[clangd] Ensure that we reply to each call exactly once. NFC (I think!)

Summary:
In debug builds, getting this wrong will trigger asserts.
In production builds, it will send an error reply if none was sent,
and drop redundant replies. (And log).

No tests because this is always a programming error.
(We did have some cases of this, but I fixed them with the new dispatcher).

Reviewers: ioeric

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, jfb, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D53399

2 files changed, 77 insertions, 31 deletions

diff --git a/clang-tools-extra/clangd/ClangdLSPServer.cpp b/clang-tools-extra/clangd/ClangdLSPServer.cpp
index c8161097d59..379fcfb81e6 100644
--- a/clang-tools-extra/clangd/ClangdLSPServer.cpp
+++ b/clang-tools-extra/clangd/ClangdLSPServer.cpp
@@ -105,16 +105,21 @@ public:
   }
 
   bool onCall(StringRef Method, json::Value Params, json::Value ID) override {
+    // Calls can be canceled by the client. Add cancellation context.
+    WithContext WithCancel(cancelableRequestContext(ID));
+    trace::Span Tracer(Method);
+    SPAN_ATTACH(Tracer, "Params", Params);
+    ReplyOnce Reply(ID, Method, &Server, Tracer.Args);
     log("<-- {0}({1})", Method, ID);
     if (!Server.Server && Method != "initialize") {
       elog("Call {0} before initialization.", Method);
-      Server.reply(ID, make_error<LSPError>("server not initialized",
-                                            ErrorCode::ServerNotInitialized));
+      Reply(make_error<LSPError>("server not initialized",
+                                 ErrorCode::ServerNotInitialized));
     } else if (auto Handler = Calls.lookup(Method))
-      Handler(std::move(Params), std::move(ID));
+      Handler(std::move(Params), std::move(Reply));
     else
-      Server.reply(ID, make_error<LSPError>("method not found",
-                                            ErrorCode::MethodNotFound));
+      Reply(
+          make_error<LSPError>("method not found", ErrorCode::MethodNotFound));
     return true;
   }
 
@@ -128,36 +133,19 @@ public:
   }
 
   // Bind an LSP method name to a call.
-  template <typename Param, typename Reply>
+  template <typename Param, typename Result>
   void bind(const char *Method,
-            void (ClangdLSPServer::*Handler)(const Param &, Callback<Reply>)) {
+            void (ClangdLSPServer::*Handler)(const Param &, Callback<Result>)) {
     Calls[Method] = [Method, Handler, this](json::Value RawParams,
-                                            json::Value ID) {
+                                            ReplyOnce Reply) {
       Param P;
-      if (!fromJSON(RawParams, P)) {
+      if (fromJSON(RawParams, P)) {
+        (Server.*Handler)(P, std::move(Reply));
+      } else {
         elog("Failed to decode {0} request.", Method);
-        Server.reply(ID, make_error<LSPError>("failed to decode request",
-                                              ErrorCode::InvalidRequest));
-        return;
+        Reply(make_error<LSPError>("failed to decode request",
+                                   ErrorCode::InvalidRequest));
       }
-      trace::Span Tracer(Method);
-      SPAN_ATTACH(Tracer, "Params", RawParams);
-      auto *Trace = Tracer.Args; // We attach reply from another thread.
-      // Calls can be canceled by the client. Add cancellation context.
-      WithContext WithCancel(cancelableRequestContext(ID));
-      // FIXME: this function should assert it's called exactly once.
-      (Server.*Handler)(P, [this, ID, Trace](Expected<Reply> Result) {
-        if (Result) {
-          if (Trace)
-            (*Trace)["Reply"] = *Result;
-          Server.reply(ID, json::Value(std::move(*Result)));
-        } else {
-          auto Err = Result.takeError();
-          if (Trace)
-            (*Trace)["Error"] = to_string(Err);
-          Server.reply(ID, std::move(Err));
-        }
-      });
     };
   }
 
@@ -178,8 +166,65 @@ public:
   }
 
 private:
+  // Function object to reply to an LSP call.
+  // Each instance must be called exactly once, otherwise:
+  //  - the bug is logged, and (in debug mode) an assert will fire
+  //  - if there was no reply, an error reply is sent
+  //  - if there were multiple replies, only the first is sent
+  class ReplyOnce {
+    std::atomic<bool> Replied = {false};
+    json::Value ID;
+    std::string Method;
+    ClangdLSPServer *Server; // Null when moved-from.
+    json::Object *TraceArgs;
+
+  public:
+    ReplyOnce(const json::Value &ID, StringRef Method, ClangdLSPServer *Server,
+              json::Object *TraceArgs)
+        : ID(ID), Method(Method), Server(Server), TraceArgs(TraceArgs) {
+      assert(Server);
+    }
+    ReplyOnce(ReplyOnce &&Other)
+        : Replied(Other.Replied.load()), ID(std::move(Other.ID)),
+          Method(std::move(Other.Method)), Server(Other.Server),
+          TraceArgs(Other.TraceArgs) {
+      Other.Server = nullptr;
+    }
+    ReplyOnce& operator=(ReplyOnce&&) = delete;
+    ReplyOnce(const ReplyOnce &) = delete;
+    ReplyOnce& operator=(const ReplyOnce&) = delete;
+
+    ~ReplyOnce() {
+      if (Server && !Replied) {
+        elog("No reply to message {0}({1})", Method, ID);
+        assert(false && "must reply to all calls!");
+        (*this)(make_error<LSPError>("server failed to reply",
+                                     ErrorCode::InternalError));
+      }
+    }
+
+    void operator()(Expected<json::Value> Reply) {
+      assert(Server && "moved-from!");
+      if (Replied.exchange(true)) {
+        elog("Replied twice to message {0}({1})", Method, ID);
+        assert(false && "must reply to each call only once!");
+        return;
+      }
+      if (TraceArgs) {
+        if (Reply)
+          (*TraceArgs)["Reply"] = *Reply;
+        else {
+          auto Err = Reply.takeError();
+          (*TraceArgs)["Error"] = to_string(Err);
+          Reply = std::move(Err);
+        }
+      }
+      Server->reply(ID, std::move(Reply));
+    }
+  };
+
   StringMap<std::function<void(json::Value)>> Notifications;
-  StringMap<std::function<void(json::Value, json::Value)>> Calls;
+  StringMap<std::function<void(json::Value, ReplyOnce)>> Calls;
 
   // Method calls may be cancelled by ID, so keep track of their state.
   // This needs a mutex: handlers may finish on a different thread, and that's
diff --git a/clang-tools-extra/clangd/Trace.h b/clang-tools-extra/clangd/Trace.h
index 0c4c461855d..4b2f72e7198 100644
--- a/clang-tools-extra/clangd/Trace.h
+++ b/clang-tools-extra/clangd/Trace.h
@@ -87,6 +87,7 @@ public:
 
   /// Mutable metadata, if this span is interested.
   /// Prefer to use SPAN_ATTACH rather than accessing this directly.
+  /// The lifetime of Args is the whole event, even if the Span dies.
   llvm::json::Object *const Args;
 
 private: