prost/encoding/
varint.rs

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
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
use core::cmp::min;
use core::num::NonZeroU64;

use ::bytes::{Buf, BufMut};

use crate::DecodeError;

/// Encodes an integer value into LEB128 variable length format, and writes it to the buffer.
/// The buffer must have enough remaining space (maximum 10 bytes).
#[inline]
pub fn encode_varint(mut value: u64, buf: &mut impl BufMut) {
    // Varints are never more than 10 bytes
    for _ in 0..10 {
        if value < 0x80 {
            buf.put_u8(value as u8);
            break;
        } else {
            buf.put_u8(((value & 0x7F) | 0x80) as u8);
            value >>= 7;
        }
    }
}

/// Returns the encoded length of the value in LEB128 variable length format.
/// The returned value will be between 1 and 10, inclusive.
#[inline]
pub fn encoded_len_varint(value: u64) -> usize {
    // Based on [VarintSize64][1].
    // [1]: https://github.com/protocolbuffers/protobuf/blob/v28.3/src/google/protobuf/io/coded_stream.h#L1744-L1756
    // Safety: value | 1 is non-zero.
    let log2value = unsafe { NonZeroU64::new_unchecked(value | 1) }.ilog2();
    ((log2value * 9 + (64 + 9)) / 64) as usize
}

/// Decodes a LEB128-encoded variable length integer from the buffer.
#[inline]
pub fn decode_varint(buf: &mut impl Buf) -> Result<u64, DecodeError> {
    let bytes = buf.chunk();
    let len = bytes.len();
    if len == 0 {
        return Err(DecodeError::new("invalid varint"));
    }

    let byte = bytes[0];
    if byte < 0x80 {
        buf.advance(1);
        Ok(u64::from(byte))
    } else if len > 10 || bytes[len - 1] < 0x80 {
        let (value, advance) = decode_varint_slice(bytes)?;
        buf.advance(advance);
        Ok(value)
    } else {
        decode_varint_slow(buf)
    }
}

/// Decodes a LEB128-encoded variable length integer from the slice, returning the value and the
/// number of bytes read.
///
/// Based loosely on [`ReadVarint64FromArray`][1] with a varint overflow check from
/// [`ConsumeVarint`][2].
///
/// ## Safety
///
/// The caller must ensure that `bytes` is non-empty and either `bytes.len() >= 10` or the last
/// element in bytes is < `0x80`.
///
/// [1]: https://github.com/google/protobuf/blob/3.3.x/src/google/protobuf/io/coded_stream.cc#L365-L406
/// [2]: https://github.com/protocolbuffers/protobuf-go/blob/v1.27.1/encoding/protowire/wire.go#L358
#[inline]
fn decode_varint_slice(bytes: &[u8]) -> Result<(u64, usize), DecodeError> {
    // Fully unrolled varint decoding loop. Splitting into 32-bit pieces gives better performance.

    // Use assertions to ensure memory safety, but it should always be optimized after inline.
    assert!(!bytes.is_empty());
    assert!(bytes.len() > 10 || bytes[bytes.len() - 1] < 0x80);

    let mut b: u8 = unsafe { *bytes.get_unchecked(0) };
    let mut part0: u32 = u32::from(b);
    if b < 0x80 {
        return Ok((u64::from(part0), 1));
    };
    part0 -= 0x80;
    b = unsafe { *bytes.get_unchecked(1) };
    part0 += u32::from(b) << 7;
    if b < 0x80 {
        return Ok((u64::from(part0), 2));
    };
    part0 -= 0x80 << 7;
    b = unsafe { *bytes.get_unchecked(2) };
    part0 += u32::from(b) << 14;
    if b < 0x80 {
        return Ok((u64::from(part0), 3));
    };
    part0 -= 0x80 << 14;
    b = unsafe { *bytes.get_unchecked(3) };
    part0 += u32::from(b) << 21;
    if b < 0x80 {
        return Ok((u64::from(part0), 4));
    };
    part0 -= 0x80 << 21;
    let value = u64::from(part0);

    b = unsafe { *bytes.get_unchecked(4) };
    let mut part1: u32 = u32::from(b);
    if b < 0x80 {
        return Ok((value + (u64::from(part1) << 28), 5));
    };
    part1 -= 0x80;
    b = unsafe { *bytes.get_unchecked(5) };
    part1 += u32::from(b) << 7;
    if b < 0x80 {
        return Ok((value + (u64::from(part1) << 28), 6));
    };
    part1 -= 0x80 << 7;
    b = unsafe { *bytes.get_unchecked(6) };
    part1 += u32::from(b) << 14;
    if b < 0x80 {
        return Ok((value + (u64::from(part1) << 28), 7));
    };
    part1 -= 0x80 << 14;
    b = unsafe { *bytes.get_unchecked(7) };
    part1 += u32::from(b) << 21;
    if b < 0x80 {
        return Ok((value + (u64::from(part1) << 28), 8));
    };
    part1 -= 0x80 << 21;
    let value = value + ((u64::from(part1)) << 28);

    b = unsafe { *bytes.get_unchecked(8) };
    let mut part2: u32 = u32::from(b);
    if b < 0x80 {
        return Ok((value + (u64::from(part2) << 56), 9));
    };
    part2 -= 0x80;
    b = unsafe { *bytes.get_unchecked(9) };
    part2 += u32::from(b) << 7;
    // Check for u64::MAX overflow. See [`ConsumeVarint`][1] for details.
    // [1]: https://github.com/protocolbuffers/protobuf-go/blob/v1.27.1/encoding/protowire/wire.go#L358
    if b < 0x02 {
        return Ok((value + (u64::from(part2) << 56), 10));
    };

