clang-tools  16.0.0git
FileCache.h
Go to the documentation of this file.
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 
17 namespace clang {
18 namespace clangd {
19 
20 /// Base class for threadsafe cache of data read from a file on disk.
21 ///
22 /// We want configuration files to be "live" as much as possible.
23 /// Reading them every time is simplest, but caching solves a few problems:
24 /// - reading and parsing is cheap but not free (and happens on hot paths)
25 /// - we can ignore invalid data and use the old value (we may see truncated
26 /// compile_commands.json from non-atomic writers)
27 /// - we avoid reporting the same errors repeatedly
28 ///
29 /// We still read and parse the data synchronously on demand, but skip as much
30 /// work as possible:
31 /// - if not enough wall-time has elapsed, assume the data is still up-to-date
32 /// - if we stat the file and it has the same mtime + size, don't read it
33 /// - obviously we only have to parse when we re-read the file
34 /// (Tracking OS change events is an alternative, but difficult to do portably.)
35 ///
36 /// Caches for particular data (e.g. compilation databases) should inherit and:
37 /// - add mutable storage for the cached parsed data
38 /// - add a public interface implemented on top of read()
39 class FileCache {
40 protected:
41  // Path must be absolute.
43 
44  // Updates the cached value if needed, then provides threadsafe access to it.
45  //
46  // Specifically:
47  // - Parse() may be called (if the cache was not up-to-date)
48  // The lock is held, so cache storage may be safely written.
49  // Parse(None) means the file doesn't exist.
50  // - Read() will always be called, to provide access to the value.
51  // The lock is again held, so the value can be copied or used.
52  //
53  // If the last Parse is newer than FreshTime, we don't check metadata.
54  // - time_point::min() means we only do IO if we never read the file before
55  // - time_point::max() means we always at least stat the file
56  // - steady_clock::now() + seconds(1) means we accept 1 second of staleness
57  void read(const ThreadsafeFS &TFS,
58  std::chrono::steady_clock::time_point FreshTime,
59  llvm::function_ref<void(llvm::Optional<llvm::StringRef>)> Parse,
60  llvm::function_ref<void()> Read) const;
61 
62  PathRef path() const { return Path; }
63 
64 private:
65  std::string Path;
66  // Members are mutable so read() can present a const interface.
67  // (It is threadsafe and approximates read-through to TFS).
68  mutable std::mutex Mu;
69  // Time when the cache was known valid (reflected disk state).
70  mutable std::chrono::steady_clock::time_point ValidTime;
71  // Filesystem metadata corresponding to the currently cached data.
72  mutable llvm::sys::TimePoint<> ModifiedTime;
73  mutable uint64_t Size;
74 };
75 
76 } // namespace clangd
77 } // namespace clang
78 
79 #endif
clang::clangd::Path
std::string Path
A typedef to represent a file path.
Definition: Path.h:26
Path.h
ThreadsafeFS.h
clang::clangd::PathRef
llvm::StringRef PathRef
A typedef to represent a ref to file path.
Definition: Path.h:29
clang::clangd::ThreadsafeFS
Wrapper for vfs::FileSystem for use in multithreaded programs like clangd.
Definition: ThreadsafeFS.h:27
clang::clangd::FileCache::path
PathRef path() const
Definition: FileCache.h:62
clang
===– Representation.cpp - ClangDoc Representation --------—*- C++ -*-===//
Definition: ApplyReplacements.h:27
clang::clangd::FileCache
Base class for threadsafe cache of data read from a file on disk.
Definition: FileCache.h:39
clang::clangd::DocumentHighlightKind::Read
@ Read
clang::clangd::FileCache::read
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
Definition: FileCache.cpp:30
clang::clangd::FileCache::FileCache
FileCache(PathRef Path)
Definition: FileCache.cpp:24