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// Copyright (c) Microsoft Corporation.
// Licensed under the MIT License.
//! Table-based decoding.
use super::StructMetadata;
use super::TableEncoder;
use crate::inplace::InplaceOption;
use crate::protobuf::FieldReader;
use crate::protobuf::MessageReader;
use crate::Error;
use crate::FieldDecode;
use crate::MessageDecode;
use core::marker::PhantomData;
use std::mem::MaybeUninit;
use std::slice;
/// Calls `f` on `item`, splitting the pointer and initialized flag out.
///
/// # Safety
///
/// The caller must ensure that on the return, the bool specifies the
/// initialized state of the item.
unsafe fn run_inplace<T, R>(
item: &mut InplaceOption<'_, T>,
f: impl FnOnce(*mut u8, &mut bool) -> R,
) -> R {
let mut initialized = item.forget();
let base = item.as_mut_ptr().cast::<u8>();
let r = f(base, &mut initialized);
if initialized {
// SAFETY: the caller ensures that `item` is initialized.
unsafe { item.set_init_unchecked() };
}
r
}
impl<'de, T, R> MessageDecode<'de, T, R> for TableEncoder
where
T: StructDecodeMetadata<'de, R>,
{
fn read_message(
item: &mut InplaceOption<'_, T>,
reader: MessageReader<'de, '_, R>,
) -> crate::Result<()> {
// SAFETY: T guarantees that the metadata is valid.
unsafe {
run_inplace(item, |base, initialized| {
read_fields(
T::NUMBERS,
T::DECODERS,
T::OFFSETS,
base,
initialized,
reader,
)
})
}
}
}
impl<'de, T, R> FieldDecode<'de, T, R> for TableEncoder
where
T: StructDecodeMetadata<'de, R>,
{
// Override the default implementation to use the table decoder directly.
// This saves code size by avoiding extra stub functions and vtables.
const ENTRY: DecoderEntry<'de, T, R> = DecoderEntry::table();
fn read_field(
item: &mut InplaceOption<'_, T>,
reader: FieldReader<'de, '_, R>,
) -> crate::Result<()> {
// SAFETY: T guarantees that the metadata is valid.
unsafe {
run_inplace(item, |base, initialized| {
read_message(
T::NUMBERS,
T::DECODERS,
T::OFFSETS,
base,
initialized,
reader,
)
})
}
}
fn default_field(item: &mut InplaceOption<'_, T>) -> crate::Result<()> {
// SAFETY: T guarantees that the metadata is valid.
unsafe {
run_inplace(item, |base, initialized| {
default_fields(T::DECODERS, T::OFFSETS, base, initialized)
})
}
}
}
/// Read a field as a message from the provided field metadata.
///
/// # Safety
///
/// The caller must ensure that `base` points to a location that can be written
/// to, that `struct_initialized` is set correctly, and that the metadata is
/// correct and complete for the type of the object pointed to by `base`.
#[doc(hidden)] // only used publicly in mesh_derive
pub unsafe fn read_message<R>(
numbers: &[u32],
decoders: &[ErasedDecoderEntry],
offsets: &[usize],
base: *mut u8,
struct_initialized: &mut bool,
reader: FieldReader<'_, '_, R>,
) -> Result<(), Error> {
assert_eq!(numbers.len(), decoders.len());
assert_eq!(numbers.len(), offsets.len());
// SAFETY: guaranteed by caller and by the assertions above.
unsafe {
// Convert the slices to pointers and a single length to shrink
// code size.
read_message_by_ptr(
numbers.len(),
numbers.as_ptr(),
decoders.as_ptr(),
offsets.as_ptr(),
base,
struct_initialized,
reader,
)
}
}
// Don't inline this since it is used by every table decoder instantiation.
#[inline(never)]
unsafe fn read_message_by_ptr<R>(
count: usize,
numbers: *const u32,
decoders: *const ErasedDecoderEntry,
offsets: *const usize,
base: *mut u8,
struct_initialized: &mut bool,
reader: FieldReader<'_, '_, R>,
) -> Result<(), Error> {
// SAFETY: guaranteed by caller.
unsafe {
read_fields_inline(
slice::from_raw_parts(numbers, count),
slice::from_raw_parts(decoders, count),
slice::from_raw_parts(offsets, count),
base,
struct_initialized,
reader.message()?,
)
}
}
/// Read a message from the provided field metadata.
