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1//===--- FileCache.h - Revalidating cache of data from disk ------*- C++-*-===//2//3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.4// See https://llvm.org/LICENSE.txt for license information.5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception6//7//===----------------------------------------------------------------------===//8 9#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_SUPPORT_FILECACHE_H10#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_SUPPORT_FILECACHE_H11 12#include "Path.h"13#include "ThreadsafeFS.h"14#include "llvm/Support/Chrono.h"15#include <mutex>16#include <optional>17 18namespace clang {19namespace clangd {20 21/// Base class for threadsafe cache of data read from a file on disk.22///23/// We want configuration files to be "live" as much as possible.24/// Reading them every time is simplest, but caching solves a few problems:25/// - reading and parsing is cheap but not free (and happens on hot paths)26/// - we can ignore invalid data and use the old value (we may see truncated27/// compile_commands.json from non-atomic writers)28/// - we avoid reporting the same errors repeatedly29///30/// We still read and parse the data synchronously on demand, but skip as much31/// work as possible:32/// - if not enough wall-time has elapsed, assume the data is still up-to-date33/// - if we stat the file and it has the same mtime + size, don't read it34/// - obviously we only have to parse when we re-read the file35/// (Tracking OS change events is an alternative, but difficult to do portably.)36///37/// Caches for particular data (e.g. compilation databases) should inherit and:38/// - add mutable storage for the cached parsed data39/// - add a public interface implemented on top of read()40class FileCache {41protected:42 // Path must be absolute.43 FileCache(PathRef Path);44 45 // Updates the cached value if needed, then provides threadsafe access to it.46 //47 // Specifically:48 // - Parse() may be called (if the cache was not up-to-date)49 // The lock is held, so cache storage may be safely written.50 // Parse(None) means the file doesn't exist.51 // - Read() will always be called, to provide access to the value.52 // The lock is again held, so the value can be copied or used.53 //54 // If the last Parse is newer than FreshTime, we don't check metadata.55 // - time_point::min() means we only do IO if we never read the file before56 // - time_point::max() means we always at least stat the file57 // - steady_clock::now() + seconds(1) means we accept 1 second of staleness58 void read(const ThreadsafeFS &TFS,59 std::chrono::steady_clock::time_point FreshTime,60 llvm::function_ref<void(std::optional<llvm::StringRef>)> Parse,61 llvm::function_ref<void()> Read) const;62 63 PathRef path() const { return Path; }64 65private:66 std::string Path;67 // Members are mutable so read() can present a const interface.68 // (It is threadsafe and approximates read-through to TFS).69 mutable std::mutex Mu;70 // Time when the cache was known valid (reflected disk state).71 mutable std::chrono::steady_clock::time_point ValidTime;72 // Filesystem metadata corresponding to the currently cached data.73 mutable llvm::sys::TimePoint<> ModifiedTime;74 mutable uint64_t Size;75};76 77} // namespace clangd78} // namespace clang79 80#endif81