Add tokio-buf and a BufStream trait (#611)

The `BufStream` trait provides an improved API for working with
asynchronous streams of bytes compared to `Stream<Item = [u8]>`

tokio-buf/src/buf_stream/from.rs

@@ -0,0 +1,110 @@
+use super::SizeHint;
+
+use bytes::{Buf, BufMut};
+
+use std::usize;
+
+/// Conversion from a `BufStream`.
+///
+/// By implementing `FromBufStream` for a type, you define how it will be
+/// created from a buf stream. This is common for types which describe byte
+/// storage of some kind.
+///
+/// `FromBufStream` is rarely called explicitly, and it is instead used through
+/// `BufStream`'s `collect` method.
+pub trait FromBufStream<T: Buf>: Sized {
+    /// Type that is used to build `Self` while the `BufStream` is being
+    /// consumed.
+    type Builder;
+
+    /// Error that might happen on conversion.
+    type Error;
+
+    /// Create a new, empty, builder. The provided `hint` can be used to inform
+    /// reserving capacity.
+    fn builder(hint: &SizeHint) -> Self::Builder;
+
+    /// Extend the builder with the `Buf`.
+    ///
+    /// This method is called whenever a new `Buf` value is obtained from the
+    /// buf stream.
+    ///
+    /// The provided size hint represents the state of the stream **after**
+    /// `buf` has been yielded. The lower bound represents the minimum amount of
+    /// data that will be provided after this call to `extend` returns.
+    fn extend(builder: &mut Self::Builder, buf: &mut T, hint: &SizeHint)
+        -> Result<(), Self::Error>;
+
+    /// Finalize the building of `Self`.
+    ///
+    /// Called once the buf stream is fully consumed.
+    fn build(builder: Self::Builder) -> Result<Self, Self::Error>;
+}
+
+/// Error returned from collecting into a `Vec<u8>`
+#[derive(Debug)]
+pub struct CollectVecError { _p: () }
+
+impl<T: Buf> FromBufStream<T> for Vec<u8> {
+    type Builder = Vec<u8>;
+    type Error = CollectVecError;
+
+    fn builder(_hint: &SizeHint) -> Vec<u8> {
+        Vec::new()
+    }
+
+    fn extend(builder: &mut Self, buf: &mut T, hint: &SizeHint) -> Result<(), Self::Error> {
+        let lower = hint.lower();
+
+        // If the lower bound is greater than `usize::MAX` then we have a
+        // problem
+        if lower > usize::MAX as u64 {
+            return Err(CollectVecError { _p: () });
+        }
+
+        let mut reserve = lower as usize;
+
+        // If `upper` is set, use this value if it is less than or equal to 64.
+        // This only really impacts the first iteration.
+        match hint.upper() {
+            Some(upper) if upper <= 64 => {
+                reserve = upper as usize;
+            }
+            _ => {},
+        }
+
+        // hint.lower() represents the minimum amount of data that will be
+        // received *after* this function call. We reserve this amount on top of
+        // the amount of data in `buf`.
+        reserve = match reserve.checked_add(buf.remaining()) {
+            Some(n) => n,
+            None => return Err(CollectVecError { _p: () }),
+        };
+
+        // Always reserve 64 bytes the first time, unless `upper` is set and is
+        // less than 64.
+        if builder.is_empty() {
+            reserve = reserve.max(match hint.upper() {
+                Some(upper) if upper < 64 => upper as usize,
+                _ => 64,
+            });
+        }
+
+        // Make sure overflow won't happen when reserving
+        if reserve.checked_add(builder.len()).is_none() {
+            return Err(CollectVecError { _p: () });
+        }
+
+        // Reserve space
+        builder.reserve(reserve);
+
+        // Copy the data
+        builder.put(buf);
+
+        Ok(())
+    }
+
+    fn build(builder: Self) -> Result<Self, Self::Error> {
+        Ok(builder)
+    }
+}