fix: add outgoing-value.finish

Without an explicit `outgoing-value.finish`, the host may not know when
the data stream has finished and when to stop writing.

Additionally, this allows for errors to be communicated to the guest,
which may have happened asynchronously

Signed-off-by: Roman Volosatovs <[email protected]>

imports.md

@@ -521,6 +521,13 @@ Soon: switch to <code>resource incoming-value { ... }</code></li>
 <li><a name="static_outgoing_value.new_outgoing_value.0"></a> own&lt;<a href="#outgoing_value"><a href="#outgoing_value"><code>outgoing-value</code></a></a>&gt;</li>
 </ul>
 <h4><a name="method_outgoing_value.outgoing_value_write_body"></a><code>[method]outgoing-value.outgoing-value-write-body: func</code></h4>
+<p>Returns a stream for writing the value contents.</p>
+<p>The returned <a href="#output_stream"><code>output-stream</code></a> is a child resource: it must be dropped
+before the parent <a href="#outgoing_value"><code>outgoing-value</code></a> resource is dropped (or finished),
+otherwise the <a href="#outgoing_value"><code>outgoing-value</code></a> drop or <code>finish</code> will trap.</p>
+<p>Returns success on the first call: the <a href="#output_stream"><code>output-stream</code></a> resource for
+this <a href="#outgoing_value"><code>outgoing-value</code></a> may be retrieved at most once. Subsequent calls
+will return error.</p>
 <h5>Params</h5>
 <ul>
 <li><a name="method_outgoing_value.outgoing_value_write_body.self"></a><code>self</code>: borrow&lt;<a href="#outgoing_value"><a href="#outgoing_value"><code>outgoing-value</code></a></a>&gt;</li>
@@ -529,6 +536,19 @@ Soon: switch to <code>resource incoming-value { ... }</code></li>
 <ul>
 <li><a name="method_outgoing_value.outgoing_value_write_body.0"></a> result&lt;own&lt;<a href="#output_stream"><a href="#output_stream"><code>output-stream</code></a></a>&gt;&gt;</li>
 </ul>
+<h4><a name="static_outgoing_value.finish"></a><code>[static]outgoing-value.finish: func</code></h4>
+<p>Finalize an outgoing value. This must be
+called to signal that the outgoing value is complete. If the <a href="#outgoing_value"><code>outgoing-value</code></a>
+is dropped without calling <code>outgoing-value.finalize</code>, the implementation
+should treat the value as corrupted.</p>
+<h5>Params</h5>
+<ul>
+<li><a name="static_outgoing_value.finish.this"></a><code>this</code>: own&lt;<a href="#outgoing_value"><a href="#outgoing_value"><code>outgoing-value</code></a></a>&gt;</li>
+</ul>
+<h5>Return values</h5>
+<ul>
+<li><a name="static_outgoing_value.finish.0"></a> result&lt;_, <a href="#error"><a href="#error"><code>error</code></a></a>&gt;</li>
+</ul>
 <h4><a name="static_incoming_value.incoming_value_consume_sync"></a><code>[static]incoming-value.incoming-value-consume-sync: func</code></h4>
 <h5>Params</h5>
 <ul>

wit/types.wit

@@ -52,7 +52,23 @@ interface types {
   // Soon: switch to `resource value { ... }`
   resource outgoing-value {
     new-outgoing-value: static func() -> outgoing-value;
+
+    /// Returns a stream for writing the value contents.
+    ///
+    /// The returned `output-stream` is a child resource: it must be dropped
+    /// before the parent `outgoing-value` resource is dropped (or finished),
+    /// otherwise the `outgoing-value` drop or `finish` will trap.
+    ///
+    /// Returns success on the first call: the `output-stream` resource for
+    /// this `outgoing-value` may be retrieved at most once. Subsequent calls
+    /// will return error.
     outgoing-value-write-body: func() -> result<output-stream>;
+
+    /// Finalize an outgoing value. This must be
+    /// called to signal that the outgoing value is complete. If the `outgoing-value`
+    /// is dropped without calling `outgoing-value.finalize`, the implementation
+    /// should treat the value as corrupted.
+    finish: static func(this: outgoing-value) -> result<_, error>;
   }
 
   /// A incoming-value is a wrapper around a value. It provides a way to read the value