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
// SPDX-License-Identifier: AGPL-3.0-or-later
pub use private::Trailered;

mod private {
    use std::{
        io::{self, BufReader, Seek, SeekFrom},
        marker::PhantomData,
    };

    use btserde::{read_from, write_to};
    use positioned_io::{ReadAt, Size, WriteAt};
    use serde::{de::DeserializeOwned, Serialize};

    use crate::{
        bterr, BoxInIoErr, Cursor, Decompose, FlushMeta, Result, Sectored, SizeExt, WriteInteg,
    };

    /// A struct which wraps a stream and which writes a trailing data structure to it when flushed.
    pub struct Trailered<T, D> {
        inner: T,
        body_len: u64,
        phantom: PhantomData<D>,
        write_buf: Vec<u8>,
    }

    impl<T: ReadAt + Size, D: DeserializeOwned> Trailered<T, D> {
        pub fn empty(inner: T) -> Trailered<T, D> {
            Trailered {
                inner,
                body_len: 0,
                phantom: PhantomData,
                write_buf: Vec::new(),
            }
        }

        /// Creates a new `Trailered<T>` containing the given `T`. This method requires that the given
        /// stream is either empty, or contains a valid serialization of `D` and the offset at which
        /// `D` is stored.
        pub fn new(inner: T) -> Result<(Trailered<T, D>, Option<D>)> {
            let end = inner.size_or_err()?;
            if 0 == end {
                return Ok((Self::empty(inner), None));
            }
            let mut reader = BufReader::new(Cursor::new(inner));
            let offset: i64 = std::mem::size_of::<i64>() as i64;
            if end < offset as u64 {
                return Err(bterr!("inner stream is non-empty but too small"));
            }
            reader.seek(SeekFrom::End(-offset))?;
            let offset: i64 = read_from(&mut reader)?;
            if end < offset.unsigned_abs() {
                return Err(bterr!("inner stream is non-empty but too small"));
            }
            let body_len = reader.seek(SeekFrom::End(offset))?;
            let trailer: D = read_from(&mut reader)?;
            let inner = reader.into_inner().into_inner();
            Ok((
                Trailered {
                    inner,
                    body_len,
                    phantom: PhantomData,
                    write_buf: Vec::new(),
                },
                Some(trailer),
            ))
        }
    }

    impl<T, D> Trailered<T, D> {
        fn update_body_len(&mut self, pos: u64, written: usize) -> usize {
            let new_pos = pos + written as u64;
            self.body_len = self.body_len.max(new_pos);
            written
        }
    }

    impl<T: ReadAt, D> ReadAt for Trailered<T, D> {
        fn read_at(&self, pos: u64, buf: &mut [u8]) -> io::Result<usize> {
            if pos > self.body_len {
                return Err(bterr!("pos {pos} is past the end of body ({})", self.body_len).into());
            }
            let available_u64 = self.body_len - pos;
            let available: usize = available_u64.try_into().box_err()?;
            let limit = buf.len().min(available);
            self.inner.read_at(pos, &mut buf[..limit])
        }
    }

    impl<T: WriteAt, D: Serialize> Trailered<T, D> {
        pub fn write_at(&mut self, pos: u64, buf: &[u8]) -> io::Result<usize> {
            let written = self.inner.write_at(pos, buf)?;
            Ok(self.update_body_len(pos, written))
        }

        /// Writes the trailer followed by the offset relative to the end of the inner stream where
        /// the trailer was written. See the `Trailered::new` method for how this is read out of
        /// the inner stream.
        fn write_trailer(&mut self, trailer: &D) -> io::Result<()> {
            self.write_buf.clear();
            write_to(trailer, &mut self.write_buf)?;
            // This is the length of `write_buf` after writing an additional `i64` into it.
            let write_buf_len = (self.write_buf.len() + std::mem::size_of::<i64>()) as u64;
            let offset: i64 = write_buf_len.try_into().box_err()?;
            write_to(&(-offset), &mut self.write_buf)?;
            self.inner.write_all_at(self.body_len, &self.write_buf)?;
            Ok(())
        }

        pub fn flush(&mut self, trailer: &D) -> io::Result<()> {
            self.write_trailer(trailer)?;
            self.inner.flush()
        }
    }

