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use crate::poll::Subscribe;
use anyhow::Result;
use bytes::Bytes;
/// Host trait for implementing the `wasi:io/streams.input-stream` resource: A
/// bytestream which can be read from.
#[async_trait::async_trait]
pub trait HostInputStream: Subscribe {
/// Reads up to `size` bytes, returning a buffer holding these bytes on
/// success.
///
/// This function does not block the current thread and is the equivalent of
/// a non-blocking read. On success all bytes read are returned through
/// `Bytes`, which is no larger than the `size` provided. If the returned
/// list of `Bytes` is empty then no data is ready to be read at this time.
///
/// # Errors
///
/// The [`StreamError`] return value communicates when this stream is
/// closed, when a read fails, or when a trap should be generated.
fn read(&mut self, size: usize) -> StreamResult<Bytes>;
/// Similar to `read`, except that it blocks until at least one byte can be
/// read.
async fn blocking_read(&mut self, size: usize) -> StreamResult<Bytes> {
self.ready().await;
self.read(size)
}
/// Same as the `read` method except that bytes are skipped.
///
/// Note that this method is non-blocking like `read` and returns the same
/// errors.
fn skip(&mut self, nelem: usize) -> StreamResult<usize> {
let bs = self.read(nelem)?;
Ok(bs.len())
}
/// Similar to `skip`, except that it blocks until at least one byte can be
/// skipped.
async fn blocking_skip(&mut self, nelem: usize) -> StreamResult<usize> {
let bs = self.blocking_read(nelem).await?;
Ok(bs.len())
}
/// Cancel any asynchronous work and wait for it to wrap up.
async fn cancel(&mut self) {}
}
/// Representation of the `error` resource type in the `wasi:io/error`
/// interface.
///
/// This is currently `anyhow::Error` to retain full type information for
/// errors.
pub type Error = anyhow::Error;
pub type StreamResult<T> = Result<T, StreamError>;
#[derive(Debug)]
pub enum StreamError {
Closed,
LastOperationFailed(anyhow::Error),
Trap(anyhow::Error),
}
impl StreamError {
pub fn trap(msg: &str) -> StreamError {
StreamError::Trap(anyhow::anyhow!("{msg}"))
}
}
impl std::fmt::Display for StreamError {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
match self {
StreamError::Closed => write!(f, "closed"),
StreamError::LastOperationFailed(e) => write!(f, "last operation failed: {e}"),
StreamError::Trap(e) => write!(f, "trap: {e}"),
}
}
}
impl std::error::Error for StreamError {
fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
match self {
StreamError::Closed => None,
StreamError::LastOperationFailed(e) | StreamError::Trap(e) => e.source(),
}
}
}
impl From<wasmtime::component::ResourceTableError> for StreamError {
fn from(error: wasmtime::component::ResourceTableError) -> Self {
Self::Trap(error.into())
}
}
/// Host trait for implementing the `wasi:io/streams.output-stream` resource:
/// A bytestream which can be written to.
#[async_trait::async_trait]
pub trait HostOutputStream: Subscribe {
/// Write bytes after obtaining a permit to write those bytes
///
/// Prior to calling [`write`](Self::write) the caller must call
/// [`check_write`](Self::check_write), which resolves to a non-zero permit
///
/// This method must never block. The [`check_write`](Self::check_write)
/// permit indicates the maximum amount of bytes that are permitted to be
/// written in a single [`write`](Self::write) following the
/// [`check_write`](Self::check_write) resolution.
///
/// # Errors
///
/// Returns a [`StreamError`] if:
/// - stream is closed
/// - prior operation ([`write`](Self::write) or [`flush`](Self::flush)) failed
/// - caller performed an illegal operation (e.g. wrote more bytes than were permitted)
fn write(&mut self, bytes: Bytes) -> StreamResult<()>;
/// Trigger a flush of any bytes buffered in this stream implementation.
///
/// This method may be called at any time and must never block.
///
/// After this method is called, [`check_write`](Self::check_write) must
/// pend until flush is complete.
///
/// When [`check_write`](Self::check_write) becomes ready after a flush,
/// that guarantees that all prior writes have been flushed from the
/// implementation successfully, or that any error associated with those
/// writes is reported in the return value of [`flush`](Self::flush) or
/// [`check_write`](Self::check_write)
///
/// # Errors
///
/// Returns a [`StreamError`] if:
/// - stream is closed
/// - prior operation ([`write`](Self::write) or [`flush`](Self::flush)) failed
/// - caller performed an illegal operation (e.g. wrote more bytes than were permitted)
fn flush(&mut self) -> StreamResult<()>;
/// Returns the number of bytes that are ready to be written to this stream.