///
/// # Safety
///
/// The caller must ensure that `base` points to a location that can be written
/// to, that `struct_initialized` is set correctly, and that the metadata is
/// correct and complete for the type of the object pointed to by `base`.
unsafe fn read_fields<R>(
numbers: &[u32],
decoders: &[ErasedDecoderEntry],
offsets: &[usize],
base: *mut u8,
struct_initialized: &mut bool,
reader: MessageReader<'_, '_, R>,
) -> Result<(), Error> {
assert_eq!(numbers.len(), decoders.len());
assert_eq!(numbers.len(), offsets.len());
// SAFETY: guaranteed by caller and by the assertions above.
unsafe {
// Convert the slices to pointers and a single length to shrink
// code size.
read_fields_by_ptr(
numbers.len(),
numbers.as_ptr(),
decoders.as_ptr(),
offsets.as_ptr(),
base,
struct_initialized,
reader,
)
}
}
// Don't inline this since it is used by every table decoder instantiation.
#[inline(never)]
unsafe fn read_fields_by_ptr<R>(
count: usize,
numbers: *const u32,
decoders: *const ErasedDecoderEntry,
offsets: *const usize,
base: *mut u8,
struct_initialized: &mut bool,
reader: MessageReader<'_, '_, R>,
) -> Result<(), Error> {
// SAFETY: guaranteed by caller.
unsafe {
read_fields_inline(
slice::from_raw_parts(numbers, count),
slice::from_raw_parts(decoders, count),
slice::from_raw_parts(offsets, count),
base,
struct_initialized,
reader,
)
}
}
/// Reads fields from the provided field metadata.
///
/// # Safety
///
/// The caller must ensure that `base` points to a location that can be written
/// to, that `initialized` is set correctly, and that the metadata is correct
/// and complete for the type of the object pointed to by `base`.
unsafe fn read_fields_inline<R>(
numbers: &[u32],
decoders: &[ErasedDecoderEntry],
offsets: &[usize],
base: *mut u8,
struct_initialized: &mut bool,
reader: MessageReader<'_, '_, R>,
) -> Result<(), Error> {
const STACK_LIMIT: usize = 32;
let mut field_init_static;
let mut field_init_dynamic;
let field_inits = if numbers.len() <= STACK_LIMIT {
field_init_static = [false; STACK_LIMIT];
field_init_static[..numbers.len()].fill(*struct_initialized);
&mut field_init_static[..numbers.len()]
} else {
field_init_dynamic = vec![*struct_initialized; numbers.len()];
&mut field_init_dynamic[..]
};
// SAFETY: guaranteed by caller.
let r = unsafe { read_fields_inner(numbers, decoders, offsets, base, field_inits, reader) };
*struct_initialized = true;
if r.is_err() && !field_inits.iter().all(|&b| b) {
// Drop any initialized fields.
for ((field_init, &offset), decoder) in field_inits.iter_mut().zip(offsets).zip(decoders) {
if *field_init {
// SAFETY: guaranteed by the caller.
unsafe {
decoder.drop_struct(base.add(offset));
}
}
}
*struct_initialized = false;
}
r
}
/// Reads fields from the provided field metadata, but does not drop any fields
/// of a partially initialized message on failure.
unsafe fn read_fields_inner<R>(
numbers: &[u32],
decoders: &[ErasedDecoderEntry],
offsets: &[usize],
base: *mut u8,
field_init: &mut [bool],
reader: MessageReader<'_, '_, R>,
) -> Result<(), Error> {
let decoders = &decoders[..numbers.len()];
let offsets = &offsets[..numbers.len()];
let field_init = &mut field_init[..numbers.len()];
for field in reader {
let (number, reader) = field?;
if let Some(index) = numbers.iter().position(|&n| n == number) {
let decoder = &decoders[index];
// SAFETY: the decoder is valid according to the caller.
unsafe {
decoder.read_struct(base.add(offsets[index]), &mut field_init[index], reader)?;
}
}
}
for ((field_init, &offset), decoder) in field_init.iter_mut().zip(offsets).zip(decoders) {
if !*field_init {
// SAFETY: the decoder is valid according to the caller.
unsafe {
decoder.default_struct(base.add(offset), field_init)?;
}
assert!(*field_init);
}
}
Ok(())
}
/// Sets fields to their default values from the provided field metadata.