    impl<T: WriteInteg + Size, D: Serialize> Trailered<T, D> {
        pub fn flush_integ(&mut self, trailer: &D, integrity: &[u8]) -> io::Result<()> {
            self.write_trailer(trailer)?;
            self.inner.flush_integ(integrity)
        }
    }

    impl<T, D> Decompose<T> for Trailered<T, D> {
        fn into_inner(self) -> T {
            self.inner
        }
    }

    impl<U, T: AsRef<U>, D> AsRef<U> for Trailered<T, D> {
        fn as_ref(&self) -> &U {
            self.inner.as_ref()
        }
    }

    impl<U, T: AsMut<U>, D> AsMut<U> for Trailered<T, D> {
        fn as_mut(&mut self) -> &mut U {
            self.inner.as_mut()
        }
    }

    impl<T: FlushMeta, D> Trailered<T, D> {
        pub fn flush_meta(&mut self) -> Result<()> {
            self.inner.flush_meta()
        }
    }

    impl<T: Sectored, D> Sectored for Trailered<T, D> {
        fn sector_sz(&self) -> usize {
            self.inner.sector_sz()
        }
    }

    impl<T, D> Size for Trailered<T, D> {
        fn size(&self) -> io::Result<Option<u64>> {
            Ok(Some(self.body_len))
        }
    }
}

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

    use crate::{test_helpers::BtCursor as Cursor, Decompose};

    /// Tests that a new `Trailered<T>` can be created from an empty stream.
    #[test]
    fn trailered_new_empty() {
        let cursor = Cursor::new(Vec::new());

        let (_, trailer): (_, Option<String>) =
            Trailered::new(cursor).expect("Trailered::new failed");

        assert_eq!(None, trailer);
    }

    /// Tests that an error is returned when an attempt is made to create a `Trailered<T>` from a
    /// non-empty stream which is too short.
    #[test]
    fn trailered_new_inner_too_short_is_error() {
        let cursor = Cursor::new([0u8; 5]);

        let result = Trailered::<_, u128>::new(cursor);

        assert!(result.is_err())
    }

    /// Checks that the trailer is persisted to the inner stream.
    #[test]
    fn trailered_trailer_persisted() {
        const EXPECTED: &str = "Everyone deserves to be remembered,";
        let cursor = {
            let cursor = Cursor::new(Vec::new());
            let (mut trailered, trailer) =
                Trailered::<_, String>::new(cursor).expect("Trailered::new failed");
            assert!(trailer.is_none());
            trailered
                .flush(&EXPECTED.to_string())
                .expect("flush failed");
            trailered.into_inner()
        };

        let (_, trailer) = Trailered::<_, String>::new(cursor).expect("Trailered::new failed");

        assert_eq!(EXPECTED, trailer.unwrap());
    }

    #[test]
    fn trailered_written_data_persisted() {
        const EXPECTED: &[u8] = b"and every life has something to teach us.";

        let cursor = {
            let (mut trailered, _) = Trailered::<_, u8>::new(Cursor::new(Vec::new()))
                .expect("failed to create first trailered");
            trailered.write_at(0, EXPECTED).expect("write failed");
            trailered.flush(&1).expect("flush failed");
            trailered.into_inner()
        };

        let (trailered, _) =
            Trailered::<_, u8>::new(cursor).expect("failed to created second trailered");

        let mut actual = vec![0u8; EXPECTED.len()];
        trailered.read_at(0, &mut actual).expect("read failed");

        assert_eq!(EXPECTED, actual);
    }

    /// Tests that a read past the end of the body in a `Trailered<T>` is not allowed.
    #[test]
    fn trailered_read_limited_to_body_len() {
        const EXPECTED: &[u8] = &[1, 1, 1, 1, 1, 0, 0, 0];
        let (mut trailered, ..) =
            Trailered::new(Cursor::new(Vec::new())).expect("failed to create Trailered");

        trailered.write_at(0, &[1u8; 5]).expect("write failed");
        trailered.flush(&1u8).expect("flush failed");
        let mut actual = vec![0u8; EXPECTED.len()];
        trailered.read_at(0, &mut actual).expect("read failed");

        assert_eq!(EXPECTED, actual);
    }
}