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
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
use std::fmt;
use std::mem::MaybeUninit;

/// A wrapper around a byte buffer that is incrementally filled and initialized.
///
/// This type is a sort of "double cursor". It tracks three regions in the
/// buffer: a region at the beginning of the buffer that has been logically
/// filled with data, a region that has been initialized at some point but not
/// yet logically filled, and a region at the end that may be uninitialized.
/// The filled region is guaranteed to be a subset of the initialized region.
///
/// In summary, the contents of the buffer can be visualized as:
///
/// ```not_rust
/// [             capacity              ]
/// [ filled |         unfilled         ]
/// [    initialized    | uninitialized ]
/// ```
///
/// It is undefined behavior to de-initialize any bytes from the uninitialized
/// region, since it is merely unknown whether this region is uninitialized or
/// not, and if part of it turns out to be initialized, it must stay initialized.
pub struct ReadBuf<'a> {
    buf: &'a mut [MaybeUninit<u8>],
    filled: usize,
    initialized: usize,
}

impl<'a> ReadBuf<'a> {
    /// Creates a new `ReadBuf` from a fully initialized buffer.
    #[inline]
    pub fn new(buf: &'a mut [u8]) -> ReadBuf<'a> {
        let initialized = buf.len();
        let buf = unsafe { slice_to_uninit_mut(buf) };
        ReadBuf {
            buf,
            filled: 0,
            initialized,
        }
    }

    /// Creates a new `ReadBuf` from a fully uninitialized buffer.
    ///
    /// Use `assume_init` if part of the buffer is known to be already initialized.
    #[inline]
    pub fn uninit(buf: &'a mut [MaybeUninit<u8>]) -> ReadBuf<'a> {
        ReadBuf {
            buf,
            filled: 0,
            initialized: 0,
        }
    }

    /// Returns the total capacity of the buffer.
    #[inline]
    pub fn capacity(&self) -> usize {
        self.buf.len()
    }

    /// Returns a shared reference to the filled portion of the buffer.
    #[inline]
    pub fn filled(&self) -> &[u8] {
        let slice = &self.buf[..self.filled];
        // safety: filled describes how far into the buffer that the
        // user has filled with bytes, so it's been initialized.
        unsafe { slice_assume_init(slice) }
    }

    /// Returns a mutable reference to the filled portion of the buffer.
    #[inline]
    pub fn filled_mut(&mut self) -> &mut [u8] {
        let slice = &mut self.buf[..self.filled];
        // safety: filled describes how far into the buffer that the
        // user has filled with bytes, so it's been initialized.
        unsafe { slice_assume_init_mut(slice) }
    }

    /// Returns a new `ReadBuf` comprised of the unfilled section up to `n`.
    #[inline]
    pub fn take(&mut self, n: usize) -> ReadBuf<'_> {
        let max = std::cmp::min(self.remaining(), n);
        // Safety: We don't set any of the `unfilled_mut` with `MaybeUninit::uninit`.
        unsafe { ReadBuf::uninit(&mut self.unfilled_mut()[..max]) }
    }

    /// Returns a shared reference to the initialized portion of the buffer.
    ///
    /// This includes the filled portion.
    #[inline]
    pub fn initialized(&self) -> &[u8] {
        let slice = &self.buf[..self.initialized];
        // safety: initialized describes how far into the buffer that the
        // user has at some point initialized with bytes.
        unsafe { slice_assume_init(slice) }
    }

    /// Returns a mutable reference to the initialized portion of the buffer.
    ///
    /// This includes the filled portion.
    #[inline]
    pub fn initialized_mut(&mut self) -> &mut [u8] {
        let slice = &mut self.buf[..self.initialized];
        // safety: initialized describes how far into the buffer that the
        // user has at some point initialized with bytes.
        unsafe { slice_assume_init_mut(slice) }
    }

    /// Returns a mutable reference to the entire buffer, without ensuring that it has been fully
    /// initialized.
    ///
    /// The elements between 0 and `self.filled().len()` are filled, and those between 0 and
    /// `self.initialized().len()` are initialized (and so can be converted to a `&mut [u8]`).
    ///
    /// The caller of this method must ensure that these invariants are upheld. For example, if the
    /// caller initializes some of the uninitialized section of the buffer, it must call
    /// [`assume_init`](Self::assume_init) with the number of bytes initialized.
    ///
    /// # Safety
    ///
    /// The caller must not de-initialize portions of the buffer that have already been initialized.
    /// This includes any bytes in the region marked as uninitialized by `ReadBuf`.
    #[inline]
    pub unsafe fn inner_mut(&mut self) -> &mut [MaybeUninit<u8>] {
        self.buf
    }

    /// Returns a mutable reference to the unfilled part of the buffer without ensuring that it has been fully
    /// initialized.
    ///
    /// # Safety
    ///
    /// The caller must not de-initialize portions of the buffer that have already been initialized.
    /// This includes any bytes in the region marked as uninitialized by `ReadBuf`.
    #[inline]
    pub unsafe fn unfilled_mut(&mut self) -> &mut [MaybeUninit<u8>] {
        &mut self.buf[self.filled..]
    }

