = std::io::Cursor::new(compressed_data);
+    let mut decomp = zstd::Decoder::with_buffer(reader).unwrap();
+
+    // Count how many bytes we decompress.
+    let mut total = 0;
+
+    // Decompress four bytes at a time (this is necessary to trigger underrun
+    // behaviour).
+    for _ in 0..(decompressed_size / 4) {
+        let mut buf = [0u8; 4];
+        let count = decomp.read(&mut buf).unwrap();
+        total += count;
+    }
+
+    // Finish reading and get the buffer back.
+    let reader = decomp.finish();
+
+    // The cursor should now be at the end of the compressed data.
+    println!("We read {total}/{decompressed_size} decompressed bytes");
+    println!(
+        "The underlying cursor is at position {} of {} compressed bytes",
+        reader.position(),
+        compressed_data.len()
+    );
+
+    assert_eq!(total, decompressed_size);
+    assert_eq!(reader.position() as usize, compressed_data.len());
+}