Offload building built-in glob matchers to worker threads

src/syntax_mapping.rs

@@ -3,7 +3,7 @@ use std::path::Path;
 use globset::{Candidate, GlobBuilder, GlobMatcher};
 
 use crate::error::Result;
-use builtin::BUILTIN_MAPPINGS;
+use builtin::OffloadIter;
 use ignored_suffixes::IgnoredSuffixes;
 
 mod builtin;
@@ -91,8 +91,7 @@ impl<'a> SyntaxMapping<'a> {
     pub fn builtin_mappings(
         &self,
     ) -> impl Iterator<Item = (&'static GlobMatcher, &'static MappingTarget<'static>)> {
-        BUILTIN_MAPPINGS
-            .iter()
+        OffloadIter::new()
             .filter_map(|(matcher, target)| matcher.as_ref().map(|glob| (glob, target)))
     }
 

src/syntax_mapping/builtin.rs

@@ -1,4 +1,4 @@
-use std::env;
+use std::{env, iter::Enumerate, slice, sync::mpsc, thread};
 
 use globset::GlobMatcher;
 use once_cell::sync::Lazy;
@@ -89,3 +89,96 @@ enum MatcherSegment {
     Text(&'static str),
     Env(&'static str),
 }
+
+/// The maximum number of offload workers.
+const MAX_OFFLOAD_WORKERS: usize = 8;
+/// The minimum number of built glob matchers remaining before offload workers
+/// start building the next batch.
+const OFFLOAD_PREEMPT_MARGIN: usize = 2;
+
+/// An iterator over the builtin mappings that offloads glob matcher building to
+/// worker threads.
+#[derive(Debug)]
+pub struct OffloadIter {
+    /// The iterator tracking the item returned by `next()`.
+    next_iter: Enumerate<slice::Iter<'static, (Lazy<Option<GlobMatcher>>, MappingTarget<'static>)>>,
+
+    /// The index of the next item that is to be fed to workers.
+    ///
+    /// Used to determine whether the next offload batch should be triggered.
+    build_idx: usize,
+    /// The iterator tracking the next item that is to be fed to workers.
+    build_iter: slice::Iter<'static, (Lazy<Option<GlobMatcher>>, MappingTarget<'static>)>,
+
+    /// Control channels for worker threads.
+    workers: Vec<mpsc::Sender<&'static Lazy<Option<GlobMatcher>>>>,
+}
+
+impl OffloadIter {
+    pub fn new() -> Self {
+        let worker_count = match thread::available_parallelism() {
+            Ok(n) => (n.get() - 1).min(MAX_OFFLOAD_WORKERS), // leave 1 for main thread
+            Err(_) => 0,
+        };
+
+        let workers = (0..worker_count)
+            .map(|_| {
+                let (cmd_tx, cmd_rx) = mpsc::channel();
+                thread::spawn(move || loop {
+                    match cmd_rx.recv() {
+                        Ok(cell) => {
+                            Lazy::force(cell);
+                        }
+                        Err(_) => break, // cmd_tx dropped; nothing more to do
+                    }
+                }); // no need for the join handle; thread will halt when cmd_tx is dropped
+                cmd_tx
+            })
+            .collect();
+
+        Self {
+            next_iter: BUILTIN_MAPPINGS.iter().enumerate(),
+            build_idx: 0,
+            build_iter: BUILTIN_MAPPINGS.iter(),
+            workers,
+        }
+    }
+}
+
+impl Iterator for OffloadIter {
+    type Item = &'static (Lazy<Option<GlobMatcher>>, MappingTarget<'static>);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let (idx, item) = self.next_iter.next()?;
+
+        if idx + OFFLOAD_PREEMPT_MARGIN < self.build_idx
+            || idx + OFFLOAD_PREEMPT_MARGIN >= BUILTIN_MAPPINGS.len()
+        {
+            // no further work needed at this point
+            return Some(item);
+        }
+
+        // feed jobs to workers
+        for cmd_tx in self.workers.iter() {
+            match self.build_iter.next() {
+                Some((unbuilt, _)) => {
+                    cmd_tx
+                        .send(unbuilt)
+                        .expect("Offload worker should not hang up before main thread");
+                    self.build_idx += 1;
+                }
+                None => {
+                    break;
+                }
+            }
+        }
+
+        // halt workers if no longer needed
+        if self.build_idx >= BUILTIN_MAPPINGS.len() {
+            // workers stop when their current job is finished and cmd_tx is dropped
+            self.workers.clear();
+        }
+
+        Some(item)
+    }
+}