gix-pack 0.76.0

Implements git packs and related data structures
Documentation 
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122

use gix_error::{ErrorExt, OptionExt, Result, bail};

fn corrupt(message: &'static str) -> gix_error::Message {
    gix_error::message!("Corrupt delta data: {message}").corrupted()
}

/// Given the decompressed pack delta `d`, decode a size in bytes (either the base object size or the result object size)
/// Equivalent to [this canonical git function](https://github.com/git/git/blob/311531c9de557d25ac087c1637818bd2aad6eb3a/delta.h#L89)
pub(crate) fn decode_header_size(d: &[u8]) -> Result<(u64, usize)> {
    let mut shift = 0;
    let mut size = 0u64;
    let mut consumed = 0;
    for cmd in d.iter() {
        if shift >= u64::BITS {
            bail!(corrupt("delta header size uses more bits than fit into u64"));
        }
        consumed += 1;
        size |= (u64::from(*cmd) & 0x7f) << shift;
        shift += 7;
        if *cmd & 0x80 == 0 {
            return Ok((size, consumed));
        }
    }
    Err(corrupt("delta header size is truncated").raise())
}

pub(crate) fn apply(base: &[u8], mut target: &mut [u8], data: &[u8]) -> Result {
    fn next_byte(data: &[u8], i: &mut usize) -> Result<u8> {
        let byte = *data
            .get(*i)
            .ok_or_raise(|| corrupt("delta copy instruction is truncated"))?;
        *i += 1;
        Ok(byte)
    }

    let mut i = 0;
    while let Some(cmd) = data.get(i) {
        i += 1;
        let bytes = match cmd {
            cmd if cmd & 0b1000_0000 != 0 => {
                let (mut ofs, mut size): (u32, u32) = (0, 0);
                if cmd & 0b0000_0001 != 0 {
                    ofs = u32::from(next_byte(data, &mut i)?);
                }
                if cmd & 0b0000_0010 != 0 {
                    ofs |= u32::from(next_byte(data, &mut i)?) << 8;
                }
                if cmd & 0b0000_0100 != 0 {
                    ofs |= u32::from(next_byte(data, &mut i)?) << 16;
                }
                if cmd & 0b0000_1000 != 0 {
                    ofs |= u32::from(next_byte(data, &mut i)?) << 24;
                }
                if cmd & 0b0001_0000 != 0 {
                    size = u32::from(next_byte(data, &mut i)?);
                }
                if cmd & 0b0010_0000 != 0 {
                    size |= u32::from(next_byte(data, &mut i)?) << 8;
                }
                if cmd & 0b0100_0000 != 0 {
                    size |= u32::from(next_byte(data, &mut i)?) << 16;
                }
                if size == 0 {
                    size = 0x10000; // 65536
                }
                let ofs = ofs as usize;
                let end = ofs
                    .checked_add(size as usize)
                    .ok_or_raise(|| corrupt("delta copy range overflows"))?;
                base.get(ofs..end)
                    .ok_or_raise(|| corrupt("delta copy range exceeds base object size"))?
            }
            0 => {
                bail!(corrupt("delta command 0 is reserved and invalid"));
            }
            size => {
                let end = i
                    .checked_add(*size as usize)
                    .ok_or_raise(|| corrupt("delta insert range overflows"))?;
                let bytes = data
                    .get(i..end)
                    .ok_or_raise(|| corrupt("delta insert data is truncated"))?;
                i = end;
                bytes
            }
        };
        let (out, rest) = target
            .split_at_mut_checked(bytes.len())
            .ok_or_raise(|| corrupt("delta instructions produced more bytes than promised"))?;
        out.copy_from_slice(bytes);
        target = rest;
    }
    debug_assert_eq!(
        i,
        data.len(),
        "delta instructions were not consumed completely, should be impossible"
    );
    if !target.is_empty() {
        bail!(corrupt("delta instructions produced fewer bytes than promised"));
    }

    Ok(())
}

#[cfg(test)]
mod tests {
    #[test]
    fn instructions_cannot_exceed_the_declared_result_size() {
        let mut error_snapshots = Vec::new();
        for instructions in [b"\x90\x02".as_slice(), b"\x02ab".as_slice()] {
            let err = super::apply(b"ab", &mut [0], instructions)
                .expect_err("neither copying nor inserting may truncate the result");
            error_snapshots.push(gix_testtools::redact_debug_snapshot(&err, &[]));
        }
        insta::assert_debug_snapshot!(error_snapshots, "instructions cannot exceed the declared result size", @"
        [
            Corrupt delta data: delta instructions produced more bytes than promised,
            Corrupt delta data: delta instructions produced more bytes than promised,
        ]
        ");
    }
}