///
/// # Safety
///
/// The caller must ensure that `base` points to a location that can be written
/// to, that `struct_initialized` is set correctly, and that the metadata is
/// correct and complete for the type of the object pointed to by `base`.
unsafe fn default_fields(
decoders: &[ErasedDecoderEntry],
offsets: &[usize],
base: *mut u8,
struct_initialized: &mut bool,
) -> Result<(), Error> {
assert_eq!(decoders.len(), offsets.len());
// SAFETY: guaranteed by caller and by the assertion above.
unsafe {
default_fields_by_ptr(
decoders.len(),
decoders.as_ptr(),
offsets.as_ptr(),
base,
struct_initialized,
)
}
}
#[inline(never)]
unsafe fn default_fields_by_ptr(
count: usize,
decoders: *const ErasedDecoderEntry,
offsets: *const usize,
base: *mut u8,
struct_initialized: &mut bool,
) -> Result<(), Error> {
// SAFETY: guaranteed by caller.
unsafe {
default_fields_inline(
slice::from_raw_parts(decoders, count),
slice::from_raw_parts(offsets, count),
base,
struct_initialized,
)
}
}
unsafe fn default_fields_inline(
decoders: &[ErasedDecoderEntry],
offsets: &[usize],
base: *mut u8,
struct_initialized: &mut bool,
) -> Result<(), Error> {
for (i, (&offset, decoder)) in offsets.iter().zip(decoders).enumerate() {
let mut field_initialized = *struct_initialized;
// SAFETY: the decoder is valid according to the caller.
let r = unsafe { decoder.default_struct(base.add(offset), &mut field_initialized) };
if let Err(err) = r {
if !field_initialized || !*struct_initialized {
// Drop initialized fields.
let initialized_until = i;
for (i, (&offset, decoder)) in offsets.iter().zip(decoders).enumerate() {
if i < initialized_until
|| (i == initialized_until && field_initialized)
|| (i > initialized_until && *struct_initialized)
{
// SAFETY: the decoder is valid according to the caller, and the field is initialized.
unsafe {
decoder.drop_struct(base.add(offset));
}
}
}
*struct_initialized = false;
}
return Err(err);
}
assert!(field_initialized);
}
*struct_initialized = true;
Ok(())
}
/// The struct metadata for decoding a struct.
///
/// # Safety
///
/// The implementor must ensure that the `DECODERS` are correct and complete for
/// `Self`, such that if every field is decoded, then the struct value is valid.
pub unsafe trait StructDecodeMetadata<'de, R>: StructMetadata {
/// The list of decoder vtables.
const DECODERS: &'static [ErasedDecoderEntry];
}
/// An entry in the decoder table.
///
/// This contains the metadata necessary to apply an decoder to a field.
///
/// This cannot be instantiated directly; use [`FieldDecode::ENTRY`] to get an
/// instance for a particular decoder.
pub struct DecoderEntry<'a, T, R>(
ErasedDecoderEntry,
PhantomData<fn(&mut T, &mut R, &'a mut ())>,
);
impl<'a, T, R> DecoderEntry<'a, T, R> {
/// # Safety
/// The caller must ensure that the erased entry is an valid entry for `T`
/// and `R`.
pub(crate) const unsafe fn new_unchecked(entry: ErasedDecoderEntry) -> Self {
Self(entry, PhantomData)
}
pub(crate) const fn custom<E: FieldDecode<'a, T, R>>() -> Self {
Self(
ErasedDecoderEntry(
std::ptr::from_ref(
const {
&StaticDecoderVtable {
read_fn: read_field_dyn::<T, R, E>,
default_fn: default_field_dyn::<T, R, E>,
drop_fn: if std::mem::needs_drop::<T>() {
Some(drop_field_dyn::<T>)
} else {
None
},
}
},
)
.cast(),
),
PhantomData,
)
}
const fn table() -> Self
where
T: StructDecodeMetadata<'a, R>,
{
Self(
ErasedDecoderEntry(
std::ptr::from_ref(
const {
&DecoderTable {
count: T::NUMBERS.len(),
numbers: T::NUMBERS.as_ptr(),
decoders: T::DECODERS.as_ptr(),
offsets: T::OFFSETS.as_ptr(),
}
},
)
.cast::<()>()
.wrapping_byte_add(ENTRY_IS_TABLE),
),
PhantomData,
)
}
/// Erases the type of the decoder entry.
pub const fn erase(&self) -> ErasedDecoderEntry {
self.0
}
}
/// An entry in a [`StructDecodeMetadata::DECODERS`] table.
//
// Internally, this is a pointer to either a vtable or a table.
// The low bit is used to distinguish between the two.
#[derive(Copy, Clone)]
pub struct ErasedDecoderEntry(*const ());
const ENTRY_IS_TABLE: usize = 1;
const _: () = assert!(align_of::<ErasedDecoderEntry>() > ENTRY_IS_TABLE);
const _: () = assert!(align_of::<StaticDecoderVtable<'_, ()>>() > ENTRY_IS_TABLE);
impl ErasedDecoderEntry {
/// Decodes the entry into either a vtable or a table.