    /// Returns a mutable reference to the unfilled part of the buffer, ensuring it is fully initialized.
    ///
    /// Since `ReadBuf` tracks the region of the buffer that has been initialized, this is effectively "free" after
    /// the first use.
    #[inline]
    pub fn initialize_unfilled(&mut self) -> &mut [u8] {
        self.initialize_unfilled_to(self.remaining())
    }

    /// Returns a mutable reference to the first `n` bytes of the unfilled part of the buffer, ensuring it is
    /// fully initialized.
    ///
    /// # Panics
    ///
    /// Panics if `self.remaining()` is less than `n`.
    #[inline]
    pub fn initialize_unfilled_to(&mut self, n: usize) -> &mut [u8] {
        assert!(self.remaining() >= n, "n overflows remaining");

        // This can't overflow, otherwise the assert above would have failed.
        let end = self.filled + n;

        if self.initialized < end {
            unsafe {
                self.buf[self.initialized..end]
                    .as_mut_ptr()
                    .write_bytes(0, end - self.initialized);
            }
            self.initialized = end;
        }

        let slice = &mut self.buf[self.filled..end];
        // safety: just above, we checked that the end of the buf has
        // been initialized to some value.
        unsafe { slice_assume_init_mut(slice) }
    }

    /// Returns the number of bytes at the end of the slice that have not yet been filled.
    #[inline]
    pub fn remaining(&self) -> usize {
        self.capacity() - self.filled
    }

    /// Clears the buffer, resetting the filled region to empty.
    ///
    /// The number of initialized bytes is not changed, and the contents of the buffer are not modified.
    #[inline]
    pub fn clear(&mut self) {
        self.filled = 0;
    }

    /// Advances the size of the filled region of the buffer.
    ///
    /// The number of initialized bytes is not changed.
    ///
    /// # Panics
    ///
    /// Panics if the filled region of the buffer would become larger than the initialized region.
    #[inline]
    pub fn advance(&mut self, n: usize) {
        let new = self.filled.checked_add(n).expect("filled overflow");
        self.set_filled(new);
    }

    /// Sets the size of the filled region of the buffer.
    ///
    /// The number of initialized bytes is not changed.
    ///
    /// Note that this can be used to *shrink* the filled region of the buffer in addition to growing it (for
    /// example, by a `AsyncRead` implementation that compresses data in-place).
    ///
    /// # Panics
    ///
    /// Panics if the filled region of the buffer would become larger than the initialized region.
    #[inline]
    pub fn set_filled(&mut self, n: usize) {
        assert!(
            n <= self.initialized,
            "filled must not become larger than initialized"
        );
        self.filled = n;
    }

    /// Asserts that the first `n` unfilled bytes of the buffer are initialized.
    ///
    /// `ReadBuf` assumes that bytes are never de-initialized, so this method does nothing when called with fewer
    /// bytes than are already known to be initialized.
    ///
    /// # Safety
    ///
    /// The caller must ensure that `n` unfilled bytes of the buffer have already been initialized.
    #[inline]
    pub unsafe fn assume_init(&mut self, n: usize) {
        let new = self.filled + n;
        if new > self.initialized {
            self.initialized = new;
        }
    }

    /// Appends data to the buffer, advancing the written position and possibly also the initialized position.
    ///
    /// # Panics
    ///
    /// Panics if `self.remaining()` is less than `buf.len()`.
    #[inline]
    pub fn put_slice(&mut self, buf: &[u8]) {
        assert!(
            self.remaining() >= buf.len(),
            "buf.len() must fit in remaining()"
        );

        let amt = buf.len();
        // Cannot overflow, asserted above
        let end = self.filled + amt;

        // Safety: the length is asserted above
        unsafe {
            self.buf[self.filled..end]
                .as_mut_ptr()
                .cast::<u8>()
                .copy_from_nonoverlapping(buf.as_ptr(), amt);
        }

        if self.initialized < end {
            self.initialized = end;
        }
        self.filled = end;
    }
}

impl fmt::Debug for ReadBuf<'_> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("ReadBuf")
            .field("filled", &self.filled)
            .field("initialized", &self.initialized)
            .field("capacity", &self.capacity())
            .finish()
    }
}

unsafe fn slice_to_uninit_mut(slice: &mut [u8]) -> &mut [MaybeUninit<u8>] {
    &mut *(slice as *mut [u8] as *mut [MaybeUninit<u8>])
}

// TODO: This could use `MaybeUninit::slice_assume_init` when it is stable.
unsafe fn slice_assume_init(slice: &[MaybeUninit<u8>]) -> &[u8] {
    &*(slice as *const [MaybeUninit<u8>] as *const [u8])
}

// TODO: This could use `MaybeUninit::slice_assume_init_mut` when it is stable.
unsafe fn slice_assume_init_mut(slice: &mut [MaybeUninit<u8>]) -> &mut [u8] {
    &mut *(slice as *mut [MaybeUninit<u8>] as *mut [u8])
}