rust: replace an unsafe use of transmute with a safe use of bytes-cast

Differential Revision: https://phab.mercurial-scm.org/D9859

rust/hg-core/src/revlog/node.rs

@@ -1,456 +1,457 b''
 // Copyright 2019-2020 Georges Racinet <[e-mail]>
 //
 // This software may be used and distributed according to the terms of the
 // GNU General Public License version 2 or any later version.
 
 //! Definitions and utilities for Revision nodes
 //!
 //! In Mercurial code base, it is customary to call "a node" the binary SHA
 //! of a revision.
 
+use bytes_cast::BytesCast;
 use hex::{self, FromHex, FromHexError};
-use std::convert::{TryFrom, TryInto};
+use std::convert::TryFrom;
 
 /// The length in bytes of a `Node`
 ///
 /// This constant is meant to ease refactors of this module, and
 /// are private so that calling code does not expect all nodes have
 /// the same size, should we support several formats concurrently in
 /// the future.
 pub const NODE_BYTES_LENGTH: usize = 20;
 
 /// Id of the null node.
 ///
 /// Used to indicate the absence of node.
 pub const NULL_NODE_ID: [u8; NODE_BYTES_LENGTH] = [0u8; NODE_BYTES_LENGTH];
 
 /// The length in bytes of a `Node`
 ///
 /// see also `NODES_BYTES_LENGTH` about it being private.
 const NODE_NYBBLES_LENGTH: usize = 2 * NODE_BYTES_LENGTH;
 
 /// Private alias for readability and to ease future change
 type NodeData = [u8; NODE_BYTES_LENGTH];
 
 /// Binary revision SHA
 ///
 /// ## Future changes of hash size
 ///
 /// To accomodate future changes of hash size, Rust callers
 /// should use the conversion methods at the boundaries (FFI, actual
 /// computation of hashes and I/O) only, and only if required.
 ///
 /// All other callers outside of unit tests should just handle `Node` values
 /// and never make any assumption on the actual length, using [`nybbles_len`]
 /// if they need a loop boundary.
 ///
 /// All methods that create a `Node` either take a type that enforces
 /// the size or fail immediately at runtime with [`ExactLengthRequired`].
 ///
 /// [`nybbles_len`]: #method.nybbles_len
 /// [`ExactLengthRequired`]: struct.NodeError#variant.ExactLengthRequired
-#[derive(Clone, Debug, PartialEq)]
+#[derive(Clone, Debug, PartialEq, BytesCast)]
 #[repr(transparent)]
 pub struct Node {
     data: NodeData,
 }
 
 /// The node value for NULL_REVISION
 pub const NULL_NODE: Node = Node {
     data: [0; NODE_BYTES_LENGTH],
 };
 
 impl From<NodeData> for Node {
     fn from(data: NodeData) -> Node {
         Node { data }
     }
 }
 
 /// Return an error if the slice has an unexpected length
 impl<'a> TryFrom<&'a [u8]> for &'a Node {
-    type Error = std::array::TryFromSliceError;
+    type Error = ();
 
     #[inline]
     fn try_from(bytes: &'a [u8]) -> Result<&'a Node, Self::Error> {
-        let data = bytes.try_into()?;
-        // Safety: `#[repr(transparent)]` makes it ok to "wrap" the target
-        // of a reference to the type of the single field.
-        Ok(unsafe { std::mem::transmute::<&NodeData, &Node>(data) })
+        match Node::from_bytes(bytes) {
+            Ok((node, rest)) if rest.is_empty() => Ok(node),
+            _ => Err(()),
+        }
     }
 }
 
 #[derive(Debug, PartialEq)]
 pub enum NodeError {
     ExactLengthRequired(usize, String),
     PrefixTooLong(String),
     HexError(FromHexError, String),
 }
 
