Reimplement io::Cursor.

The `bitcoin::io` Cursor will miss a bunch of functionality that we were
using that we will reimplement here, most critically `set_position`.

Author: arik-so

lightning/src/io/mod.rs

@@ -4,3 +4,75 @@ pub use core2::io::*;
 #[cfg(feature = "std")]
 /// Re-export of either `core2::io` or `std::io`, depending on the `std` feature flag.
 pub use std::io::*;
+
+/// Emulation of std::io::Cursor
+#[derive(Clone, Debug, Default, Eq, PartialEq)]
+pub struct Cursor<T> {
+	inner: T,
+	pos: u64,
+}
+
+impl<T> Cursor<T> {
+	/// Creates a `Cursor` by wrapping `inner`.
+	#[inline]
+	pub fn new(inner: T) -> Cursor<T> {
+		Cursor { pos: 0, inner }
+	}
+
+	/// Returns the position read up to thus far.
+	#[inline]
+	pub fn position(&self) -> u64 {
+		self.pos
+	}
+
+	/// Returns the inner buffer.
+	///
+	/// This is the whole wrapped buffer, including the bytes already read.
+	#[inline]
+	pub fn into_inner(self) -> T {
+		self.inner
+	}
+
+	/// Gets a reference to the underlying value in this cursor.
+	pub fn get_ref(&self) -> &T {
+		&self.inner
+	}
+
+	/// Gets a mutable reference to the underlying value in this cursor.
+	///
+	/// Care should be taken to avoid modifying the internal I/O state of the
+	/// underlying value as it may corrupt this cursor's position.
+	pub fn get_mut(&mut self) -> &mut T {
+		&mut self.inner
+	}
+
+	/// Sets the position of this cursor.
+	pub fn set_position(&mut self, pos: u64) {
+		self.pos = pos;
+	}
+}
+
+impl<T: AsRef<[u8]>> Read for Cursor<T> {
+	fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
+		let n = Read::read(&mut self.fill_buf()?, buf)?;
+		self.pos += n as u64;
+		Ok(n)
+	}
+
+	fn read_exact(&mut self, buf: &mut [u8]) -> Result<()> {
+		let n = buf.len();
+		Read::read_exact(&mut self.fill_buf()?, buf)?;
+		self.pos += n as u64;
+		Ok(())
+	}
+}
+
+impl<T: AsRef<[u8]>> BufRead for Cursor<T> {
+	fn fill_buf(&mut self) -> Result<&[u8]> {
+		let amt = core::cmp::min(self.pos, self.inner.as_ref().len() as u64);
+		Ok(&self.inner.as_ref()[(amt as usize)..])
+	}
+	fn consume(&mut self, amt: usize) {
+		self.pos += amt as u64;
+	}
+}