clang-tools-extra/clangd/support/FileCache.h

   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
  18 namespace clang {
  19 namespace 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()
  40 class FileCache {
  41 protected:
  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
  65 private:
  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