tls: stop the event-loop spin when a handshake stalls on WANT_READ

packages/bun-usockets/src/crypto/openssl.c

@@ -2032,7 +2032,15 @@ static void ssl_update_handshake(struct us_socket_t *s) {
     }
     s->ssl_handshake_state = HANDSHAKE_PENDING;
     s->ssl_write_wants_read = 1;
-    s->flags.last_write_failed = 1;
+    /* Keep writable interest only for a blocked write. Setting this for
+     * WANT_READ too kept the always-writable socket's writable event firing
+     * every tick with zero progress (100% CPU) whenever writable interest
+     * existed while the handshake stalled, e.g. pause() mid-handshake.
+     * WANT_WRITE's blocked BIO write normally sets it in us_socket_raw_write
+     * already; kept for BIO retry paths that buffer without a send. */
+    if (err == SSL_ERROR_WANT_WRITE) {
+      s->flags.last_write_failed = 1;
+    }
     return;
   }
 

test/js/bun/net/socket.test.ts

@@ -362,6 +362,13 @@ describe.concurrent("socket", () => {
     expect(await bunRun(fileURLToPath(new URL("./kqueue-filter-coalesce-fixture.ts", import.meta.url)))).toSpawn();
   });
 
+  // ssl_update_handshake set last_write_failed on SSL_ERROR_WANT_READ, so a
+  // paused (writable-armed) socket with a stalled handshake re-fired writable
+  // every tick at 100% CPU until the peer's handshake bytes arrived.
+  it("paused TLS socket with a stalled handshake must not spin the event loop", async () => {
+    expect(await bunRun(fileURLToPath(new URL("./tls-handshake-pause-spin-fixture.ts", import.meta.url)))).toSpawn();
+  });
+
   it("reload() should preserve active_connections (no UAF / counter underflow)", async () => {
     await using proc = Bun.spawn({
       cmd: [bunExe(), fileURLToPath(new URL("./socket-reload-fixture.ts", import.meta.url))],