 /// Low level utility function, also for prefixes
 fn get_nybble(s: &[u8], i: usize) -> u8 {
     if i % 2 == 0 {
         s[i / 2] >> 4
     } else {
         s[i / 2] & 0x0f
     }
 }
 
 impl Node {
     /// Retrieve the `i`th half-byte of the binary data.
     ///
     /// This is also the `i`th hexadecimal digit in numeric form,
     /// also called a [nybble](https://en.wikipedia.org/wiki/Nibble).
     pub fn get_nybble(&self, i: usize) -> u8 {
         get_nybble(&self.data, i)
     }
 
     /// Length of the data, in nybbles
     pub fn nybbles_len(&self) -> usize {
         // public exposure as an instance method only, so that we can
         // easily support several sizes of hashes if needed in the future.
         NODE_NYBBLES_LENGTH
     }
 
     /// Convert from hexadecimal string representation
     ///
     /// Exact length is required.
     ///
     /// To be used in FFI and I/O only, in order to facilitate future
     /// changes of hash format.
     pub fn from_hex(hex: impl AsRef<[u8]>) -> Result<Node, NodeError> {
         Ok(NodeData::from_hex(hex.as_ref())
             .map_err(|e| NodeError::from((e, hex)))?
             .into())
     }
 
     /// Convert to hexadecimal string representation
     ///
     /// To be used in FFI and I/O only, in order to facilitate future
     /// changes of hash format.
     pub fn encode_hex(&self) -> String {
         hex::encode(self.data)
     }
 
     /// Provide access to binary data
     ///
     /// This is needed by FFI layers, for instance to return expected
     /// binary values to Python.
     pub fn as_bytes(&self) -> &[u8] {
         &self.data
     }
 }
 
 impl<T: AsRef<[u8]>> From<(FromHexError, T)> for NodeError {
     fn from(err_offender: (FromHexError, T)) -> Self {
         let (err, offender) = err_offender;
         let offender = String::from_utf8_lossy(offender.as_ref()).into_owned();
         match err {
             FromHexError::InvalidStringLength => {
                 NodeError::ExactLengthRequired(NODE_NYBBLES_LENGTH, offender)
             }
             _ => NodeError::HexError(err, offender),
         }
     }
 }
 
 /// The beginning of a binary revision SHA.
 ///
 /// Since it can potentially come from an hexadecimal representation with
 /// odd length, it needs to carry around whether the last 4 bits are relevant
 /// or not.
 #[derive(Debug, PartialEq)]
 pub struct NodePrefix {
     buf: Vec<u8>,
     is_odd: bool,
 }
 
 impl NodePrefix {
     /// Convert from hexadecimal string representation
     ///
     /// Similarly to `hex::decode`, can be used with Unicode string types
     /// (`String`, `&str`) as well as bytes.
     ///
     /// To be used in FFI and I/O only, in order to facilitate future
     /// changes of hash format.
     pub fn from_hex(hex: impl AsRef<[u8]>) -> Result<Self, NodeError> {
         let hex = hex.as_ref();
         let len = hex.len();
         if len > NODE_NYBBLES_LENGTH {
             return Err(NodeError::PrefixTooLong(
                 String::from_utf8_lossy(hex).to_owned().to_string(),
             ));
         }
 
         let is_odd = len % 2 == 1;
         let even_part = if is_odd { &hex[..len - 1] } else { hex };
         let mut buf: Vec<u8> =
             Vec::from_hex(&even_part).map_err(|e| (e, hex))?;
 
         if is_odd {
             let latest_char = char::from(hex[len - 1]);
             let latest_nybble = latest_char.to_digit(16).ok_or_else(|| {
                 (
                     FromHexError::InvalidHexCharacter {
                         c: latest_char,
                         index: len - 1,
                     },
                     hex,
                 )
             })? as u8;
             buf.push(latest_nybble << 4);
         }
         Ok(NodePrefix { buf, is_odd })
     }
 
     pub fn borrow(&self) -> NodePrefixRef {
         NodePrefixRef {
             buf: &self.buf,
             is_odd: self.is_odd,
         }
     }
 }
 
