Fail a missing checker server at startup instead of at its first read

The POSIX launcher backgrounded a subshell that blocked opening its input FIFO, so a missing executable looked started and its transport stayed tracked with an exit hook. Check the executable before creating the FIFOs.

Co-Authored-By: Claude 5.5 <noreply@anthropic.com>

packages/compiler/src/frontend/ts7/rpc-process.test.ts

@@ -30,13 +30,12 @@ test.skipIf(!posix)("a FIFO server echoes, and exits when its transport closes",
   expect(alive(io.pid)).toBe(false);
 });
 
-test.skipIf(!posix)("a FIFO server that cannot start fails its first read", () => {
-  const io = new Ts7FifoIo("/nonexistent/scriptc-ts7-server", []);
-  try {
-    expect(io.read(Buffer.alloc(1), 0, 1)).toBe(0);
-  } finally {
-    io.close();
-  }
+test.skipIf(!posix)("a missing FIFO server fails at construction and tracks nothing", () => {
+  const before = process.listenerCount("exit");
+  expect(() => new Ts7FifoIo("/nonexistent/scriptc-ts7-server", [])).toThrow(
+    "Unable to start TypeScript server",
+  );
+  expect(process.listenerCount("exit")).toBe(before);
 });
 
 test.skipIf(!posix)(

packages/compiler/src/frontend/ts7/rpc-process.ts

@@ -135,9 +135,12 @@ class Ts7NamedPipeIo implements Ts7WireIo {
  * this process would stay a zombie after it exits: Node reaps children only
  * from its event loop, which a synchronous compile never returns to. The
  * subshell points its standard streams at /dev/null before it opens the
- * FIFOs, so spawnSync's output pipe closes as soon as the shell exits. */
+ * FIFOs, so spawnSync's output pipe closes as soon as the shell exits. A
+ * missing or non-executable server fails here, before any FIFO exists: the
+ * background subshell would otherwise block opening its input and the failure
+ * would only surface as end of file on the first read. */
 const FIFO_LAUNCH =
-  'i=$1 o=$2; shift 2; mkfifo -m 600 "$i" "$o" || exit 1; ' +
+  'i=$1 o=$2; shift 2; command -v "$1" >/dev/null || exit 127; mkfifo -m 600 "$i" "$o" || exit 1; ' +
   '( exec </dev/null >/dev/null; exec "$@" <"$i" >"$o" ) & echo $!';
 
 /** How long a starting server may take to open its input, in milliseconds. */