Faster startup by offloading glob matcher building to a worker thread

CHANGELOG.md

@@ -25,6 +25,7 @@
 - Update git-version dependency to use Syn v2, see #2816 (@dtolnay)
 - Update git2 dependency to v0.18.2, see #2852 (@eth-p)
 - Apply clippy fixes #2864 (@cyqsimon)
+- Faster startup by offloading glob matcher building to a worker thread #2868 (@cyqsimon)
 
 ## Syntaxes
 

src/bin/bat/app.rs

@@ -122,6 +122,10 @@ impl App {
         };
 
         let mut syntax_mapping = SyntaxMapping::new();
+        // start building glob matchers for builtin mappings immediately
+        // this is an appropriate approach because it's statistically likely that
+        // all the custom mappings need to be checked
+        syntax_mapping.start_offload_build_all();
 
         if let Some(values) = self.matches.get_many::<String>("ignored-suffix") {
             for suffix in values {

src/syntax_mapping.rs

@@ -1,6 +1,14 @@
-use std::path::Path;
+use std::{
+    path::Path,
+    sync::{
+        atomic::{AtomicBool, Ordering},
+        Arc,
+    },
+    thread,
+};
 
 use globset::{Candidate, GlobBuilder, GlobMatcher};
+use once_cell::sync::Lazy;
 
 use crate::error::Result;
 use builtin::BUILTIN_MAPPINGS;
@@ -44,14 +52,45 @@ pub struct SyntaxMapping<'a> {
     ///
     /// Rules in front have precedence.
     custom_mappings: Vec<(GlobMatcher, MappingTarget<'a>)>,
+
     pub(crate) ignored_suffixes: IgnoredSuffixes<'a>,
+
+    /// A flag to halt glob matcher building, which is offloaded to another thread.
+    ///
+    /// We have this so that we can signal the thread to halt early when appropriate.
+    halt_glob_build: Arc<AtomicBool>,
+}
+
+impl<'a> Drop for SyntaxMapping<'a> {
+    fn drop(&mut self) {
+        // signal the offload thread to halt early
+        self.halt_glob_build.store(true, Ordering::Relaxed);
+    }
 }
 
 impl<'a> SyntaxMapping<'a> {
     pub fn new() -> SyntaxMapping<'a> {
         Default::default()
     }
 
+    /// Start a thread to build the glob matchers for all builtin mappings.
+    ///
+    /// The use of this function while not necessary, is useful to speed up startup
+    /// times by starting this work early in parallel.
+    ///
+    /// The thread halts if/when `halt_glob_build` is set to true.
+    pub fn start_offload_build_all(&self) {
+        let halt = Arc::clone(&self.halt_glob_build);
+        thread::spawn(move || {
+            for (matcher, _) in BUILTIN_MAPPINGS.iter() {
+                if halt.load(Ordering::Relaxed) {
+                    break;
+                }
+                Lazy::force(matcher);
+            }
+        });
+    }
+
     pub fn insert(&mut self, from: &str, to: MappingTarget<'a>) -> Result<()> {
         let matcher = make_glob_matcher(from)?;
         self.custom_mappings.push((matcher, to));