[mlir-lsp] Support outgoing requests

Add support for outgoing requests to `lsp::MessageHandler`. Much like
`MessageHandler::outgoingNotification`, this allows for the message
handler to send outgoing messages via its JSON transport, but in this
case, those messages are requests, not notifications.

Requests receive responses (also referred to as "replies" in
`MLIRLspServerSupportLib`). These were previously unsupported, and
`lsp::MessageHandler` would log an error each time it processed a JSON
message that appeared to be a response (something with an "id" field,
but no "method" field). However, the `outgoingRequest` method now
handles response callbacks: an outgoing request with a given ID is set
up such that a callback function is invoked when a response with that ID
is received.

Author: modocache

mlir/include/mlir/Tools/lsp-server-support/Transport.h

@@ -15,6 +15,7 @@
 #ifndef MLIR_TOOLS_LSPSERVERSUPPORT_TRANSPORT_H
 #define MLIR_TOOLS_LSPSERVERSUPPORT_TRANSPORT_H
 
+#include "mlir/Support/DebugStringHelper.h"
 #include "mlir/Support/LLVM.h"
 #include "mlir/Support/LogicalResult.h"
 #include "mlir/Tools/lsp-server-support/Logging.h"
@@ -100,6 +101,18 @@ using Callback = llvm::unique_function<void(llvm::Expected<T>)>;
 template <typename T>
 using OutgoingMessage = llvm::unique_function<void(const T &)>;
 
+/// An OutgoingRequest<T> is a function used for outgoing requests to send to
+/// the client.
+template <typename T>
+using OutgoingRequest =
+    llvm::unique_function<void(const T &, llvm::json::Value id)>;
+
+/// An `OutgoingRequestCallback` is invoked when an outgoing request to the
+/// client receives a response in turn. It is passed the original request's ID,
+/// as well as the result JSON.
+using OutgoingRequestCallback =
+    std::function<void(llvm::json::Value, llvm::Expected<llvm::json::Value>)>;
+
 /// A handler used to process the incoming transport messages.
 class MessageHandler {
 public:
@@ -171,6 +184,26 @@ class MessageHandler {
     };
   }
 
+  /// Create an OutgoingRequest function that, when called, sends a request with
+  /// the given method via the transport. Should the outgoing request be
+  /// met with a response, the response callback is invoked to handle that
+  /// response.
+  template <typename T>
+  OutgoingRequest<T> outgoingRequest(llvm::StringLiteral method,
+                                     OutgoingRequestCallback callback) {
+    return [&, method](const T &params, llvm::json::Value id) {
+      {
+        std::lock_guard<std::mutex> lock(responseHandlersMutex);
+        responseHandlers.insert(
+            {debugString(id), std::make_pair(method.str(), callback)});
+      }
+
+      std::lock_guard<std::mutex> transportLock(transportOutputMutex);
+      Logger::info("--> {0}({1})", method, id);
+      transport.call(method, llvm::json::Value(params), id);
+    };
+  }
+
 private:
   template <typename HandlerT>
   using HandlerMap = llvm::StringMap<llvm::unique_function<HandlerT>>;
@@ -179,6 +212,14 @@ class MessageHandler {
   HandlerMap<void(llvm::json::Value, Callback<llvm::json::Value>)>
       methodHandlers;
 
+  /// A pair of (1) the original request's method name, and (2) the callback
+  /// function to be invoked for responses.
+  using ResponseHandlerTy = std::pair<std::string, OutgoingRequestCallback>;
+  /// A mapping from request/response ID to response handler.
+  llvm::StringMap<ResponseHandlerTy> responseHandlers;
+  /// Mutex to guard insertion into the response handler map.
+  std::mutex responseHandlersMutex;
+
   JSONTransport &transport;
 
   /// Mutex to guard sending output messages to the transport.

mlir/lib/Tools/lsp-server-support/Transport.cpp

@@ -117,21 +117,17 @@ bool MessageHandler::onCall(llvm::StringRef method, llvm::json::Value params,
 
 bool MessageHandler::onReply(llvm::json::Value id,
                              llvm::Expected<llvm::json::Value> result) {
-  // TODO: Add support for reply callbacks when support for outgoing messages is
-  // added. For now, we just log an error on any replies received.
-  Callback<llvm::json::Value> replyHandler =
-      [&id](llvm::Expected<llvm::json::Value> result) {
-        Logger::error(
-            "received a reply with ID {0}, but there was no such call", id);
-        if (!result)
-          llvm::consumeError(result.takeError());
-      };
-
-  // Log and run the reply handler.
-  if (result)
-    replyHandler(std::move(result));
-  else
-    replyHandler(result.takeError());
+  auto it = responseHandlers.find(debugString(id));
+  if (it != responseHandlers.end()) {
+    Logger::info("--> reply:{0}({1})", it->second.first, id);
+    it->second.second(std::move(id), std::move(result));
+  } else {
+    Logger::error(
+        "received a reply with ID {0}, but there was no such outgoing request",
+        id);
+    if (!result)
+      llvm::consumeError(result.takeError());
+  }
   return true;
 }
 

mlir/unittests/Tools/lsp-server-support/Transport.cpp

@@ -125,4 +125,39 @@ TEST_F(TransportInputTest, OutgoingNotification) {
   notifyFn(CompletionList{});
   EXPECT_THAT(getOutput(), HasSubstr("\"method\":\"outgoing-notification\""));
 }
+
+TEST_F(TransportInputTest, ResponseHandlerNotFound) {
+  // Unhandled responses are only reported via error logging. As a result, this
+  // test can't make any expectations -- but it prints the output anyway, by way
+  // of demonstration.
+  Logger::setLogLevel(Logger::Level::Error);
+  writeInput("{\"jsonrpc\":\"2.0\",\"id\":81,\"result\":null}\n");
+  runTransport();
+}
+
+TEST_F(TransportInputTest, OutgoingRequest) {
+  // Make some outgoing requests.
+  bool responseCallbackInvoked = false;
+  auto callFn = getMessageHandler().outgoingRequest<CompletionList>(
+      "outgoing-request",
+      [&responseCallbackInvoked](llvm::json::Value id,
+                                 llvm::Expected<llvm::json::Value> value) {
+        // Make expectations on the expected response.
+        EXPECT_EQ(id, 83);
+        ASSERT_TRUE((bool)value);
+        EXPECT_EQ(debugString(*value), "{\"foo\":6}");
+        responseCallbackInvoked = true;
+      });
+  callFn({}, 82);
+  callFn({}, 83);
+  callFn({}, 84);
+  EXPECT_THAT(getOutput(), HasSubstr("\"method\":\"outgoing-request\""));
+  EXPECT_FALSE(responseCallbackInvoked);
+
+  // One of the requests receives a response. The message handler handles this
+  // response by invoking the callback from above.
+  writeInput("{\"jsonrpc\":\"2.0\",\"id\":83,\"result\":{\"foo\":6}}\n");
+  runTransport();
+  EXPECT_TRUE(responseCallbackInvoked);
+}
 } // namespace