f: Convert BufReader to single-byte-increments.

TODO: Squash commit if this looks reasonable.

Author: arik-so

lightning/src/util/ser.rs

@@ -67,21 +67,22 @@ impl<W: Write> Writer for W {
 /// Wrap buffering support for implementations of Read.
 /// A [`Read`]er which keeps an internal buffer to avoid hitting the underlying stream directly for
 /// every read, implementing [`BufRead`].
-pub struct BufReader<'a, R: Read> {
+///
+/// In order to avoid reading bytes past the first object, and those bytes then ending up getting
+/// dropped, this BufReader operates in one-byte-increments.
+struct BufReader<'a, R: Read> {
 	inner: &'a mut R,
-	buf: [u8; 4096],
-	pos: usize,
-	limit: usize,
+	buf: [u8; 1],
+	is_consumed: bool
 }
 
 impl<'a, R: Read> BufReader<'a, R> {
 	/// Creates a [`BufReader`] which will read from the given `inner`.
 	pub fn new(inner: &'a mut R) -> Self {
 		BufReader {
 			inner,
-			buf: [0; 4096],
-			pos: 0,
-			limit: 0,
+			buf: [0; 1],
+			is_consumed: true
 		}
 	}
 }
@@ -100,21 +101,24 @@ impl<'a, R: Read> Read for BufReader<'a, R> {
 impl<'a, R: Read> BufRead for BufReader<'a, R> {
 	#[inline]
 	fn fill_buf(&mut self) -> io::Result<&[u8]> {
-		if self.pos < self.limit {
-			Ok(&self.buf[self.pos..self.limit])
-		} else {
+		if self.is_consumed {
 			let count = self.inner.read(&mut self.buf[..])?;
-			debug_assert!(count <= self.buf.len(), "read gave us a garbage length");
-			self.pos = 0;
-			self.limit = cmp::min(count, self.buf.len());
-			Ok(&self.buf[..self.limit])
+			debug_assert!(count <= 1, "read gave us a garbage length");
+
+			// upon hitting EOF, assume the byte is already consumed
+			self.is_consumed = count == 0;
 		}
+
+		Ok(&self.buf[..])
 	}
 
 	#[inline]
 	fn consume(&mut self, amount: usize) {
-		debug_assert!(self.pos.saturating_add(amount) <= self.limit, "Cannot consume more than was provided");
-		self.pos += amount;
+		if amount >= 1 {
+			debug_assert_eq!(amount, 1, "Can only consume one byte");
+			debug_assert!(!self.is_consumed, "Cannot consume more than had been read");
+			self.is_consumed = true;
+		}
 	}
 }