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
//! This module exposes two fundamental concepts:
//!
//! [PbBufferReader]
//! A [PbBufferReader] is something which a [Message] can be deserialized from. In the common case,
//! this means that the relevant bytes are copied out of the underlying store and copied into an
//! appropriate struct which implements [Message].
//!
//! [PbBufferWriter]
//! A [PbBufferWriter] is something which a [Message] can be serialized to. In the common case, this
//! means that the relevant bytes are copied out of the concrete struct and into the underlying
//! data store.
//!
//! # Zerocopy serialization and deserialization and `Lazy`
//!
//! There are cases where we want to minimize the number of times we copy the data contained within
//! a message. Especially on resource-constrained hardware (mostly MP OSDs), we want to avoid the cost
//! of copying large buffers during serialization and deserialization.
//!
//! To support this, messages may contain zerocopy fields using the [`Lazy`] type.
//! `pb-jelly-gen` may generate these using the `blob`, `grpc_slices`, or `zero_copy` options; they
//! use different underlying types, which must implement [PbBuffer], but they all behave similarly.
//!
//! [PbBufferReader] and a [PbBufferWriter] have the opportunity to recognize [Lazy] fields.
//! At deserialization time, if [PbBufferReader] is used with a compatible [Lazy] field, instead of
//! allocating, it may simply store a reference to its underlying input buffer in the [Lazy].
//! Similarly, at serialization time, a [PbBufferWriter] used with a compatible [Lazy] may copy a
//! reference to the [Lazy] field into its output buffer, rather than copying its content.
//!
//! Request (bytes on the wire)
//! |
//! v
//! RPC Framework (with an underlying allocator, e.g. blob::Blob or grpc::Slice)
//! |
//! v
//! BR: [PbBufferReader] deserializes the struct using RPC framework's allocator.
//! |
//! v
//! Request (concrete struct containing a [Lazy] field)
//! |
//! v
//! RPC handler (doesn't modify the [Lazy] field)
//! |
//! v
//! Response (concrete struct containing a [Lazy] field)
//! |
//! v
//! BW: [PbBufferWriter] serializes the struct using RPC framework's allocator.
//! |
//! v
//! RPC Framework (with an underlying allocator, e.g. blob::Blob or grpc::Slice)
//! |
//! v
//! Response (bytes on the wire).
//!
//!
//! In the status quo, the behavior is as follows:
//!
//! `blob_pb::WrappedBlob` and `blob_pb::VecSlice` allow zero-copy deserialization -> serialization,
//! provided that their respective [PbBufferWriter]s are used.
//! Converting from `blob_pb::WrappedBlob` to a `blob_pb::VecSlice` is zero-copy.
//! Converting from a `blob_pb::VecSlice` to a `blob_pb::Blob` requires a single copy.
use std::any::Any;
use std::fmt::{
self,
Debug,
};
use std::io::{
Cursor,
Result,
Write,
};
use bytes::{
Buf,
Bytes,
};
use super::{
Message,
Reflection,
};
/// A stand-in trait for any backing buffer store.
/// `PbBuffer`s are expected to own references to the data they reference, and should be cheap
/// (constant-time) to clone.
#[allow(clippy::len_without_is_empty)]
pub trait PbBuffer: Any + Sized {
/// Returns the length of the data contained in this buffer.
fn len(&self) -> usize;
/// Fallback method to read the contents of `self`. This method is expected to write exactly
/// `self.len()` bytes into `writer`, or fail with an error.
///
/// This method is used to write `Lazy` fields to incompatible [`PbBufferWriter`]s.
fn copy_to_writer<W: Write + ?Sized>(&self, writer: &mut W) -> Result<()>;
/// Fallback method to create an instance of this `PbBuffer`.
///
/// This method is used to read `Lazy` fields from incompatible [`PbBufferReader`]s.
fn copy_from_reader<B: Buf + ?Sized>(reader: &mut B) -> Result<Self>;
}
/// If `B1` and `B2` are the same type, returns a function to cast `B1 -> B2`; otherwise None.
/// Used to implement [PbBuffer] casting.
pub fn type_is<B1: 'static, B2: 'static>() -> Option<fn(B1) -> B2> {
let f: fn(B1) -> B1 = |x| x;
// If B1 = B2, then this cast should succeed!
(&f as &dyn Any).downcast_ref::<fn(B1) -> B2>().copied()
}
/// All concrete types which are used for deserialization should implement
/// [PbBufferReader], which includes functions to convert to and from [PbBuffer].
pub trait PbBufferReader: Buf {
/// Attempt to read into a compatible [PbBuffer], avoiding a copy if possible.
/// The implementation should dispatch on the type `B`. If unsupported,
/// the reader may fall back to [PbBuffer::copy_from_reader].
fn read_buffer<B: PbBuffer>(&mut self) -> Result<B> {
B::copy_from_reader(self)
}
/// Advance the interal cursor by `at`, and return a [PbBufferReader] corresponding to the
/// traversed indices (i.e. self.position..self.position + at).
fn split(&mut self, at: usize) -> Self;
}
/// All concrete types used for serialization should implement [PbBufferWriter] in order to support
/// serializing [Lazy] fields without copies.
pub trait PbBufferWriter: Write {
/// Attempt to write a zerocopy buffer into `self`. If `B` is not zero-copy-supported
/// by the [PbBufferWriter], this may read/copy the bytes out from `buf`.
fn write_buffer<B: PbBuffer>(&mut self, buf: &B) -> Result<()>;
}
/// A wrapper around a [PbBuffer], which implements [Message].
