text: // Copyright (c) 2011 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.
  
  // The cache is stored on disk as a collection of block-files, plus an index
  // file plus a collection of external files.
  //
  // Any data blob bigger than kMaxBlockSize (net/addr.h) will be stored on a
  // separate file named f_xxx where x is a hexadecimal number. Shorter data will
  // be stored as a series of blocks on a block-file. In any case, CacheAddr
  // represents the address of the data inside the cache.
  //
  // The index file is just a simple hash table that maps a particular entry to
  // a CacheAddr value. Linking for a given hash bucket is handled internally
  // by the cache entry.
  //
  // The last element of the cache is the block-file. A block file is a file
  // designed to store blocks of data of a given size. It is able to store data
  // that spans from one to four consecutive "blocks", and it grows as needed to
  // store up to approximately 65000 blocks. It has a fixed size header used for
  // book keeping such as tracking free of blocks on the file. For example, a
  // block-file for 1KB blocks will grow from 8KB when totally empty to about 64MB
  // when completely full. At that point, data blocks of 1KB will be stored on a
  // second block file that will store the next set of 65000 blocks. The first
  // file contains the number of the second file, and the second file contains the
  // number of a third file, created when the second file reaches its limit. It is
  // important to remember that no matter how long the chain of files is, any
  // given block can be located directly by its address, which contains the file
  // number and starting block inside the file.
  //
  // A new cache is initialized with four block files (named data_0 through
  // data_3), each one dedicated to store blocks of a given size. The number at
  // the end of the file name is the block file number (in decimal).
  //
  // There are two "special" types of blocks: an entry and a rankings node. An
  // entry keeps track of all the information related to the same cache entry,
  // such as the key, hash value, data pointers etc. A rankings node keeps track
  // of the information that is updated frequently for a given entry, such as its
  // location on the LRU lists, last access time etc.
  //
  // The files that store internal information for the cache (blocks and index)
  // are at least partially memory mapped. They have a location that is signaled
  // every time the internal structures are modified, so it is possible to detect
  // (most of the time) when the process dies in the middle of an update.
  //
  // In order to prevent dirty data to be used as valid (after a crash), every
  // cache entry has a dirty identifier. Each running instance of the cache keeps
  // a separate identifier (maintained on the "this_id" header field) that is used
  // to mark every entry that is created or modified. When the entry is closed,
  // and all the data can be trusted, the dirty flag is cleared from the entry.
  // When the cache encounters an entry whose identifier is different than the one
  // being currently used, it means that the entry was not properly closed on a
  // previous run, so it is discarded.
  
  #ifndef NET_DISK_CACHE_DISK_FORMAT_H_
  #define NET_DISK_CACHE_DISK_FORMAT_H_
  #pragma once
  
  #include "base/basictypes.h"
  #include "net/base/net_export.h"
  
  namespace disk_cache {
  
  typedef uint32 CacheAddr;
  
  const int kIndexTablesize = 0x10000;
  const uint32 kIndexMagic = 0xC103CAC3;
  const uint32 kCurrentVersion = 0x20000;  // Version 2.0.
  
  struct LruData {
    int32     pad1[2];
    int32     filled;          // Flag to tell when we filled the cache.
    int32     sizes[5];
    CacheAddr heads[5];
    CacheAddr tails[5];
    CacheAddr transaction;     // In-flight operation target.
    int32     operation;       // Actual in-flight operation.
    int32     operation_list;  // In-flight operation list.
    int32     pad2[7];
  };
  
  // Header for the master index file.
  struct NET_EXPORT_PRIVATE IndexHeader {
    IndexHeader();
  
    uint32      magic;
    uint32      version;
    int32       num_entries;   // Number of entries currently stored.
    int32       num_bytes;     // Total size of the stored data.
    int32       last_file;     // Last external file created.
    int32       this_id;       // Id for all entries being changed (dirty flag).
    CacheAddr   stats;         // Storage for usage data.
    int32       table_len;     // Actual size of the table (0 == kIndexTablesize).
    int32       crash;         // Signals a previous crash.
    int32       experiment;    // Id of an ongoing test.
    uint64      create_time;   // Creation time for this set of files.
    int32       pad[52];
    LruData     lru;           // Eviction control data.
  };
  
  // The structure of the whole index file.
  struct Index {
    IndexHeader header;
    CacheAddr   table[kIndexTablesize];  // Default size. Actual size controlled
                                         // by header.table_len.
  };
  
