io-thread impl for pre-vthread release

src/main/clojure/clojure/core/async.clj

@@ -465,28 +465,24 @@ to catch and handle."
 (defonce ^:private ^Executor thread-macro-executor
   (Executors/newCachedThreadPool (conc/counted-thread-factory "async-thread-macro-%d" true)))
 
-(def ^ExecutorService io-thread-exec
-  (if (= "21" (System/getProperty "java.vm.specification.version"))
-    (eval '(Executors/newThreadPerTaskExecutor (-> (Thread/ofVirtual)
-                                                   (java.lang.Thread$Builder/.name "io-thread-" 0)
-                                                   .factory)))
-    thread-macro-executor))
+(defonce ^:private ^ExecutorService io-thread-exec thread-macro-executor)
 
 (defmacro io-thread
-  "Asynchronously executes the body in a virtual thread, returning immediately
-  to the calling thread.
+  "Asynchronously executes the body in a thread compatible with I/O workload,
+  returning immediately to the calling thread. Only blocking operations should
+  be used in io-thread bodies.
 
   io-thread blocks should not (either directly or indirectly) perform operations
-  that may block indefinitely. Doing so risks pinning the virtual thread
-  to its carrier thread.
+  that never block and run pure compute operations. Parking ops
+  (i.e. <!, >! and alt!/alts!) used in io-thread bodies will throw at
+  runtime.
 
   Returns a channel which will receive the result of the body when
   completed"
   [& body]
   `(let [c# (chan 1)
          captured-bindings# (Var/getThreadBindingFrame)]
-     (.execute
-      io-thread-exec
+     (.execute ^ExecutorService @#'io-thread-exec
       (^:once fn* []
        (Var/resetThreadBindingFrame captured-bindings#)
        (try

src/test/clojure/clojure/core/async_test.clj

@@ -185,6 +185,15 @@
     (binding [test-dyn true]
       (is (<!! (thread test-dyn))))))
 
+(deftest io-thread-tests
+  (testing "io-thread"
+    (let [c1 (chan)
+          c2 (chan)
+          c3 (chan)]
+      (io-thread (>!! c2 (clojure.string/upper-case (<!! c1))))
+      (io-thread (>!! c3 (clojure.string/reverse (<!! c2))))
+      (>!! c1 "loop")
+      (is (= "POOL" (<!! c3))))))
 
 (deftest ops-tests
   (testing "map<"