net/disk_cache/simple/simple_file_tracker.h

// Copyright 2017 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef NET_DISK_CACHE_SIMPLE_SIMPLE_FILE_TRACKER_H_
#define NET_DISK_CACHE_SIMPLE_SIMPLE_FILE_TRACKER_H_

#include <stdint.h>
#include <algorithm>
#include <memory>
#include <unordered_map>
#include <vector>

#include "base/files/file.h"
#include "base/macros.h"
#include "base/memory/ref_counted.h"
#include "base/synchronization/lock.h"
#include "net/base/net_export.h"
#include "net/disk_cache/simple/simple_entry_format.h"

namespace disk_cache {

class SimpleSynchronousEntry;

// This keeps track of all the files SimpleCache has open, across all the
// backend instancess, in order to prevent us from running out of file
// descriptors.
// TODO(morlovich): Actually implement closing and re-opening of things if we
// run out.
//
// This class is thread-safe.
class NET_EXPORT_PRIVATE SimpleFileTracker {
 public:
  enum class SubFile { FILE_0, FILE_1, FILE_SPARSE };

  // A RAII helper that guards access to a file grabbed for use from
  // SimpleFileTracker::Acquire().  While it's still alive, if IsOK() is true,
  // then using the underlying base::File via get() or the -> operator will be
  // safe.
  //
  // This class is movable but not copyable.  It should only be used from a
  // single logical sequence of execution, and should not outlive the
  // corresponding SimpleSynchronousEntry.
  class NET_EXPORT_PRIVATE FileHandle {
   public:
    FileHandle();
    FileHandle(FileHandle&& other);
    ~FileHandle();
    FileHandle& operator=(FileHandle&& other);
    base::File* operator->() const;
    base::File* get() const;
    // Returns true if this handle points to a valid file. This should normally
    // be the first thing called on the object, after getting it from
    // SimpleFileTracker::Acquire.
    bool IsOK() const;

   private:
    friend class SimpleFileTracker;
    FileHandle(SimpleFileTracker* file_tracker,
               const SimpleSynchronousEntry* entry,
               SimpleFileTracker::SubFile subfile,
               base::File* file);

    // All the pointer fields are nullptr in the default/moved away from form.
    SimpleFileTracker* file_tracker_;
    const SimpleSynchronousEntry* entry_;
    SimpleFileTracker::SubFile subfile_;
    base::File* file_;
    DISALLOW_COPY_AND_ASSIGN(FileHandle);
  };

  struct EntryFileKey {
    EntryFileKey() : entry_hash(0), doom_generation(0) {}
    explicit EntryFileKey(uint64_t hash)
        : entry_hash(hash), doom_generation(0) {}

    uint64_t entry_hash;

    // In case of a hash collision, there may be multiple SimpleEntryImpl's
    // around which have the same entry_hash but different key. In that case,
    // we doom all but the most recent one and this number will eventually be
    // used to name the files for the doomed ones.
    // 0 here means the entry is the active one, and is the only value that's
    // presently in use here.
    uint32_t doom_generation;
  };

  SimpleFileTracker();
  ~SimpleFileTracker();

  // Established |file| as what's backing |subfile| for |owner|. This is
  // intended to be called when SimpleSynchronousEntry first sets up the file to
  // transfer its ownership to SimpleFileTracker. Any Register() call must be
  // eventually followed by a corresponding Close() call before the |owner| is
  // destroyed.
  void Register(const SimpleSynchronousEntry* owner,
                SubFile subfile,
                std::unique_ptr<base::File> file);

  // Lends out a file to SimpleSynchronousEntry for use. SimpleFileTracker
  // will ensure that it doesn't close the file until the handle is destroyed.
  // The caller should check .IsOK() on the returned value before using it, as
  // it's possible that the file had to be closed and re-opened due to FD
  // pressure, and that open may have failed. This should not be called twice
  // with the exact same arguments until the handle returned from the previous
  // such call is destroyed.
  FileHandle Acquire(const SimpleSynchronousEntry* owner, SubFile subfile);

  // Tells SimpleFileTracker that SimpleSynchronousEntry will not be interested
  // in the file further, so it can be closed and forgotten about.  It's OK to
  // call this while a handle to the file is alive, in which case the effect
  // takes place after the handle is destroyed.
  // If Close() has been called and the handle to the file is no longer alive,
  // a new backing file can be established by calling Register() again.
  void Close(const SimpleSynchronousEntry* owner, SubFile file);

  // Returns true if there is no in-memory state around, e.g. everything got
  // cleaned up. This is a test-only method since this object is expected to be
  // shared between multiple threads, in which case its return value may be
  // outdated the moment it's returned.
  bool IsEmptyForTesting();

 private:
  struct TrackedFiles {
    // We can potentially run through this state machine multiple times for
    // FILE_1, as that's often missing, so SimpleSynchronousEntry can sometimes
    // close and remove the file for an empty stream, then re-open it on actual
    // data.
    enum State {
      TF_NO_REGISTRATION = 0,
      TF_REGISTERED = 1,
      TF_ACQUIRED = 2,
      TF_ACQUIRED_PENDING_CLOSE = 3,
    };

    TrackedFiles() {
      std::fill(state, state + kSimpleEntryTotalFileCount, TF_NO_REGISTRATION);
    }

    bool Empty() const;

    // We use pointers to SimpleSynchronousEntry two ways:
    // 1) As opaque keys. This is handy as it avoids having to compare paths in
    //    case multiple backends use the same key. Since we access the
    //    bookkeeping under |lock_|
    //
    // 2) To get info on the caller of our operation.
    //    Accessing |owner| from any other TrackedFiles would be unsafe (as it
    //    may be doing its own thing in a different thread).
    const SimpleSynchronousEntry* owner;
    EntryFileKey key;

    // Some of these may be !IsValid(), if they are not open.
    // Note that these are stored indirect since we hand out pointers to these,
    // and we don't want those to become invalid if some other thread appends
    // things here.
    std::unique_ptr<base::File> files[kSimpleEntryTotalFileCount];

    State state[kSimpleEntryTotalFileCount];
  };

  // Marks the file that was previously returned by Acquire as eligible for
  // closing again. Called by ~FileHandle.
  void Release(const SimpleSynchronousEntry* owner, SubFile subfile);

  // |*found| will be set to whether the entry was found or not.
  std::vector<TrackedFiles>::iterator Find(const SimpleSynchronousEntry* owner);

  // Handles state transition of closing file (when we are not deferring it),
  // and moves the file out. Note that this may invalidate |owners_files|.
  std::unique_ptr<base::File> PrepareClose(
      std::vector<TrackedFiles>::iterator owners_files,
      int file_index);

  base::Lock lock_;
  std::unordered_map<uint64_t, std::vector<TrackedFiles>> tracked_files_;

  DISALLOW_COPY_AND_ASSIGN(SimpleFileTracker);
};

}  // namespace disk_cache

#endif  // NET_DISK_CACHE_SIMPLE_SIMPLE_FILE_TRACKER_H_