The `bytes()` / `bytes_mut()` name implies the method returns the full
set of bytes represented by `Buf`/`BufMut`. To rectify this, the methods
are renamed to `chunk()` and `chunk_mut()` to reflect the partial nature
of the returned byte slice.
`bytes_vectored()` is renamed `chunks_vectored()`.
Closes#447
The way BufMut uses MaybeUninit can lead to unsoundness. This replaces
MaybeUnit with a type owned by bytes so we can ensure the usage patterns
are sound.
Refs: #328
Use case:
```
let bytes: Bytes = ...
let subbytes = bytes.slice(a..b); // where a == b
let slice = &subbytes[..];
let slice_bytes = bytes.slice_ref(slice);
```
Last line should not panic, because `slice` object is derived from
the original `Bytes` object.
Before this commit it panics, because `Bytes::slice` returns a fresh
`Bytes` object when `begin == end`.
This brings `BytesMut` in line with `Vec<u8>` behavior.
This also fixes an existing bug in BytesMut::bytes_mut that exposes
invalid slices. The bug was recently introduced and was only on master
and never released to `crates.io`.
In order to fix a test, `BufMutExt::chain_mut` is provided. Withou this,
it is not possible to chain two `&mut [u8]`.
Closes#170
- The return type of `BufMut::bytes_mut` is now
`&mut [MaybeUninit<u8>]`.
- The argument type of `BufMut::bytes_vectored_mut` is now
`&mut [bytes::buf::IoSliceMut]`.
- `bytes::buf::IoSliceMut` is a `repr(transparent)` wrapper around an
`std::io::IoSliceMut`, but does not expose the inner bytes with a safe
API, since they might be uninitialized.
- `BufMut::bytesMut` and `BufMut::bytes_vectored_mut` are no longer
`unsafe fn`, since the types encapsulate the unsafety instead.
Bytes is a useful tool for managing multiple slices into the same region
of memory, and the other things it used to have been removed to reduce
complexity. The exact strategy for managing the multiple references is
no longer hard-coded, but instead backing by a customizable vtable.
- Removed ability to mutate the underlying memory from the `Bytes` type.
- Removed the "inline" (SBO) mechanism in `Bytes`. The reduces a large
amount of complexity, and improves performance when accessing the
slice of bytes, since a branch is no longer needed to check if the
data is inline.
- Removed `Bytes` knowledge of `BytesMut` (`BytesMut` may grow that
knowledge back at a future point.)
As consequence Buf::collect is removed as well, which is replaced with `Buf::into_bytes`. The advantage of `Buf::into_bytes` is that it can be optimized in cases where converting a `T: Buf` into a `Bytes` instance is efficient.
With this if foo is a mutable slice, it is possible to do
foo.into_buf().put_u32_le(42);
Before this patch into_buf would create a Cursor<&'a [u8]> and it
would not be possible to write into it.
With this if foo is a mutable slice, it is possible to do
foo.into_buf().put_u32_le(42);
Before this patch into_buf would create a Cursor<&'a [u8]> and it
would not be possible to write into it.
* Recycle space when reserving from Vec-backed Bytes
BytesMut::reserve, when called on a BytesMut instance which is backed by
a non-shared Vec<u8>, would previously just delegate to Vec::reserve
regardless of the current location in the buffer. If the Bytes is
actually the trailing component of a larger Vec, then the unused space
won't be recycled. In applications which continually move the pointer
forward to consume data as it comes in, this can cause the underlying
buffer to get extremely large.
This commit checks whether there's extra space at the start of the
backing Vec in this case, and reuses the unused space if possible
instead of allocating.
* Avoid excessive copying when reusing Vec space
Only reuse space in a Vec-backed Bytes when doing so would gain back
more than half of the current capacity. This avoids excessive copy
operations when a large buffer is almost (but not completely) full.
* Recycle space when reserving from Vec-backed Bytes
BytesMut::reserve, when called on a BytesMut instance which is backed by
a non-shared Vec<u8>, would previously just delegate to Vec::reserve
regardless of the current location in the buffer. If the Bytes is
actually the trailing component of a larger Vec, then the unused space
won't be recycled. In applications which continually move the pointer
forward to consume data as it comes in, this can cause the underlying
buffer to get extremely large.
This commit checks whether there's extra space at the start of the
backing Vec in this case, and reuses the unused space if possible
instead of allocating.
* Avoid excessive copying when reusing Vec space
Only reuse space in a Vec-backed Bytes when doing so would gain back
more than half of the current capacity. This avoids excessive copy
operations when a large buffer is almost (but not completely) full.