    // We have overrun the maximum size of a varint (10 bytes) or the final byte caused an overflow.
    // Assume the data is corrupt.
    Err(DecodeError::new("invalid varint"))
}

/// Decodes a LEB128-encoded variable length integer from the buffer, advancing the buffer as
/// necessary.
///
/// Contains a varint overflow check from [`ConsumeVarint`][1].
///
/// [1]: https://github.com/protocolbuffers/protobuf-go/blob/v1.27.1/encoding/protowire/wire.go#L358
#[inline(never)]
#[cold]
fn decode_varint_slow(buf: &mut impl Buf) -> Result<u64, DecodeError> {
    let mut value = 0;
    for count in 0..min(10, buf.remaining()) {
        let byte = buf.get_u8();
        value |= u64::from(byte & 0x7F) << (count * 7);
        if byte <= 0x7F {
            // Check for u64::MAX overflow. See [`ConsumeVarint`][1] for details.
            // [1]: https://github.com/protocolbuffers/protobuf-go/blob/v1.27.1/encoding/protowire/wire.go#L358
            if count == 9 && byte >= 0x02 {
                return Err(DecodeError::new("invalid varint"));
            } else {
                return Ok(value);
            }
        }
    }

    Err(DecodeError::new("invalid varint"))
}

#[cfg(test)]
mod test {
    use super::*;

    #[test]
    fn varint() {
        fn check(value: u64, encoded: &[u8]) {
            // Small buffer.
            let mut buf = Vec::with_capacity(1);
            encode_varint(value, &mut buf);
            assert_eq!(buf, encoded);

            // Large buffer.
            let mut buf = Vec::with_capacity(100);
            encode_varint(value, &mut buf);
            assert_eq!(buf, encoded);

            assert_eq!(encoded_len_varint(value), encoded.len());

            // See: https://github.com/tokio-rs/prost/pull/1008 for copying reasoning.
            let mut encoded_copy = encoded;
            let roundtrip_value = decode_varint(&mut encoded_copy).expect("decoding failed");
            assert_eq!(value, roundtrip_value);

            let mut encoded_copy = encoded;
            let roundtrip_value =
                decode_varint_slow(&mut encoded_copy).expect("slow decoding failed");
            assert_eq!(value, roundtrip_value);
        }

        check(2u64.pow(0) - 1, &[0x00]);
        check(2u64.pow(0), &[0x01]);

        check(2u64.pow(7) - 1, &[0x7F]);
        check(2u64.pow(7), &[0x80, 0x01]);
        check(300, &[0xAC, 0x02]);

        check(2u64.pow(14) - 1, &[0xFF, 0x7F]);
        check(2u64.pow(14), &[0x80, 0x80, 0x01]);

        check(2u64.pow(21) - 1, &[0xFF, 0xFF, 0x7F]);
        check(2u64.pow(21), &[0x80, 0x80, 0x80, 0x01]);

        check(2u64.pow(28) - 1, &[0xFF, 0xFF, 0xFF, 0x7F]);
        check(2u64.pow(28), &[0x80, 0x80, 0x80, 0x80, 0x01]);

        check(2u64.pow(35) - 1, &[0xFF, 0xFF, 0xFF, 0xFF, 0x7F]);
        check(2u64.pow(35), &[0x80, 0x80, 0x80, 0x80, 0x80, 0x01]);

        check(2u64.pow(42) - 1, &[0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x7F]);
        check(2u64.pow(42), &[0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x01]);

        check(
            2u64.pow(49) - 1,
            &[0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x7F],
        );
        check(
            2u64.pow(49),
            &[0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x01],
        );

        check(
            2u64.pow(56) - 1,
            &[0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x7F],
        );
        check(
            2u64.pow(56),
            &[0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x01],
        );

        check(
            2u64.pow(63) - 1,
            &[0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x7F],
        );
        check(
            2u64.pow(63),
            &[0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x01],
        );

        check(
            u64::MAX,
            &[0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x01],
        );
    }

    const U64_MAX_PLUS_ONE: &[u8] = &[0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x02];

    #[test]
    fn varint_overflow() {
        let mut copy = U64_MAX_PLUS_ONE;
        decode_varint(&mut copy).expect_err("decoding u64::MAX + 1 succeeded");
    }

    #[test]
    fn variant_slow_overflow() {
        let mut copy = U64_MAX_PLUS_ONE;
        decode_varint_slow(&mut copy).expect_err("slow decoding u64::MAX + 1 succeeded");
    }
}