Prevent macOS Server worker stack overflow

src/bin/webcodex-server.rs

@@ -1,9 +1,22 @@
 use webcodex::{server_binary_action, ServerBinaryAction};
 
-#[tokio::main]
-async fn main() -> Result<(), Box<dyn std::error::Error>> {
+// The default macOS Tokio worker stack is too small for the deepest MCP request
+// path exercised by the local Server. Keep this explicit instead of requiring
+// callers to set RUST_MIN_STACK for `webcodex share`.
+#[cfg(target_os = "macos")]
+const MACOS_SERVER_RUNTIME_STACK_SIZE: usize = 8 * 1024 * 1024;
+
+fn build_server_runtime() -> std::io::Result<tokio::runtime::Runtime> {
+    let mut builder = tokio::runtime::Builder::new_multi_thread();
+    builder.enable_all();
+    #[cfg(target_os = "macos")]
+    builder.thread_stack_size(MACOS_SERVER_RUNTIME_STACK_SIZE);
+    builder.build()
+}
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
     match server_binary_action(std::env::args().skip(1)) {
-        ServerBinaryAction::Run => webcodex::run_server().await,
+        ServerBinaryAction::Run => build_server_runtime()?.block_on(webcodex::run_server()),
         ServerBinaryAction::Exit {
             code,
             stdout,
@@ -19,3 +32,26 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         }
     }
 }
+
+#[cfg(all(test, target_os = "macos"))]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn server_runtime_uses_large_worker_stack() {
+        let runtime = build_server_runtime().expect("build WebCodex Server runtime");
+        let stack_size = runtime.block_on(async {
+            tokio::spawn(async {
+                // SAFETY: pthread_self returns the current worker thread and
+                // pthread_get_stacksize_np only queries that thread's stack metadata.
+                unsafe { libc::pthread_get_stacksize_np(libc::pthread_self()) }
+            })
+            .await
+            .expect("observe WebCodex Server worker")
+        });
+        assert!(
+            stack_size >= MACOS_SERVER_RUNTIME_STACK_SIZE,
+            "WebCodex Server worker stack was {stack_size} bytes; expected at least {MACOS_SERVER_RUNTIME_STACK_SIZE}"
+        );
+    }
+}