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//! Structures for managing the state of a queue.
use std::cmp::{Ordering, PartialOrd};
use std::fs::*;
use std::io::{self, BufWriter, Read, Write};
use std::path::{Path, PathBuf};
/// The internal state of one side of the queue.
#[derive(Debug, Clone, Copy, PartialEq)]
pub struct QueueState {
/// The number of the actual segment.
pub segment: u64,
/// The byte position within the segment (the position that can be reached
/// by using Seek::seek).
pub position: u64,
}
impl Default for QueueState {
fn default() -> QueueState {
QueueState {
segment: 0,
position: 0,
}
}
}
impl PartialOrd for QueueState {
fn partial_cmp(&self, other: &QueueState) -> Option<Ordering> {
if self.segment > other.segment {
Some(Ordering::Greater)
} else if self.segment < other.segment {
Some(Ordering::Less)
} else {
Some(self.position.cmp(&other.position))
}
}
}
impl QueueState {
/// Guesses the send metadata for a given queue. This equals to the top
/// position in the greatest segment present in the directory. This function
/// will substitute the current send metadata by this guess upon acquiring
/// the send lock on this queue.
///
/// # Panics
///
/// This function panics if there is a file in the queue folder with extension
/// `.q` whose name is not an integer, such as `foo.q`.
pub fn for_send_metadata<P: AsRef<Path>>(base: P) -> io::Result<QueueState> {
match read_dir(base.as_ref()) {
Ok(read_dir) => {
log::trace!("base {:?} exists", base.as_ref());
// Find greatest segment:
let mut max_segment = None;
for maybe_entry in read_dir {
let path = maybe_entry?.path();
if path.extension().map(|ext| ext == "q").unwrap_or(false) {
let segment = path
.file_stem()
.expect("has extension, therefore has stem")
.to_string_lossy()
.parse::<u64>()
.expect("failed to parse segment filename");
max_segment = if let Some(max_segment) = max_segment {
Some(u64::max(segment, max_segment))
} else {
Some(segment)
};
}
}
// May not have found any segment. In this case, queue is empty.
if let Some(max_segment) = max_segment {
// Find top position in the segment:
let segment_metadata =
metadata(base.as_ref().join(format!("{}.q", max_segment)))?;
let position = segment_metadata.len();
// Generate new queue state:
let queue_state = QueueState {
segment: max_segment,
position,
..QueueState::default()
};
Ok(queue_state)
} else {
log::trace!("queue {:?} was empty", base.as_ref());
Ok(QueueState::default())
}
}
Err(err) if err.kind() == io::ErrorKind::NotFound => {
log::trace!("base {:?} doesn't exist", base.as_ref());
Ok(QueueState::default())
}
Err(err) => Err(err),
}
}
/// Advances to the next segment.
pub fn advance_segment(&mut self) -> u64 {
self.position = 0;
self.segment += 1;
self.segment
}
// /// Goes back to the last segment.
// pub fn retreat_segment(&mut self) {
// self.segment -= 1;
// }
/// Advances the position in the segment.'
pub fn advance_position(&mut self, offset: u64) {
self.position += offset;
}
}
/// An implementation of persistence using the filesystem itself.
#[derive(Default)]
pub struct QueueStatePersistence {
path: Option<PathBuf>,
}
/// The name of the file inside the queue folder.
fn recv_persistence_filename<P: AsRef<Path>>(base: P) -> PathBuf {
base.as_ref().join("recv-metadata")
}
impl QueueStatePersistence {
/// Creates a new file persistence.
pub fn new() -> QueueStatePersistence {
QueueStatePersistence::default()
}
pub fn open<P: AsRef<Path>>(&mut self, base: P) -> io::Result<QueueState> {
let path = recv_persistence_filename(base);
self.path = Some(path.clone());
let mut u64_buffer = [0; 8];
match File::open(&path) {
Ok(mut file) => {
let mut read_u64 = move || -> io::Result<_> {
file.read_exact(&mut u64_buffer)?;
Ok(u64::from_be_bytes(u64_buffer))
};
Ok(QueueState {
segment: read_u64()?,
position: read_u64()?,
})
}
Err(err) if err.kind() == io::ErrorKind::NotFound => Ok(QueueState::default()),
Err(err) => Err(err),
}
}
/// Saves the queue state.
pub fn save(&mut self, queue_state: &QueueState) -> io::Result<()> {
let mut file = BufWriter::new(File::create(
self.path
.as_ref()
.expect("save should be called *after* open"),
)?);
file.write_all(&queue_state.segment.to_be_bytes())?;
file.write_all(&queue_state.position.to_be_bytes())?;
file.flush()?;
Ok(())
}
}