refactor(ext/http): Use const thread-local initializer for slightly better perf (#19881)

Benchmarking shows numbers are pretty close, however this is recommended
for the best possible thread-local performance and may improve in future
Rust compiler revisions.

2 files changed, 18 insertions, 1 deletions

diff --git a/ext/http/http_next.rs b/ext/http/http_next.rs
index e95839005..5635fbd4b 100644
--- a/ext/http/http_next.rs
+++ b/ext/http/http_next.rs
@@ -12,6 +12,7 @@ use crate::response_body::ResponseBytesInner;
 use crate::response_body::V8StreamHttpResponseBody;
 use crate::slab::slab_drop;
 use crate::slab::slab_get;
+use crate::slab::slab_init;
 use crate::slab::slab_insert;
 use crate::slab::SlabId;
 use crate::websocket_upgrade::WebSocketUpgrade;
@@ -836,6 +837,8 @@ pub fn op_http_serve<HTTP>(
 where
   HTTP: HttpPropertyExtractor,
 {
+  slab_init();
+
   let listener =
     HTTP::get_listener_for_rid(&mut state.borrow_mut(), listener_rid)?;
 
@@ -886,6 +889,8 @@ pub fn op_http_serve_on<HTTP>(
 where
   HTTP: HttpPropertyExtractor,
 {
+  slab_init();
+
   let connection =
     HTTP::get_connection_for_rid(&mut state.borrow_mut(), connection_rid)?;
 
diff --git a/ext/http/slab.rs b/ext/http/slab.rs
index 9f7c1f3e9..dbe1a6635 100644
--- a/ext/http/slab.rs
+++ b/ext/http/slab.rs
@@ -32,7 +32,7 @@ pub struct HttpSlabRecord {
 }
 
 thread_local! {
-  static SLAB: RefCell<Slab<HttpSlabRecord>> = RefCell::new(Slab::with_capacity(1024));
+  static SLAB: RefCell<Slab<HttpSlabRecord>> = const { RefCell::new(Slab::new()) };
 }
 
 macro_rules! http_trace {
@@ -56,6 +56,18 @@ pub struct SlabEntry(
   RefMut<'static, Slab<HttpSlabRecord>>,
 );
 
+const SLAB_CAPACITY: usize = 1024;
+
+pub fn slab_init() {
+  SLAB.with(|slab: &RefCell<Slab<HttpSlabRecord>>| {
+    // Note that there might already be an active HTTP server, so this may just
+    // end up adding room for an additional SLAB_CAPACITY items. All HTTP servers
+    // on a single thread share the same slab.
+    let mut slab = slab.borrow_mut();
+    slab.reserve(SLAB_CAPACITY);
+  })
+}
+
 pub fn slab_get(index: SlabId) -> SlabEntry {
   http_trace!(index, "slab_get");
   let mut lock: RefMut<'static, Slab<HttpSlabRecord>> = SLAB.with(|x| {