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use std::io;
use std::time::{Duration, Instant};
use crate::backend::Storage;
use crate::error::{Error, Result};
use crate::header::{
read_u32_at, validate_capacity, verify_header, wait_slice, write_u32_at, HEADER_SIZE,
OFF_CLOSED, OFF_HEAD, OFF_TAIL,
};
use crate::options::Options;
/// The consumer side of a ring buffer. A `Reader` must only be used from a
/// single thread at a time.
pub struct Reader<S: Storage> {
storage: S,
capacity: u64,
mask: u64,
options: Options,
head: u32, // local authoritative copy; persisted to storage on every read
cached_tail: u32, // last tail value observed from storage
}
impl<S: Storage> Reader<S> {
/// Attaches to a ring buffer previously initialized by
/// [`Writer::new`](crate::Writer::new) on the same storage. `capacity`
/// must match the value the writer used.
///
/// This is the low-level entry point used by [`open_shm`](crate::open_shm);
/// use it directly to run the ring buffer over a custom [`Storage`].
pub fn new(storage: S, capacity: u64, options: Options) -> Result<Self> {
validate_capacity(capacity)?;
if storage.size() < HEADER_SIZE + capacity {
return Err(Error::StorageTooSmall);
}
verify_header(&storage, capacity)?;
Ok(Reader {
storage,
capacity,
mask: capacity - 1,
options,
head: 0,
cached_tail: 0,
})
}
/// Reads as many bytes as are currently available into `buf` without
/// blocking, returning the number of bytes read. Returns `Ok(0)` if the
/// buffer is empty and still open. Once the buffer is empty and the
/// writer has been closed, returns `Err(Error::Eof)`.
pub fn try_read(&mut self, buf: &mut [u8]) -> Result<usize> {
if buf.is_empty() {
return Ok(0);
}
let mut avail = self.cached_tail.wrapping_sub(self.head) as i64;
if avail < buf.len() as i64 {
// The cached tail may be stale (the writer has produced more
// than we've observed); refresh it before concluding there's
// no data.
self.cached_tail = read_u32_at(&self.storage, OFF_TAIL)?;
avail = self.cached_tail.wrapping_sub(self.head) as i64;
}
if avail <= 0 {
let closed = read_u32_at(&self.storage, OFF_CLOSED)?;
if closed != 0 {
return Err(Error::Eof);
}
return Ok(0);
}
let mut n = buf.len() as i64;
if n > avail {
n = avail;
}
let n = n as u64;
let start = self.head as u64 & self.mask;
if start + n <= self.capacity {
self.storage
.read_at(&mut buf[..n as usize], HEADER_SIZE + start)?;
} else {
let first = self.capacity - start;
self.storage
.read_at(&mut buf[..first as usize], HEADER_SIZE + start)?;
self.storage
.read_at(&mut buf[first as usize..n as usize], HEADER_SIZE)?;
}
self.head = self.head.wrapping_add(n as u32);
write_u32_at(&self.storage, OFF_HEAD, self.head)?;
Ok(n as usize)
}
/// Blocks until at least one byte is available and reads into `buf`, or
/// returns `Err(Error::Timeout)` if `timeout` elapses first. Returns
/// `Err(Error::Eof)` once the writer has closed and all buffered data
/// has been drained. Mirrors Go's `ReadContext`.
pub fn read_timeout(&mut self, buf: &mut [u8], timeout: Duration) -> Result<usize> {
self.read_until(buf, Some(Instant::now() + timeout))
}
fn read_until(&mut self, buf: &mut [u8], deadline: Option<Instant>) -> Result<usize> {
let can_wait = self.storage.supports_wait();
let mut wait = self.options.min_poll;
loop {
let n = self.try_read(buf)?;
if n > 0 {
return Ok(n);
}
if let Some(deadline) = deadline {
if Instant::now() >= deadline {
return Err(Error::Timeout);
}
}
if can_wait {
// Block on a real wakeup tied to OFF_TAIL instead of
// sleeping; see Writer::write_until's wait_slice for why
// the wait is bounded when there's a deadline.
self.storage
.wait_u32_at(OFF_TAIL, self.cached_tail, wait_slice(deadline));
continue;
}
std::thread::sleep(wait);
wait = (wait * 2).min(self.options.max_poll);
}
}
/// Releases the reader's handle on the underlying storage. For OS
/// shared memory this unmaps the segment (it does not remove it; only
/// the creating writer's `close_storage` does that).
pub fn close(self) -> Result<()> {
self.storage.close()
}
/// The last tail value this `Reader` has observed from `storage`
/// (refreshed by `try_read` whenever the cached one looked
/// insufficient). crate-internal: used by
/// [`wasm_api`](crate::wasm_api) to drive `Atomics.waitAsync` on
/// `OFF_TAIL` without duplicating the ring buffer's own bookkeeping.
#[cfg_attr(
not(all(target_arch = "wasm32", target_os = "unknown")),
allow(dead_code)
)]
pub(crate) fn cached_tail(&self) -> u32 {
self.cached_tail
}
/// crate-internal accessor to the underlying storage, for the same
/// reason as [`cached_tail`](Reader::cached_tail).
#[cfg_attr(
not(all(target_arch = "wasm32", target_os = "unknown")),
allow(dead_code)
)]
pub(crate) fn storage(&self) -> &S {
&self.storage
}
}
impl<S: Storage> std::fmt::Debug for Reader<S> {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
f.debug_struct("Reader")
.field("capacity", &self.capacity)
.finish()
}
}
impl<S: Storage> io::Read for Reader<S> {
/// Blocks until at least one byte is available and reads into `buf`,
/// implementing [`io::Read`]. Returns `Ok(0)` once the writer has
/// closed and all buffered data has been drained (this trait's
/// end-of-stream convention).
fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
if buf.is_empty() {
return Ok(0);
}
let can_wait = self.storage.supports_wait();
let mut wait = self.options.min_poll;
loop {
match self.try_read(buf) {
Ok(0) => {
if can_wait {
self.storage.wait_u32_at(OFF_TAIL, self.cached_tail, None);
} else {
std::thread::sleep(wait);
wait = (wait * 2).min(self.options.max_poll);
}
}
Ok(n) => return Ok(n),
Err(Error::Eof) => return Ok(0),
Err(Error::Io(e)) => return Err(e),
Err(other) => return Err(io::Error::other(other)),
}
}
}
}