 #[derive(Clone, Debug, PartialEq)]
 pub struct NodePrefixRef<'a> {
     buf: &'a [u8],
     is_odd: bool,
 }
 
 impl<'a> NodePrefixRef<'a> {
     pub fn len(&self) -> usize {
         if self.is_odd {
             self.buf.len() * 2 - 1
         } else {
             self.buf.len() * 2
         }
     }
 
     pub fn is_empty(&self) -> bool {
         self.len() == 0
     }
 
     pub fn is_prefix_of(&self, node: &Node) -> bool {
         if self.is_odd {
             let buf = self.buf;
             let last_pos = buf.len() - 1;
             node.data.starts_with(buf.split_at(last_pos).0)
                 && node.data[last_pos] >> 4 == buf[last_pos] >> 4
         } else {
             node.data.starts_with(self.buf)
         }
     }
 
     /// Retrieve the `i`th half-byte from the prefix.
     ///
     /// This is also the `i`th hexadecimal digit in numeric form,
     /// also called a [nybble](https://en.wikipedia.org/wiki/Nibble).
     pub fn get_nybble(&self, i: usize) -> u8 {
         assert!(i < self.len());
         get_nybble(self.buf, i)
     }
 
     /// Return the index first nybble that's different from `node`
     ///
     /// If the return value is `None` that means that `self` is
     /// a prefix of `node`, but the current method is a bit slower
     /// than `is_prefix_of`.
     ///
     /// Returned index is as in `get_nybble`, i.e., starting at 0.
     pub fn first_different_nybble(&self, node: &Node) -> Option<usize> {
         let buf = self.buf;
         let until = if self.is_odd {
             buf.len() - 1
         } else {
             buf.len()
         };
         for (i, item) in buf.iter().enumerate().take(until) {
             if *item != node.data[i] {
                 return if *item & 0xf0 == node.data[i] & 0xf0 {
                     Some(2 * i + 1)
                 } else {
                     Some(2 * i)
                 };
             }
         }
         if self.is_odd && buf[until] & 0xf0 != node.data[until] & 0xf0 {
             Some(until * 2)
         } else {
             None
         }
     }
 }
 
 /// A shortcut for full `Node` references
 impl<'a> From<&'a Node> for NodePrefixRef<'a> {
     fn from(node: &'a Node) -> Self {
         NodePrefixRef {
             buf: &node.data,
             is_odd: false,
         }
     }
 }
 
 impl PartialEq<Node> for NodePrefixRef<'_> {
     fn eq(&self, other: &Node) -> bool {
         !self.is_odd && self.buf == other.data
     }
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
 
     fn sample_node() -> Node {
         let mut data = [0; NODE_BYTES_LENGTH];
         data.copy_from_slice(&[
             0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef, 0xfe, 0xdc, 0xba,
             0x98, 0x76, 0x54, 0x32, 0x10, 0xde, 0xad, 0xbe, 0xef,
         ]);
         data.into()
     }
 
     /// Pad an hexadecimal string to reach `NODE_NYBBLES_LENGTH`
     ///check_hash
     /// The padding is made with zeros
     pub fn hex_pad_right(hex: &str) -> String {
         let mut res = hex.to_string();
         while res.len() < NODE_NYBBLES_LENGTH {
             res.push('0');
         }
         res
     }
 
     fn sample_node_hex() -> String {
         hex_pad_right("0123456789abcdeffedcba9876543210deadbeef")
     }
 
     #[test]
     fn test_node_from_hex() {
         assert_eq!(Node::from_hex(&sample_node_hex()), Ok(sample_node()));
 
         let mut short = hex_pad_right("0123");
         short.pop();
         short.pop();
         assert_eq!(
             Node::from_hex(&short),
             Err(NodeError::ExactLengthRequired(NODE_NYBBLES_LENGTH, short)),
         );
 
         let not_hex = hex_pad_right("012... oops");
         assert_eq!(
             Node::from_hex(&not_hex),
             Err(NodeError::HexError(
                 FromHexError::InvalidHexCharacter { c: '.', index: 3 },
                 not_hex,
             )),
         );
     }
 
