fill in unimplemented logic in resolve_exported_promise()

capnp-rpc/src/rpc.rs

@@ -278,6 +278,11 @@ impl<VatId> Answer<VatId> {
 
 pub struct Export {
     refcount: u32,
+
+    /// If true, this is the canonical export entry for this clientHook, that is,
+    /// `exports_by_cap[clientHook]` points to this entry.
+    canonical: bool,
+
     client_hook: Box<dyn ClientHook>,
 
     // If this export is a promise (not a settled capability), the `resolve_op` represents the
@@ -289,6 +294,7 @@ impl Export {
     fn new(client_hook: Box<dyn ClientHook>) -> Self {
         Self {
             refcount: 1,
+            canonical: false,
             client_hook,
             resolve_op: Promise::err(Error::failed("no resolve op".to_string())),
         }
@@ -1184,8 +1190,10 @@ impl<VatId> ConnectionState<VatId> {
         e.refcount -= refcount;
         if e.refcount == 0 {
             let client_ptr = e.client_hook.get_ptr();
+            if e.canonical {
+                self.exports_by_cap.borrow_mut().remove(&client_ptr);
+            }
             exports.erase(id);
-            self.exports_by_cap.borrow_mut().remove(&client_ptr);
         }
         Ok(())
     }
@@ -1286,7 +1294,8 @@ impl<VatId> ConnectionState<VatId> {
         promise: Promise<Box<dyn ClientHook>, Error>,
     ) -> Promise<(), Error> {
         let weak_connection_state = Rc::downgrade(state);
-        state.eagerly_evaluate(promise.map(move |resolution_result| {
+        state.eagerly_evaluate(Promise::from_future(async move {
+            let resolution_result = promise.await;
             let connection_state = weak_connection_state
                 .upgrade()
                 .expect("dangling connection state?");
@@ -1300,29 +1309,68 @@ impl<VatId> ConnectionState<VatId> {
                     // Update the export table to point at this object instead. We know that our
                     // entry in the export table is still live because when it is destroyed the
                     // asynchronous resolution task (i.e. this code) is canceled.
-                    if let Some(exp) = connection_state.exports.borrow_mut().find(export_id) {
+                    let mut exports = connection_state.exports.borrow_mut();
+                    let Some(exp) = exports.find(export_id) else {
+                        return Err(Error::failed("export table entry not found".to_string()));
+                    };
+
+                    if exp.canonical {
                         connection_state
                             .exports_by_cap
                             .borrow_mut()
                             .remove(&exp.client_hook.get_ptr());
-                        exp.client_hook = resolution.clone();
-                    } else {
-                        return Err(Error::failed("export table entry not found".to_string()));
                     }
+                    exp.client_hook = resolution.clone();
+
+                    // The export now points to `resolution`, but it is not necessarily the
+                    // canonical export for `resolution`. The export itself still represents
+                    // the promise that ended up resolving to `resolution`, but `resolution`
+                    // itself also needs to be exported under a separate export ID to
+                    // distinguish from the promise. (Unless it's also a promise, see the next
+                    // bit...)
+                    exp.canonical = false;
 
                     if brand != connection_state.get_brand() {
                         // We're resolving to a local capability. If we're resolving to a promise,
                         // we might be able to reuse our export table entry and avoid sending a
                         // message.
-                        if let Some(_promise) = resolution.when_more_resolved() {
+                        if let Some(promise) = resolution.when_more_resolved() {
                             // We're replacing a promise with another local promise. In this case,
                             // we might actually be able to just reuse the existing export table
                             // entry to represent the new promise -- unless it already has an entry.
                             // Let's check.
 
