return pending event if present instead of cancelling reads/writes

Fixes #137

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

Author: dicej

crates/wasmtime/src/runtime/component/concurrent/futures_and_streams.rs

@@ -1841,6 +1841,15 @@ impl ComponentInstance {
         let transmit_id = TableId::<TransmitState>::new(rep);
         let transmit = self.get_mut(transmit_id)?;
 
+        if let Some(event) = Waitable::Transmit(transmit.write_handle).take_event(self)? {
+            let (Event::FutureWrite { code, .. } | Event::StreamWrite { code, .. }) = event else {
+                unreachable!();
+            };
+            return Ok(code);
+        }
+
+        let transmit = self.get_mut(transmit_id)?;
+
         match &transmit.write {
             WriteState::GuestReady { .. } | WriteState::HostReady { .. } => {
                 transmit.write = WriteState::Open;
@@ -1858,6 +1867,15 @@ impl ComponentInstance {
         let transmit_id = TableId::<TransmitState>::new(rep);
         let transmit = self.get_mut(transmit_id)?;
 
+        if let Some(event) = Waitable::Transmit(transmit.read_handle).take_event(self)? {
+            let (Event::FutureRead { code, .. } | Event::StreamRead { code, .. }) = event else {
+                unreachable!();
+            };
+            return Ok(code);
+        }
+
+        let transmit = self.get_mut(transmit_id)?;
+
         match &transmit.read {
             ReadState::GuestReady { .. } | ReadState::HostReady { .. } => {
                 transmit.read = ReadState::Open;

tests/misc_testsuite/component-model-async/future-cancel-read-closed.wast

@@ -0,0 +1,60 @@
+;;! component_model_async = true
+;;! reference_types = true
+;;! gc_types = true
+;;! multi_memory = true
+
+;; Create a future, start a read, close the write end, and cancel the read.
+(component
+  (type $f (future))
+  (core func $new (canon future.new $f))
+  (core module $libc (memory (export "mem") 1))
+  (core instance $libc (instantiate $libc))
+  (core func $read (canon future.read $f async (memory $libc "mem")))
+  (core func $cancel (canon future.cancel-read $f))
+  (core func $close-write (canon future.close-writable $f))
+  (core module $m
+    (import "" "new" (func $new (result i64)))
+    (import "" "read" (func $read (param i32 i32) (result i32)))
+    (import "" "cancel" (func $cancel (param i32) (result i32)))
+    (import "" "close-write" (func $close-write (param i32)))
+
+    (func (export "f") (result i32)
+      (local $read i32)
+      (local $write i32)
+      (local $new i64)
+
+      (local.set $new (call $new))
+      (local.set $read (i32.wrap_i64 (local.get $new)))
+      (local.set $write (i32.wrap_i64 (i64.shr_u (local.get $new) (i64.const 32))))
+
+      ;; start a read
+      local.get $read
+      i32.const 0
+      call $read
+      i32.const -1
+      i32.ne
+      if unreachable end
+
+      ;; close the write end
+      local.get $write
+      call $close-write
+
+      ;; cancel the read, returning the result
+      local.get $read
+      call $cancel
+    )
+  )
+
+  (core instance $i (instantiate $m
+    (with "" (instance
+      (export "new" (func $new))
+      (export "read" (func $read))
+      (export "cancel" (func $cancel))
+      (export "close-write" (func $close-write))
+    ))
+  ))
+
+  (func (export "f") (result u32) (canon lift (core func $i "f")))
+)
+
+(assert_return (invoke "f") (u32.const 1)) ;; expect CLOSED status (not CANCELLED)

tests/misc_testsuite/component-model-async/future-cancel-write-closed.wast

@@ -0,0 +1,60 @@
+;;! component_model_async = true
+;;! reference_types = true
+;;! gc_types = true
+;;! multi_memory = true
+
+;; Create a future, start a write, close the read end, and cancel the write.
+(component
+  (type $f (future))
+  (core func $new (canon future.new $f))
+  (core module $libc (memory (export "mem") 1))
+  (core instance $libc (instantiate $libc))
+  (core func $write (canon future.write $f async (memory $libc "mem")))
+  (core func $cancel (canon future.cancel-write $f))
+  (core func $close-read (canon future.close-readable $f))
+  (core module $m
+    (import "" "new" (func $new (result i64)))
+    (import "" "write" (func $write (param i32 i32) (result i32)))
+    (import "" "cancel" (func $cancel (param i32) (result i32)))
+    (import "" "close-read" (func $close-read (param i32)))
+
+    (func (export "f") (result i32)
+      (local $read i32)
+      (local $write i32)
+      (local $new i64)
+
+      (local.set $new (call $new))
+      (local.set $read (i32.wrap_i64 (local.get $new)))
+      (local.set $write (i32.wrap_i64 (i64.shr_u (local.get $new) (i64.const 32))))
+
+      ;; start a write
+      local.get $write
+      i32.const 0
+      call $write
+      i32.const -1
+      i32.ne
+      if unreachable end
+
+      ;; close the read end
+      local.get $read
+      call $close-read
+
+      ;; cancel the write, returning the result
+      local.get $write
+      call $cancel
+    )
+  )
+
+  (core instance $i (instantiate $m
+    (with "" (instance
+      (export "new" (func $new))
+      (export "write" (func $write))
+      (export "cancel" (func $cancel))
+      (export "close-read" (func $close-read))
+    ))
+  ))
+
+  (func (export "f") (result u32) (canon lift (core func $i "f")))
+)
+
+(assert_return (invoke "f") (u32.const 1)) ;; expect CLOSED status (not CANCELLED)