* Compact Bytes original capacity representation
In order to avoid unnecessary allocations, a `Bytes` structure remembers
the capacity with which it was first created. When a reserve operation
is issued, this original capacity value is used to as a baseline for
reallocating new storage.
Previously, this original capacity value was stored in its raw form. In
other words, the original capacity `usize` was stored as is. In order to
reclaim some `Bytes` internal storage space for additional features,
this original capacity value is compressed from requiring 16 bits to 3.
To do this, instead of storing the exact original capacity. The original
capacity is rounded down to the nearest power of two. If the original
capacity is less than 1024, then it is rounded down to zero. This
roughly means that the original capacity is now stored as a table:
0 => 0
1 => 1k
2 => 2k
3 => 4k
4 => 8k
5 => 16k
6 => 32k
7 => 64k
For the purposes that the original capacity feature was introduced, this
is sufficient granularity.
* Provide `advance` on Bytes and BytesMut
This is the `advance` function that would be part of a `Buf`
implementation. However, `Bytes` and `BytesMut` cannot impl `Buf` until
the next breaking release.
The implementation uses the additional storage made available by the
previous commit to store the number of bytes that the view was advanced.
The `ptr` pointer will point to the start of the window, avoiding any
pointer arithmetic when dereferencing the `Bytes` handle.
Saves the cognitive load of having to wrap them in slices to compare
them when that seems like what one would expect.
Signed-off-by: Clint Byrum <clint@fewbar.com>
Return empty `Bytes` object
Bench for `slice_empty` difference is
```
55 ns/iter (+/- 1) # before this patch
17 ns/iter (+/- 5) # with this patch
```
Bench for `slice_not_empty` is
```
25,058 ns/iter (+/- 1,099) # before this patch
25,072 ns/iter (+/- 1,593) # with this patch
```
Round up to power of 2 is not necessary, because `reserve` already
doubles previous capacity in
```
new_cap = cmp::max(
cmp::max(v.capacity() << 1, new_cap),
original_capacity);
```
which makes `reserve` calls constant in average. Avoiding rounding
up prevents `reserve` from wasting space when caller knows exactly
what space they need.
Patch adds three tests which would fail before this test. The most
important is this:
```
#[test]
fn reserve_in_arc_unique_does_not_overallocate() {
let mut bytes = BytesMut::with_capacity(1000);
bytes.take();
// now bytes is Arc and refcount == 1
assert_eq!(1000, bytes.capacity());
bytes.reserve(2001);
assert_eq!(2001, bytes.capacity());
}
```
It asserts that when user requests more than double of current
capacity, exactly the requested amount of memory is allocated and
is not wasted to next power of two.
`extend_with_slice` is super-convenient operation on `Bytes`.
While `put_u8` would be expensive on `Bytes`, `extend_from_slice`
is OK, because it is batch, and it checks for kind only once.
Patch also adds `impl Extend for Bytes`.
cc #116
The shared debug_assert is to ensure that the internal Bytes
representation is such that offset views are supported. The only
representation that does not support offset views is vec.
Fixes#97
Before this commit `Bytes::split_{off,to}` always created a shallow copy if `self` is arc or vec.
However, in certain cases `split_off` or `split_to` is called with `len` or `0` parameter. E. g. if you are reading a frame from buffered stream, it is likely that buffer contains exactly the frame size bytes, so `split_to` will be called with `len` param.
Although, `split_off` and `split_to` functions are `O(1)`, shallow copy have downsides:
* shallow copy on vector does malloc and atomic cmpxchg
* after shallow copy, following operations (e. g. `drop`) on both `bytes` objects require atomics
* memory will be probably released to the system later
* `try_mut` will fail
* [into_vec](https://github.com/carllerche/bytes/issues/86) will copy
Standard `Debug` implementation for `[u8]` is comma separated list
of numbers. Since large amount of byte strings are in fact ASCII
strings or contain a lot of ASCII strings (e. g. HTTP), it is
convenient to print strings as ASCII when possible.
Limit the number of threads when using qemu to 1. Also, don't bother
running the stress test as this will trigger qemu bugs. Finally, also
make the stress test actually stress test.
This change tracks the original capacity requested when `BytesMut` is
first created. This capacity is used when a `reserve` needs to allocate
due to the current view being too small. The newly allocated buffer will
be sized the same as the original allocation.
The previous implementation didn't factor in a single `Bytes` handle
being stored in an `Arc`. This new implementation correctly impelments
both `Bytes` and `BytesMut` such that both are `Sync`.
The rewrite also increases the number of bytes that can be stored
inline.
Ralf Jung