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//===--- 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/ADT/ScopeExit.h"
#include "llvm/Support/Chrono.h"
#include "llvm/Support/VirtualFileSystem.h"
#include <mutex>
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 {
protected:
// Path must be absolute.
FileCache(PathRef Path);
// Updates the cached value if needed, then provides threadsafe access to it.
//
// Specifically:
// - Parse() may be called (if the cache was not up-to-date)
// The lock is held, so cache storage may be safely written.
// Parse(None) means the file doesn't exist.
// - Read() will always be called, to provide access to the value.
// The lock is again held, so the value can be copied or used.
//
// If the last Parse is newer than FreshTime, we don't check metadata.
// - time_point::min() means we only do IO if we never read the file before
// - time_point::max() means we always at least stat the file
// - steady_clock::now() + seconds(1) means we accept 1 second of staleness
void read(const ThreadsafeFS &TFS,
std::chrono::steady_clock::time_point FreshTime,
llvm::function_ref<void(llvm::Optional<llvm::StringRef>)> Parse,
llvm::function_ref<void()> Read) const;
PathRef path() const { return Path; }
private:
std::string Path;
// Members are mutable so read() can present a const interface.
// (It is threadsafe and approximates read-through to TFS).
mutable std::mutex Mu;
// Time when the cache was known valid (reflected disk state).
mutable std::chrono::steady_clock::time_point ValidTime;
// Filesystem metadata corresponding to the currently cached data.
mutable llvm::sys::TimePoint<> ModifiedTime;
mutable uint64_t Size;
};
} // namespace clangd
} // namespace clang
#endif
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