-                            unimplemented!()
+                            let mut exports_by_cap = connection_state.exports_by_cap.borrow_mut();
+
+                            let replacement_export_id =
+                                match exports_by_cap.entry(exp.client_hook.get_ptr()) {
+                                    hash_map::Entry::Occupied(occ) => *occ.get(),
+                                    hash_map::Entry::Vacant(vac) => {
+                                        // The replacement capability isn't previously exported,
+                                        // so assign it to the existing table entry.
+                                        vac.insert(export_id);
+                                        export_id
+                                    }
+                                };
+                            if replacement_export_id == export_id {
+                                // The new promise was not already in the table, therefore the existing
+                                // export table entry has now been repurposed to represent it. There is
+                                // no need to send a resolve message at all. We do, however, have to
+                                // start resolving the next promise.
+                                exp.canonical = true;
+                                drop(exports);
+                                drop(exports_by_cap);
+                                return Self::resolve_exported_promise(
+                                    &connection_state,
+                                    export_id,
+                                    promise,
+                                )
+                                .await;
+                            }
                         }
                     }
+                    // Prevent a double borrow in write_descriptor() below.
+                    drop(exports);
 
                     // OK, we have to send a `Resolve` message.
                     let mut message = connection_state.new_outgoing_message(15)?;
@@ -1383,7 +1431,8 @@ impl<VatId> ConnectionState<VatId> {
             } else {
                 // This is the first time we've seen this capability.
 
-                let exp = Export::new(inner.clone());
+                let mut exp = Export::new(inner.clone());
+                exp.canonical = true;
                 let export_id = state.exports.borrow_mut().push(exp);
                 state.exports_by_cap.borrow_mut().insert(ptr, export_id);
                 match inner.when_more_resolved() {

capnp-rpc/test/impls.rs

@@ -21,13 +21,14 @@
 
 use crate::test_capnp::{
     bootstrap, test_call_order, test_capability_server_set, test_extends, test_handle,
-    test_interface, test_more_stuff, test_pipeline, test_streaming,
+    test_interface, test_more_stuff, test_pipeline, test_promise_resolve, test_streaming,
 };
 
 use capnp::capability::{FromClientHook, Promise};
 use capnp::Error;
 use capnp_rpc::pry;
 
+use futures::channel::oneshot;
 use futures::{FutureExt, TryFutureExt};
 
 use std::cell::{Cell, RefCell};
@@ -113,6 +114,17 @@ impl bootstrap::Server for Bootstrap {
             .set_cap(capnp_rpc::new_client(TestCapabilityServerSet::new()));
         Promise::ok(())
     }
+
+    fn test_promise_resolve(
+        &mut self,
+        _params: bootstrap::TestPromiseResolveParams,
+        mut results: bootstrap::TestPromiseResolveResults,
+    ) -> Promise<(), Error> {
+        results
+            .get()
+            .set_cap(capnp_rpc::new_client(TestPromiseResolveImpl {}));
+        Promise::ok(())
+    }
 }
 
 #[derive(Default)]
@@ -575,12 +587,12 @@ impl Drop for Handle {
 impl test_handle::Server for Handle {}
 
 pub struct TestCapDestructor {
-    fulfiller: Option<::futures::channel::oneshot::Sender<()>>,
+    fulfiller: Option<oneshot::Sender<()>>,
     imp: TestInterface,
 }
 
