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

rust-nodemap: accounting for dead blocks

By the very append-only nature of the `NodeTree`, inserting
new blocks has the effect of making some of the older ones
useless as they become unreachable.

Therefore some automatic housekeeping will need to be provided.
This is standard procedure in the word of databases, under names
such as "repack" or "vacuum".

The new `masked_readonly_blocks()` will provide callers with
useful information to decide if the nodetree is ripe for
repacking, but all the `NodeTree` can provide is how many
blocks have been masked in the currently mutable part. Analysing
the readonly part would be way too long to do it for each
transaction and defeat the whole purpose of nodemap persistence.
Serializing callers (from the Python layer) will get this figure
before each extraction and maintain an aggregate counter of
unreachable blocks separately.

Note: at this point, the most efficient repacking is just to restart
afresh with a full rescan.

Differential Revision: https://phab.mercurial-scm.org/D8097

// 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 hex::{self, FromHex, FromHexError};

/// 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.
const NODE_BYTES_LENGTH: usize = 20;

/// 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)]
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 }
    }
}

#[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: &str) -> Result<Node, NodeError> {
        Ok(NodeData::from_hex(hex)
            .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<str>> From<(FromHexError, T)> for NodeError {
    fn from(err_offender: (FromHexError, T)) -> Self {
        let (err, offender) = err_offender;
        match err {
            FromHexError::InvalidStringLength => {
                NodeError::ExactLengthRequired(
                    NODE_NYBBLES_LENGTH,
                    offender.as_ref().to_owned(),
                )
            }
            _ => NodeError::HexError(err, offender.as_ref().to_owned()),
        }
    }
}

/// 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, String::from_utf8_lossy(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,
                    },
                    String::from_utf8_lossy(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_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 in 0..until {
            if buf[i] != node.data[i] {
                if buf[i] & 0xf0 == node.data[i] & 0xf0 {
                    return Some(2 * i + 1);
                } else {
                    return 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,
        }
    }
}

#[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`
    ///
    /// 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;