project search: Stream search results to improve TTFB (#16923)

This is a prototype change to improve latency of local project searches.
It refactors the matcher to keep paths "in-order" so that we don't need
to wait for all matching files to display the first result.

On a test (searching for `<` in zed.dev) it changes the time until first
result from about 2s to about 50ms. The tail latency seems to increase
slightly (from 5s to 7s) so we may want to do more tuning before hitting
merge.

Release Notes:

- reduces latency for first project search result

---------

Co-authored-by: Thorsten Ball <mrnugget@gmail.com>
Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Thorsten <thorsten@zed.dev>

crates/project/src/buffer_store.rs

@@ -6,7 +6,7 @@ use crate::{
 use anyhow::{anyhow, Context as _, Result};
 use collections::{hash_map, HashMap, HashSet};
 use fs::Fs;
-use futures::{channel::oneshot, stream::FuturesUnordered, StreamExt as _};
+use futures::{channel::oneshot, stream::FuturesUnordered, StreamExt};
 use git::blame::Blame;
 use gpui::{
     AppContext, AsyncAppContext, Context as _, EventEmitter, Model, ModelContext, Task, WeakModel,
@@ -784,95 +784,57 @@ impl BufferStore {
     pub fn find_search_candidates(
         &mut self,
         query: &SearchQuery,
-        limit: usize,
+        mut limit: usize,
         fs: Arc<dyn Fs>,
         cx: &mut ModelContext<Self>,
     ) -> Receiver<Model<Buffer>> {
         let (tx, rx) = smol::channel::unbounded();
-        let open_buffers = self.find_open_search_candidates(query, cx);
-        let skip_entries: HashSet<_> = open_buffers
-            .iter()
-            .filter_map(|buffer| buffer.read(cx).entry_id(cx))
-            .collect();
-
-        let limit = limit.saturating_sub(open_buffers.len());
-        for open_buffer in open_buffers {
-            tx.send_blocking(open_buffer).ok();
+        let mut open_buffers = HashSet::default();
+        let mut unnamed_buffers = Vec::new();
+        for handle in self.buffers() {
+            let buffer = handle.read(cx);
+            if let Some(entry_id) = buffer.entry_id(cx) {
+                open_buffers.insert(entry_id);
+            } else {
+                limit = limit.saturating_sub(1);
+                unnamed_buffers.push(handle)
+            };
         }
 
-        let match_rx = self.worktree_store.update(cx, |worktree_store, cx| {
-            worktree_store.find_search_candidates(query.clone(), limit, skip_entries, fs, cx)
-        });
+        const MAX_CONCURRENT_BUFFER_OPENS: usize = 64;
+        let mut project_paths_rx = self
+            .worktree_store
+            .update(cx, |worktree_store, cx| {
+                worktree_store.find_search_candidates(query.clone(), limit, open_buffers, fs, cx)
+            })
+            .chunks(MAX_CONCURRENT_BUFFER_OPENS);
 
-        const MAX_CONCURRENT_BUFFER_OPENS: usize = 8;
+        cx.spawn(|this, mut cx| async move {
+            for buffer in unnamed_buffers {
+                tx.send(buffer).await.ok();
+            }
 
-        for _ in 0..MAX_CONCURRENT_BUFFER_OPENS {
-            let mut match_rx = match_rx.clone();
-            let tx = tx.clone();
-            cx.spawn(|this, mut cx| async move {
-                while let Some(project_path) = match_rx.next().await {
-                    let buffer = this
-                        .update(&mut cx, |this, cx| this.open_buffer(project_path, cx))?
-                        .await
-                        .log_err();
-                    if let Some(buffer) = buffer {
-                        tx.send_blocking(buffer).ok();
+            while let Some(project_paths) = project_paths_rx.next().await {
+                let buffers = this.update(&mut cx, |this, cx| {
+                    project_paths
+                        .into_iter()
+                        .map(|project_path| this.open_buffer(project_path, cx))
+                        .collect::<Vec<_>>()
+                })?;
+                for buffer_task in buffers {
+                    if let Some(buffer) = buffer_task.await.log_err() {
+                        if tx.send(buffer).await.is_err() {
+                            return anyhow::Ok(());
+                        }
                     }
                 }
-                anyhow::Ok(())
-            })
-            .detach();
-        }
+            }
+            anyhow::Ok(())
+        })
+        .detach();
         rx
     }
 
-    /// Returns open buffers filtered by filename
-    /// Does *not* check the buffer content, the caller must do that
-    fn find_open_search_candidates(
-        &self,
-        query: &SearchQuery,
-        cx: &ModelContext<Self>,
-    ) -> Vec<Model<Buffer>> {
-        let include_root = self
-            .worktree_store
-            .read(cx)
-            .visible_worktrees(cx)
-            .collect::<Vec<_>>()
-            .len()
-            > 1;
-        self.buffers()
-            .filter_map(|buffer| {
-                let handle = buffer.clone();
-                buffer.read_with(cx, |buffer, cx| {
-                    let worktree_store = self.worktree_store.read(cx);
-                    let entry_id = buffer.entry_id(cx);
-                    let is_ignored = entry_id
-                        .and_then(|entry_id| worktree_store.entry_for_id(entry_id, cx))
-                        .map_or(false, |entry| entry.is_ignored);
-
-                    if is_ignored && !query.include_ignored() {
-                        return None;
-                    }
-                    if let Some(file) = buffer.file() {
-                        let matched_path = if include_root {
-                            query.file_matches(Some(&file.full_path(cx)))
-                        } else {
-                            query.file_matches(Some(file.path()))
-                        };
-
-                        if matched_path {
-                            Some(handle)
-                        } else {
-                            None
-                        }
-                    } else {
-                        Some(handle)
-                    }
-                })
-            })
-            .collect()
-    }
-
     fn on_buffer_event(
         &mut self,
         buffer: Model<Buffer>,