proxy: Call shutdownWrite() in Connection destructor

Flush pending Cap'n Proto release messages before closing the stream.
When one side of a socket pair closes, the other side does not receive
an onDisconnect event, so it relies on receiving release messages from
the closing side to free its ProxyServer objects and shut down cleanly.
Without this, Server objects are not freed by Cap'n Proto on
disconnection.

Author: ryanofsky

src/mp/proxy.cpp

@@ -99,6 +99,28 @@ Connection::~Connection()
     // after the calls finish.
     m_rpc_system.reset();
 
+    // shutdownWrite is needed on Windows so pending data in the m_stream socket
+    // will be sent instead of discarded when m_stream is destroyed. On unix,
+    // this doesn't seem to be needed because data is sent more reliably.
+    //
+    // Sending pending data is important if the connection is a socketpair
+    // because when one side of the socketpair is closed, the other side doesn't
+    // seem to receive any onDisconnect event. So it is important for the other
+    // side to instead receive Cap'n Proto "release" messages (see `struct
+    // Release` in capnp/rpc.capnp) from local Client objects being destroyed so
+    // the remote side can free resources and shut down cleanly. Without this,
+    // when one side of a socket pair is closed the other side may not receive
+    // these messages, preventing the remote side from freeing ProxyServer
+    // resources and shutting down cleanly.
+    try {
+        m_stream->shutdownWrite();
+    } catch (const kj::Exception& e) {
+        // Ignore ENOTCONN: on macOS/FreeBSD (unlike Linux), shutdown(SHUT_WR)
+        // returns ENOTCONN if the peer already closed the connection. This is
+        // expected when the destructor is triggered by a remote disconnect.
+        if (e.getType() != kj::Exception::Type::DISCONNECTED) throw;
+    }
+
     // ProxyClient cleanup handlers are in sync list, and ProxyServer cleanup
     // handlers are in the async list.
     //