     #[test]
     fn test_node_encode_hex() {
         assert_eq!(sample_node().encode_hex(), sample_node_hex());
     }
 
     #[test]
     fn test_prefix_from_hex() -> Result<(), NodeError> {
         assert_eq!(
             NodePrefix::from_hex("0e1")?,
             NodePrefix {
                 buf: vec![14, 16],
                 is_odd: true
             }
         );
         assert_eq!(
             NodePrefix::from_hex("0e1a")?,
             NodePrefix {
                 buf: vec![14, 26],
                 is_odd: false
             }
         );
 
         // checking limit case
         let node_as_vec = sample_node().data.iter().cloned().collect();
         assert_eq!(
             NodePrefix::from_hex(sample_node_hex())?,
             NodePrefix {
                 buf: node_as_vec,
                 is_odd: false
             }
         );
 
         Ok(())
     }
 
     #[test]
     fn test_prefix_from_hex_errors() {
         assert_eq!(
             NodePrefix::from_hex("testgr"),
             Err(NodeError::HexError(
                 FromHexError::InvalidHexCharacter { c: 't', index: 0 },
                 "testgr".to_string()
             ))
         );
         let mut long = NULL_NODE.encode_hex();
         long.push('c');
         match NodePrefix::from_hex(&long)
             .expect_err("should be refused as too long")
         {
             NodeError::PrefixTooLong(s) => assert_eq!(s, long),
             err => panic!(format!("Should have been TooLong, got {:?}", err)),
         }
     }
 
     #[test]
     fn test_is_prefix_of() -> Result<(), NodeError> {
         let mut node_data = [0; NODE_BYTES_LENGTH];
         node_data[0] = 0x12;
         node_data[1] = 0xca;
         let node = Node::from(node_data);
         assert!(NodePrefix::from_hex("12")?.borrow().is_prefix_of(&node));
         assert!(!NodePrefix::from_hex("1a")?.borrow().is_prefix_of(&node));
         assert!(NodePrefix::from_hex("12c")?.borrow().is_prefix_of(&node));
         assert!(!NodePrefix::from_hex("12d")?.borrow().is_prefix_of(&node));
         Ok(())
     }
 
     #[test]
     fn test_get_nybble() -> Result<(), NodeError> {
         let prefix = NodePrefix::from_hex("dead6789cafe")?;
         assert_eq!(prefix.borrow().get_nybble(0), 13);
         assert_eq!(prefix.borrow().get_nybble(7), 9);
         Ok(())
     }
 
     #[test]
     fn test_first_different_nybble_even_prefix() {
         let prefix = NodePrefix::from_hex("12ca").unwrap();
         let prefref = prefix.borrow();
         let mut node = Node::from([0; NODE_BYTES_LENGTH]);
         assert_eq!(prefref.first_different_nybble(&node), Some(0));
         node.data[0] = 0x13;
         assert_eq!(prefref.first_different_nybble(&node), Some(1));
         node.data[0] = 0x12;
         assert_eq!(prefref.first_different_nybble(&node), Some(2));
         node.data[1] = 0xca;
         // now it is a prefix
         assert_eq!(prefref.first_different_nybble(&node), None);
     }
 
     #[test]
     fn test_first_different_nybble_odd_prefix() {
         let prefix = NodePrefix::from_hex("12c").unwrap();
         let prefref = prefix.borrow();
         let mut node = Node::from([0; NODE_BYTES_LENGTH]);
         assert_eq!(prefref.first_different_nybble(&node), Some(0));
         node.data[0] = 0x13;
         assert_eq!(prefref.first_different_nybble(&node), Some(1));
         node.data[0] = 0x12;
         assert_eq!(prefref.first_different_nybble(&node), Some(2));
         node.data[1] = 0xca;
         // now it is a prefix
         assert_eq!(prefref.first_different_nybble(&node), None);
     }
 }
 
 #[cfg(test)]
 pub use tests::hex_pad_right;