Merge pull request #22 from carlsverre/faster_bytes

faster Slice::from_reader for bytes feature

Author: marvin-j97

src/lib.rs

@@ -91,14 +91,15 @@
 
 #![doc(html_logo_url = "https://raw.githubusercontent.com/fjall-rs/value-log/main/logo.png")]
 #![doc(html_favicon_url = "https://raw.githubusercontent.com/fjall-rs/value-log/main/logo.png")]
-#![forbid(unsafe_code)]
 #![deny(clippy::all, missing_docs, clippy::cargo)]
 #![deny(clippy::unwrap_used)]
 #![deny(clippy::indexing_slicing)]
 #![warn(clippy::pedantic, clippy::nursery)]
 #![warn(clippy::expect_used)]
 #![allow(clippy::missing_const_for_fn)]
 #![warn(clippy::multiple_crate_versions)]
+// the bytes feature uses unsafe to improve from_reader performance
+#![cfg_attr(not(feature = "bytes"), forbid(unsafe_code))]
 
 mod blob_cache;
 mod coding;

src/slice/slice_bytes.rs

@@ -37,10 +37,27 @@ impl Slice {
     }
 
     /// Constructs a [`Slice`] from an I/O reader by pulling in `len` bytes.
+    ///
+    /// The reader may not read the existing buffer.
     #[doc(hidden)]
     pub fn from_reader<R: std::io::Read>(reader: &mut R, len: usize) -> std::io::Result<Self> {
-        let mut builder = BytesMut::zeroed(len);
+        // Use `with_capacity` & `set_len`` to avoid zeroing the buffer
+        let mut builder = BytesMut::with_capacity(len);
+
+        // SAFETY: we just allocated `len` bytes, and `read_exact` will fail if
+        // it doesn't fill the buffer, subsequently dropping the uninitialized
+        // BytesMut object
+        unsafe {
+            builder.set_len(len);
+        }
+
+        /// SAFETY: Normally, read_exact over an uninitialized buffer is UB,
+        /// however we know that in lsm-tree etc. only I/O readers or cursors over Vecs are used
+        /// so it's safe
+        ///
+        /// The safe API is unstable: https://github.com/rust-lang/rust/issues/78485
         reader.read_exact(&mut builder)?;
+
         Ok(Self(builder.freeze()))
     }
 }