Rollup merge of rust-lang#81546 - hyd-dev:libtest-run-out-of-threads, r=Mark-Simulacrum

[libtest] Run the test synchronously when hitting thread limit

libtest currently panics if it hits the thread limit. This often results in spurious test failures (<code>thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Os { code: 11, kind: WouldBlock, message: "Resource temporarily unavailable" }'</code> ... `error: test failed, to rerun pass '--lib'`). This PR makes it continue to run the test synchronously if it runs out of threads.

Closes rust-lang#78165.

``@rustbot`` label: A-libtest T-libs

Author: Dylan-DPC

test/src/lib.rs

@@ -506,7 +506,18 @@ pub fn run_test(
         let supports_threads = !cfg!(target_os = "emscripten") && !cfg!(target_arch = "wasm32");
         if concurrency == Concurrent::Yes && supports_threads {
             let cfg = thread::Builder::new().name(name.as_slice().to_owned());
-            Some(cfg.spawn(runtest).unwrap())
+            let mut runtest = Arc::new(Mutex::new(Some(runtest)));
+            let runtest2 = runtest.clone();
+            match cfg.spawn(move || runtest2.lock().unwrap().take().unwrap()()) {
+                Ok(handle) => Some(handle),
+                Err(e) if e.kind() == io::ErrorKind::WouldBlock => {
+                    // `ErrorKind::WouldBlock` means hitting the thread limit on some
+                    // platforms, so run the test synchronously here instead.
+                    Arc::get_mut(&mut runtest).unwrap().get_mut().unwrap().take().unwrap()();
+                    None
+                }
+                Err(e) => panic!("failed to spawn thread to run test: {}", e),
+            }
         } else {
             runtest();
             None