#[derive(Clone, PartialEq)]
pub struct Lazy<B> {
// TODO: Make this not an `Option` by giving `VecSlice` a cheap `Default` impl
contents: Option<B>,
}
impl<B> Default for Lazy<B> {
fn default() -> Self {
Self { contents: None }
}
}
impl<B> Lazy<B> {
pub fn new(r: B) -> Self {
Self { contents: Some(r) }
}
pub fn into_buffer(self) -> B
where
B: Default,
{
self.contents.unwrap_or_default()
}
}
impl<B> Debug for Lazy<B> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Lazy")
.field("contents", &self.contents.as_ref().map(|_| "_"))
.finish()
}
}
impl<B: PbBuffer + PartialEq> Message for Lazy<B> {
fn compute_size(&self) -> usize {
self.contents.as_ref().map(PbBuffer::len).unwrap_or(0)
}
fn compute_grpc_slices_size(&self) -> usize {
self.contents.as_ref().map(PbBuffer::len).unwrap_or(0)
}
fn serialize<W: PbBufferWriter>(&self, w: &mut W) -> Result<()> {
if let Some(ref contents) = self.contents {
w.write_buffer(contents)?;
}
Ok(())
}
fn deserialize<R: PbBufferReader>(&mut self, r: &mut R) -> Result<()> {
self.contents = Some(r.read_buffer()?);
Ok(())
}
}
impl<B: PbBuffer + PartialEq> Reflection for Lazy<B> {}
impl<'a> PbBufferReader for Cursor<&'a [u8]> {
fn split(&mut self, at: usize) -> Self {
let pos = self.position() as usize;
self.advance(at);
let new_slice = &self.get_ref()[pos..pos + at];
Self::new(new_slice)
}
}
impl PbBuffer for Bytes {
#[inline]
fn len(&self) -> usize {
self.len()
}
fn copy_to_writer<W: Write + ?Sized>(&self, writer: &mut W) -> Result<()> {
writer.write_all(&self)
}
fn copy_from_reader<B: Buf + ?Sized>(reader: &mut B) -> Result<Self> {
let len = reader.remaining();
Ok(reader.copy_to_bytes(len))
}
}
impl PbBufferReader for Cursor<Bytes> {
fn read_buffer<B: PbBuffer>(&mut self) -> Result<B> {
if let Some(cast) = type_is::<Bytes, B>() {
let bytes = self.get_ref().slice((self.position() as usize)..);
Ok(cast(bytes))
} else {
B::copy_from_reader(self)
}
}
#[inline]
fn split(&mut self, at: usize) -> Self {
let pos = self.position() as usize;
self.advance(at);
let new_slice = self.get_ref().slice(pos..(pos + at));
Self::new(new_slice)
}
}
impl<'a> PbBufferWriter for Cursor<&'a mut Vec<u8>> {
/// Note: this implementation freely copies the data out of `buf`.
#[inline]
fn write_buffer<B: PbBuffer>(&mut self, buf: &B) -> Result<()> {
buf.copy_to_writer(self)
}
}
impl<'a> PbBufferWriter for Cursor<&'a mut [u8]> {
/// Note: this implementation freely copies the data out of `buf`.
#[inline]
fn write_buffer<B: PbBuffer>(&mut self, buf: &B) -> Result<()> {
buf.copy_to_writer(self)
}
}
/// A wrapper around a [Write] which copies all [Lazy] data into the underlying [Write]r.
pub struct CopyWriter<'a, W: Write> {
pub writer: &'a mut W,
}
impl<'a, W: Write + 'a> Write for CopyWriter<'a, W> {
#[inline]
fn write(&mut self, buf: &[u8]) -> Result<usize> {
self.writer.write(buf)
}
#[inline]
fn flush(&mut self) -> Result<()> {
self.writer.flush()
}
}
impl<'a, W: Write + 'a> PbBufferWriter for CopyWriter<'a, W> {
/// Note: this implementation freely copies the data out of `buf`.
#[inline]
fn write_buffer<B: PbBuffer>(&mut self, buf: &B) -> Result<()> {
buf.copy_to_writer(self.writer)
}
}
#[test]
fn test_lazy_bytes_deserialize() {
let mut lazy = Lazy::<Bytes>::default();
let bytes = Bytes::from_static(b"asdfasdf");
lazy.deserialize(&mut Cursor::new(bytes.clone()))
.expect("failed to deserialize");
let deserialized = lazy.into_buffer();
assert_eq!(deserialized, bytes, "The entire buffer should be copied");
assert_eq!(
deserialized.as_ptr(),
bytes.as_ptr(),
"The Bytes instance should be cloned"
)
}