///
/// # Safety
/// The caller must ensure that the encoder was for resource type `R`.
unsafe fn decode<'de, R>(&self) -> Result<&StaticDecoderVtable<'de, R>, &DecoderTable> {
// SAFETY: guaranteed by caller.
unsafe {
if self.0 as usize & ENTRY_IS_TABLE == 0 {
Ok(&*self.0.cast::<StaticDecoderVtable<'_, R>>())
} else {
Err(&*self
.0
.wrapping_byte_sub(ENTRY_IS_TABLE)
.cast::<DecoderTable>())
}
}
}
/// Reads a struct using the decoder metadata.
///
/// # Safety
/// The caller must ensure that the decoder was for resource type `R` and
/// the object type matching what `ptr` is pointing to.
unsafe fn read_struct<R>(
&self,
ptr: *mut u8,
init: &mut bool,
reader: FieldReader<'_, '_, R>,
) -> Result<(), Error> {
// SAFETY: guaranteed by caller.
unsafe {
match self.decode::<R>() {
Ok(vtable) => (vtable.read_fn)(ptr, init, reader),
Err(table) => read_message_by_ptr(
table.count,
table.numbers,
table.decoders,
table.offsets,
ptr,
init,
reader,
),
}
}
}
/// Initializes a struct to its default state using the decoder metadata.
///
/// # Safety
/// The caller must ensure that the decoder was for the object type matching
/// what `ptr` is pointing to.
unsafe fn default_struct(&self, ptr: *mut u8, init: &mut bool) -> Result<(), Error> {
// SAFETY: guaranteed by caller.
unsafe {
match self.decode::<()>() {
Ok(vtable) => (vtable.default_fn)(ptr, init),
Err(table) => {
default_fields_by_ptr(table.count, table.decoders, table.offsets, ptr, init)
}
}
}
}
/// Drops a struct in place using the decoder metadata.
///
/// # Safety
/// The caller must ensure that the decoder was for the object type matching
/// what `ptr` is pointing to, and that `ptr` is ready to be dropped.
unsafe fn drop_struct(&self, ptr: *mut u8) {
// SAFETY: guaranteed by caller.
unsafe {
match self.decode::<()>() {
Ok(vtable) => {
if let Some(drop_fn) = vtable.drop_fn {
drop_fn(ptr);
}
}
Err(table) => {
for i in 0..table.count {
let offset = *table.offsets.add(i);
let decoder = &*table.decoders.add(i);
decoder.drop_struct(ptr.add(offset));
}
}
}
}
}
}
struct DecoderTable {
count: usize,
numbers: *const u32,
decoders: *const ErasedDecoderEntry,
offsets: *const usize,
}
/// A vtable for decoding a message.
#[repr(C)] // to ensure the layout is the same regardless of R
struct StaticDecoderVtable<'de, R> {
read_fn: unsafe fn(*mut u8, init: *mut bool, FieldReader<'de, '_, R>) -> Result<(), Error>,
default_fn: unsafe fn(*mut u8, init: *mut bool) -> Result<(), Error>,
drop_fn: Option<unsafe fn(*mut u8)>,
}
unsafe fn read_field_dyn<'a, T, R, E: FieldDecode<'a, T, R>>(
field: *mut u8,
init: *mut bool,
reader: FieldReader<'a, '_, R>,
) -> Result<(), Error> {
// SAFETY: `init` is valid according to the caller.
let init = unsafe { &mut *init };
// SAFETY: `field` is valid and points to a valid `MaybeUninit<T>` according
// to the caller.
let field = unsafe { &mut *field.cast::<MaybeUninit<T>>() };
let mut field = if *init {
// SAFETY: the caller attests that the field is initialized.
unsafe { InplaceOption::new_init_unchecked(field) }
} else {
InplaceOption::uninit(field)
};
let r = E::read_field(&mut field, reader);
*init = field.forget();
r
}
unsafe fn default_field_dyn<'a, T, R, E: FieldDecode<'a, T, R>>(
field: *mut u8,
init: *mut bool,
) -> Result<(), Error> {
// SAFETY: `init` is valid according to the caller.
let init = unsafe { &mut *init };
// SAFETY: `field` is valid and points to a valid `MaybeUninit<T>` according
// to the caller.
let field = unsafe { &mut *field.cast::<MaybeUninit<T>>() };
let mut field = if *init {
// SAFETY: the caller attests that the field is initialized.
unsafe { InplaceOption::new_init_unchecked(field) }
} else {
InplaceOption::uninit(field)
};
let r = E::default_field(&mut field);
*init = field.forget();
r
}
unsafe fn drop_field_dyn<T>(field: *mut u8) {
let field = field.cast::<T>();
// SAFETY: `field` is valid and points to a valid `T` according to the
// caller.
unsafe { field.drop_in_place() }
}