push and test the search queue datastructure

meilisearch/src/search_queue.rs

@@ -0,0 +1,88 @@
+use std::time::Duration;
+
+use rand::{rngs::StdRng, Rng, SeedableRng};
+use tokio::sync::{mpsc, oneshot};
+
+use crate::error::MeilisearchHttpError;
+
+#[derive(Debug)]
+pub struct SearchQueue {
+    sender: mpsc::Sender<oneshot::Sender<Permit>>,
+    capacity: usize,
+}
+
+#[derive(Debug)]
+pub struct Permit {
+    sender: mpsc::Sender<()>,
+}
+
+impl Drop for Permit {
+    fn drop(&mut self) {
+        // if the channel is closed then the whole instance is down
+        let _ = futures::executor::block_on(self.sender.send(()));
+    }
+}
+
+impl SearchQueue {
+    pub fn new(capacity: usize, paralellism: usize) -> Self {
+        // We can make the search requests wait until we're available.
+        // they're going to wait anyway right after, so let's not allocate any
+        // RAM by keeping a capacity of 1.
+        let (sender, receiver) = mpsc::channel(1);
+        tokio::task::spawn(Self::run(capacity, paralellism, receiver));
+        Self { sender, capacity }
+    }
+
+    async fn run(
+        capacity: usize,
+        parallelism: usize,
+        mut receive_new_searches: mpsc::Receiver<oneshot::Sender<Permit>>,
+    ) {
+        let mut queue: Vec<oneshot::Sender<Permit>> = Default::default();
+        let mut rng: StdRng = StdRng::from_entropy();
+        let mut searches_running = 0;
+        // by having a capacity of parallelism we ensures that every time a search finish it can release its RAM asap
+        let (sender, mut search_finished) = mpsc::channel(parallelism);
+
+        loop {
+            tokio::select! {
+                search_request = receive_new_searches.recv() => {
+                    let search_request = search_request.unwrap();
+                    println!("queue contains {} elements and already running {}", queue.len(), searches_running);
+                    if searches_running < parallelism && queue.is_empty() {
+                        println!("We can process the search straight away");
+                        searches_running += 1;
+                        // if the search requests die it's not a hard error on our side
+                        let _ = search_request.send(Permit { sender: sender.clone() });
+                        continue;
+                    }
+                    if queue.len() >= capacity {
+                        println!("we're above capacity, dropping a random request");
+                        let remove = rng.gen_range(0..queue.len());
+                        let thing = queue.swap_remove(remove); // this will drop the channel and notify the search that it won't be processed
+                        drop(thing);
+                    }
+                    println!("pushed a new search request to the queue {}", queue.len());
+                    queue.push(search_request);
+                },
+                _ = search_finished.recv() => {
+                    searches_running = searches_running.saturating_sub(1);
+                    if !queue.is_empty() {
+                        println!("processed an element in the queue");
+                        let remove = rng.gen_range(0..queue.len());
+                        let channel = queue.swap_remove(remove);
+                        let _ = channel.send(Permit { sender: sender.clone() });
+                    }
+                },
+            }
+        }
+    }
+
+    pub async fn register_search(&self) -> Result<Permit, MeilisearchHttpError> {
+        let (sender, receiver) = oneshot::channel();
+        self.sender.send(sender).await.map_err(|_| MeilisearchHttpError::SearchLimiterIsDown)?;
+        receiver.await.map_err(|_| {
+            MeilisearchHttpError::TooManySearchRequests(self.capacity, Duration::from_secs(10))
+        })
+    }
+}