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-exception
6//
7//===----------------------------------------------------------------------===//
8
9#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_SUPPORT_FILECACHE_H
10#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_SUPPORT_FILECACHE_H
11
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 truncated
27/// compile_commands.json from non-atomic writers)
28/// - we avoid reporting the same errors repeatedly
29///
30/// We still read and parse the data synchronously on demand, but skip as much
31/// work as possible:
32/// - if not enough wall-time has elapsed, assume the data is still up-to-date
33/// - if we stat the file and it has the same mtime + size, don't read it
34/// - obviously we only have to parse when we re-read the file
35/// (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 data
39/// - 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 before
56 // - time_point::max() means we always at least stat the file
57 // - steady_clock::now() + seconds(1) means we accept 1 second of staleness
58 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 clangd
78} // namespace clang
79
80#endif
81

source code of clang-tools-extra/clangd/support/FileCache.h