llvm/clang-tools-extra/clangd/support/FileCache.h

//===--- FileCache.h - Revalidating cache of data from disk ------*- C++-*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_SUPPORT_FILECACHE_H
#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_SUPPORT_FILECACHE_H

#include "Path.h"
#include "ThreadsafeFS.h"
#include "llvm/Support/Chrono.h"
#include <mutex>
#include <optional>

namespace clang {
namespace clangd {

/// Base class for threadsafe cache of data read from a file on disk.
///
/// We want configuration files to be "live" as much as possible.
/// Reading them every time is simplest, but caching solves a few problems:
///  - reading and parsing is cheap but not free (and happens on hot paths)
///  - we can ignore invalid data and use the old value (we may see truncated
///    compile_commands.json from non-atomic writers)
///  - we avoid reporting the same errors repeatedly
///
/// We still read and parse the data synchronously on demand, but skip as much
/// work as possible:
///  - if not enough wall-time has elapsed, assume the data is still up-to-date
///  - if we stat the file and it has the same mtime + size, don't read it
///  - obviously we only have to parse when we re-read the file
/// (Tracking OS change events is an alternative, but difficult to do portably.)
///
/// Caches for particular data (e.g. compilation databases) should inherit and:
///  - add mutable storage for the cached parsed data
///  - add a public interface implemented on top of read()
class FileCache { … };

} // namespace clangd
} // namespace clang

#endif