///
/// Zero bytes indicates that this stream is not currently ready for writing
/// and `ready()` must be awaited first.
///
/// Note that this method does not block.
///
/// # Errors
///
/// Returns an [`StreamError`] if:
/// - stream is closed
/// - prior operation ([`write`](Self::write) or [`flush`](Self::flush)) failed
fn check_write(&mut self) -> StreamResult<usize>;
/// Perform a write of up to 4096 bytes, and then flush the stream. Block
/// until all of these operations are complete, or an error occurs.
///
/// This is a convenience wrapper around the use of `check-write`,
/// `subscribe`, `write`, and `flush`, and is implemented with the
/// following pseudo-code:
///
/// ```text
/// let pollable = this.subscribe();
/// while !contents.is_empty() {
/// // Wait for the stream to become writable
/// pollable.block();
/// let Ok(n) = this.check-write(); // eliding error handling
/// let len = min(n, contents.len());
/// let (chunk, rest) = contents.split_at(len);
/// this.write(chunk ); // eliding error handling
/// contents = rest;
/// }
/// this.flush();
/// // Wait for completion of `flush`
/// pollable.block();
/// // Check for any errors that arose during `flush`
/// let _ = this.check-write(); // eliding error handling
/// ```
async fn blocking_write_and_flush(&mut self, mut bytes: Bytes) -> StreamResult<()> {
loop {
let permit = self.write_ready().await?;
let len = bytes.len().min(permit);
let chunk = bytes.split_to(len);
self.write(chunk)?;
if bytes.is_empty() {
break;
}
}
self.flush()?;
self.write_ready().await?;
Ok(())
}
/// Repeatedly write a byte to a stream.
/// Important: this write must be non-blocking!
/// Returning an Err which downcasts to a [`StreamError`] will be
/// reported to Wasm as the empty error result. Otherwise, errors will trap.
fn write_zeroes(&mut self, nelem: usize) -> StreamResult<()> {
// TODO: We could optimize this to not allocate one big zeroed buffer, and instead write
// repeatedly from a 'static buffer of zeros.
let bs = Bytes::from_iter(core::iter::repeat(0).take(nelem));
self.write(bs)?;
Ok(())
}
/// Perform a write of up to 4096 zeroes, and then flush the stream.
/// Block until all of these operations are complete, or an error
/// occurs.
///
/// This is a convenience wrapper around the use of `check-write`,
/// `subscribe`, `write-zeroes`, and `flush`, and is implemented with
/// the following pseudo-code:
///
/// ```text
/// let pollable = this.subscribe();
/// while num_zeroes != 0 {
/// // Wait for the stream to become writable
/// pollable.block();
/// let Ok(n) = this.check-write(); // eliding error handling
/// let len = min(n, num_zeroes);
/// this.write-zeroes(len); // eliding error handling
/// num_zeroes -= len;
/// }
/// this.flush();
/// // Wait for completion of `flush`
/// pollable.block();
/// // Check for any errors that arose during `flush`
/// let _ = this.check-write(); // eliding error handling
/// ```
async fn blocking_write_zeroes_and_flush(&mut self, nelem: usize) -> StreamResult<()> {
// TODO: We could optimize this to not allocate one big zeroed buffer, and instead write
// repeatedly from a 'static buffer of zeros.
let bs = Bytes::from_iter(core::iter::repeat(0).take(nelem));
self.blocking_write_and_flush(bs).await
}
/// Simultaneously waits for this stream to be writable and then returns how
/// much may be written or the last error that happened.
async fn write_ready(&mut self) -> StreamResult<usize> {
self.ready().await;
self.check_write()
}
/// Cancel any asynchronous work and wait for it to wrap up.
async fn cancel(&mut self) {}
}
#[async_trait::async_trait]
impl Subscribe for Box<dyn HostOutputStream> {
async fn ready(&mut self) {
(**self).ready().await
}
}
#[async_trait::async_trait]
impl Subscribe for Box<dyn HostInputStream> {
async fn ready(&mut self) {
(**self).ready().await
}
}
pub type InputStream = Box<dyn HostInputStream>;
pub type OutputStream = Box<dyn HostOutputStream>;