forked from OSchip/llvm-project
82 lines
3.3 KiB
C++
82 lines
3.3 KiB
C++
//===--- FileCache.h - Revalidating cache of data from disk ------*- C++-*-===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_SUPPORT_FILECACHE_H
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#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_SUPPORT_FILECACHE_H
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#include "Path.h"
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#include "ThreadsafeFS.h"
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#include "llvm/ADT/ScopeExit.h"
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#include "llvm/Support/Chrono.h"
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#include "llvm/Support/VirtualFileSystem.h"
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#include <mutex>
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namespace clang {
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namespace clangd {
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/// Base class for threadsafe cache of data read from a file on disk.
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///
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/// We want configuration files to be "live" as much as possible.
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/// Reading them every time is simplest, but caching solves a few problems:
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/// - reading and parsing is cheap but not free (and happens on hot paths)
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/// - we can ignore invalid data and use the old value (we may see truncated
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/// compile_commands.json from non-atomic writers)
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/// - we avoid reporting the same errors repeatedly
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///
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/// We still read and parse the data synchronously on demand, but skip as much
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/// work as possible:
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/// - if not enough wall-time has elapsed, assume the data is still up-to-date
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/// - if we stat the file and it has the same mtime + size, don't read it
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/// - obviously we only have to parse when we re-read the file
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/// (Tracking OS change events is an alternative, but difficult to do portably.)
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///
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/// Caches for particular data (e.g. compilation databases) should inherit and:
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/// - add mutable storage for the cached parsed data
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/// - add a public interface implemented on top of read()
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class FileCache {
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protected:
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// Path must be absolute.
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FileCache(PathRef Path);
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// Updates the cached value if needed, then provides threadsafe access to it.
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//
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// Specifically:
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// - Parse() may be called (if the cache was not up-to-date)
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// The lock is held, so cache storage may be safely written.
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// Parse(None) means the file doesn't exist.
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// - Read() will always be called, to provide access to the value.
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// The lock is again held, so the value can be copied or used.
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//
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// If the last Parse is newer than FreshTime, we don't check metadata.
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// - time_point::min() means we only do IO if we never read the file before
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// - time_point::max() means we always at least stat the file
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// - steady_clock::now() + seconds(1) means we accept 1 second of staleness
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void read(const ThreadsafeFS &TFS,
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std::chrono::steady_clock::time_point FreshTime,
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llvm::function_ref<void(llvm::Optional<llvm::StringRef>)> Parse,
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llvm::function_ref<void()> Read) const;
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PathRef path() const { return Path; }
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private:
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std::string Path;
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// Members are mutable so read() can present a const interface.
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// (It is threadsafe and approximates read-through to TFS).
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mutable std::mutex Mu;
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// Time when the cache was known valid (reflected disk state).
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mutable std::chrono::steady_clock::time_point ValidTime;
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// Filesystem metadata corresponding to the currently cached data.
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mutable llvm::sys::TimePoint<> ModifiedTime;
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mutable uint64_t Size;
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};
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} // namespace clangd
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} // namespace clang
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#endif
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