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//===--- Transport.h - sending and receiving LSP messages -------*- C++ -*-===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// The language server protocol is usually implemented by writing messages as
// JSON-RPC over the stdin/stdout of a subprocess. However other communications
// mechanisms are possible, such as XPC on mac.
// The Transport interface allows the mechanism to be replaced, and the JSONRPC
// Transport is the standard implementation.
#include "Feature.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/Support/JSON.h"
#include "llvm/Support/raw_ostream.h"
namespace clang {
namespace clangd {
// A transport is responsible for maintaining the connection to a client
// application, and reading/writing structured messages to it.
// Transports have limited thread safety requirements:
// - messages will not be sent concurrently
// - messages MAY be sent while loop() is reading, or its callback is active
class Transport {
virtual ~Transport() = default;
// Called by Clangd to send messages to the client.
virtual void notify(llvm::StringRef Method, llvm::json::Value Params) = 0;
virtual void call(llvm::StringRef Method, llvm::json::Value Params,
llvm::json::Value ID) = 0;
virtual void reply(llvm::json::Value ID,
llvm::Expected<llvm::json::Value> Result) = 0;
// Implemented by Clangd to handle incoming messages. (See loop() below).
class MessageHandler {
virtual ~MessageHandler() = default;
// Handler returns true to keep processing messages, or false to shut down.
virtual bool onNotify(llvm::StringRef Method, llvm::json::Value) = 0;
virtual bool onCall(llvm::StringRef Method, llvm::json::Value Params,
llvm::json::Value ID) = 0;
virtual bool onReply(llvm::json::Value ID,
llvm::Expected<llvm::json::Value> Result) = 0;
// Called by Clangd to receive messages from the client.
// The transport should in turn invoke the handler to process messages.
// If handler returns false, the transport should immediately exit the loop.
// (This is used to implement the `exit` notification).
// Otherwise, it returns an error when the transport becomes unusable.
virtual llvm::Error loop(MessageHandler &) = 0;
// Controls the way JSON-RPC messages are encoded (both input and output).
enum JSONStreamStyle {
// Encoding per the LSP specification, with mandatory Content-Length header.
// Messages are delimited by a '---' line. Comment lines start with #.
// Returns a Transport that speaks JSON-RPC over a pair of streams.
// The input stream must be opened in binary mode.
// If InMirror is set, data read will be echoed to it.
// The use of C-style std::FILE* input deserves some explanation.
// Previously, std::istream was used. When a debugger attached on MacOS, the
// process received EINTR, the stream went bad, and clangd exited.
// A retry-on-EINTR loop around reads solved this problem, but caused clangd to
// sometimes hang rather than exit on other OSes. The interaction between
// istreams and signals isn't well-specified, so it's hard to get this right.
// The C APIs seem to be clearer in this respect.
newJSONTransport(std::FILE *In, llvm::raw_ostream &Out,
llvm::raw_ostream *InMirror, bool Pretty,
JSONStreamStyle = JSONStreamStyle::Standard);
// Returns a Transport for macOS based on XPC.
// Clangd with this transport is meant to be run as bundled XPC service.
std::unique_ptr<Transport> newXPCTransport();
} // namespace clangd
} // namespace clang