 impl TestCapDestructor {
-    pub fn new(fulfiller: ::futures::channel::oneshot::Sender<()>) -> Self {
+    pub fn new(fulfiller: oneshot::Sender<()>) -> Self {
         Self {
             fulfiller: Some(fulfiller),
             imp: TestInterface::new(),
@@ -718,3 +730,58 @@ impl test_capability_server_set::Server for TestCapabilityServerSet {
         })
     }
 }
+
+pub struct ResolverImpl {
+    sender: Option<oneshot::Sender<test_interface::Client>>,
+}
+
+impl test_promise_resolve::resolver::Server for ResolverImpl {
+    fn resolve_to_another_promise(
+        &mut self,
+        _params: test_promise_resolve::resolver::ResolveToAnotherPromiseParams,
+        _results: test_promise_resolve::resolver::ResolveToAnotherPromiseResults,
+    ) -> Promise<(), Error> {
+        let Some(sender) = self.sender.take() else {
+            return Promise::err(Error::failed("no sender".into()));
+        };
+        let (snd, rcv) = oneshot::channel();
+        let _ = sender.send(capnp_rpc::new_promise_client(
+            rcv.map_err(|_| Error::failed("oneshot was canceled".to_string()))
+                .map_ok(|x: test_interface::Client| x.client),
+        ));
+        self.sender = Some(snd);
+        Promise::ok(())
+    }
+
+    fn resolve_to_cap(
+        &mut self,
+        _params: test_promise_resolve::resolver::ResolveToCapParams,
+        _results: test_promise_resolve::resolver::ResolveToCapResults,
+    ) -> Promise<(), Error> {
+        let Some(sender) = self.sender.take() else {
+            return Promise::err(Error::failed("no sender".into()));
+        };
+        let _ = sender.send(capnp_rpc::new_client(TestInterface::new()));
+        Promise::ok(())
+    }
+}
+
+pub struct TestPromiseResolveImpl {}
+
+impl test_promise_resolve::Server for TestPromiseResolveImpl {
+    fn foo(
+        &mut self,
+        _params: test_promise_resolve::FooParams,
+        mut results: test_promise_resolve::FooResults,
+    ) -> Promise<(), Error> {
+        let (snd, rcv) = oneshot::channel();
+        let resolver = ResolverImpl { sender: Some(snd) };
+        let mut results_root = results.get();
+        results_root.set_cap(capnp_rpc::new_promise_client(
+            rcv.map_err(|_| Error::failed("oneshot was canceled".to_string()))
+                .map_ok(|x| x.client),
+        ));
+        results_root.set_resolver(capnp_rpc::new_client(resolver));
+        Promise::ok(())
+    }
+}

capnp-rpc/test/test.capnp

@@ -79,6 +79,7 @@ interface Bootstrap {
   testCallOrder @4 () -> (cap: TestCallOrder);
   testMoreStuff @5 () -> (cap: TestMoreStuff);
   testCapabilityServerSet @6 () -> (cap: TestCapabilityServerSet);
+  testPromiseResolve @7 () -> (cap: TestPromiseResolve);
 }
 
 interface TestInterface {
@@ -195,3 +196,14 @@ interface TestCapabilityServerSet {
   createHandle @0 () -> (handle :Handle);
   checkHandle @1 (handle: Handle) -> (isOurs :Bool);
 }
+
+interface TestPromiseResolve {
+  interface Resolver {
+    resolveToAnotherPromise @0 ();
+    resolveToCap @1 ();
+  }
+
+  foo @0 () -> (cap: TestInterface, resolver: Resolver);
+  # Teturns a promise capability whose resolution can
+  # be triggered by a `resolver` capability.
+}

capnp-rpc/test/test.rs

@@ -1230,3 +1230,31 @@ fn stream_error_gets_reported() {
         Ok(())
     });
 }
+
+#[test]
+fn promise_resolve_twice() {
+    rpc_top_level(|_spawner, client| async move {
+        let response1 = client.test_promise_resolve_request().send().promise.await?;
+        let client1 = response1.get()?.get_cap()?;
+
+        let response = client1.foo_request().send().promise.await?;
+        let resolver = response.get()?.get_resolver()?;
+
+        resolver
+            .resolve_to_another_promise_request()
+            .send()
+            .promise
+            .await?;
+
+        resolver.resolve_to_cap_request().send().promise.await?;
+
+        let cap = response.get()?.get_cap()?;
+        let mut request = cap.foo_request();
+        request.get().set_i(123);
+        request.get().set_j(true);
+        let response2 = request.send().promise.await?;
+        let x = response2.get()?.get_x()?.to_str()?;
+        assert_eq!(x, "foo");
+        Ok(())
+    });
+}