  // Main structure for an entry on the backing storage. If the key is longer than
  // what can be stored on this structure, it will be extended on consecutive
  // blocks (adding 256 bytes each time), up to 4 blocks (1024 - 32 - 1 chars).
  // After that point, the whole key will be stored as a data block or external
  // file.
  struct EntryStore {
    uint32      hash;               // Full hash of the key.
    CacheAddr   next;               // Next entry with the same hash or bucket.
    CacheAddr   rankings_node;      // Rankings node for this entry.
    int32       reuse_count;        // How often is this entry used.
    int32       refetch_count;      // How often is this fetched from the net.
    int32       state;              // Current state.
    uint64      creation_time;
    int32       key_len;
    CacheAddr   long_key;           // Optional address of a long key.
    int32       data_size[4];       // We can store up to 4 data streams for each
    CacheAddr   data_addr[4];       // entry.
    uint32      flags;              // Any combination of EntryFlags.
    int32       pad[4];
    uint32      self_hash;          // The hash of EntryStore up to this point.
    char        key[256 - 24 * 4];  // null terminated
  };
  
  COMPILE_ASSERT(sizeof(EntryStore) == 256, bad_EntyStore);
  const int kMaxInternalKeyLength = 4 * sizeof(EntryStore) -
                                    offsetof(EntryStore, key) - 1;
  
  // Possible states for a given entry.
  enum EntryState {
    ENTRY_NORMAL = 0,
    ENTRY_EVICTED,    // The entry was recently evicted from the cache.
    ENTRY_DOOMED      // The entry was doomed.
  };
  
  // Flags that can be applied to an entry.
  enum EntryFlags {
    PARENT_ENTRY = 1,         // This entry has children (sparse) entries.
    CHILD_ENTRY = 1 << 1      // Child entry that stores sparse data.
  };
  
  #pragma pack(push, 4)
  // Rankings information for a given entry.
  struct RankingsNode {
    uint64      last_used;        // LRU info.
    uint64      last_modified;    // LRU info.
    CacheAddr   next;             // LRU list.
    CacheAddr   prev;             // LRU list.
    CacheAddr   contents;         // Address of the EntryStore.
    int32       dirty;            // The entry is being modifyied.
    uint32      self_hash;        // RankingsNode's hash.
  };
  #pragma pack(pop)
  
  COMPILE_ASSERT(sizeof(RankingsNode) == 36, bad_RankingsNode);
  
  const uint32 kBlockMagic = 0xC104CAC3;
  const int kBlockHeaderSize = 8192;  // Two pages: almost 64k entries
  const int kMaxBlocks = (kBlockHeaderSize - 80) * 8;
  
  // Bitmap to track used blocks on a block-file.
  typedef uint32 AllocBitmap[kMaxBlocks / 32];
  
  // A block-file is the file used to store information in blocks (could be
  // EntryStore blocks, RankingsNode blocks or user-data blocks).
  // We store entries that can expand for up to 4 consecutive blocks, and keep
  // counters of the number of blocks available for each type of entry. For
  // instance, an entry of 3 blocks is an entry of type 3. We also keep track of
  // where did we find the last entry of that type (to avoid searching the bitmap
  // from the beginning every time).
  // This Structure is the header of a block-file:
  struct NET_EXPORT_PRIVATE BlockFileHeader {
    BlockFileHeader();
  
    uint32          magic;
    uint32          version;
    int16           this_file;    // Index of this file.
    int16           next_file;    // Next file when this one is full.
    int32           entry_size;   // Size of the blocks of this file.
    int32           num_entries;  // Number of stored entries.
    int32           max_entries;  // Current maximum number of entries.
    int32           empty[4];     // Counters of empty entries for each type.
    int32           hints[4];     // Last used position for each entry type.
    volatile int32  updating;     // Keep track of updates to the header.
    int32           user[5];
    AllocBitmap     allocation_map;
  };
  
  COMPILE_ASSERT(sizeof(BlockFileHeader) == kBlockHeaderSize, bad_header);
  
  // Sparse data support:
  // We keep a two level hierarchy to enable sparse data for an entry: the first
  // level consists of using separate "child" entries to store ranges of 1 MB,
  // and the second level stores blocks of 1 KB inside each child entry.
  //
  // Whenever we need to access a particular sparse offset, we first locate the
  // child entry that stores that offset, so we discard the 20 least significant
  // bits of the offset, and end up with the child id. For instance, the child id
  // to store the first megabyte is 0, and the child that should store offset
  // 0x410000 has an id of 4.
  //
  // The child entry is stored the same way as any other entry, so it also has a
  // name (key). The key includes a signature to be able to identify children
  // created for different generations of the same resource. In other words, given
  // that a given sparse entry can have a large number of child entries, and the
  // resource can be invalidated and replaced with a new version at any time, it
  // is important to be sure that a given child actually belongs to certain entry.
  //
  // The full name of a child entry is composed with a prefix ("Range_"), and two
  // hexadecimal 64-bit numbers at the end, separated by semicolons. The first
  // number is the signature of the parent key, and the second number is the child
  // id as described previously. The signature itself is also stored internally by
  // the child and the parent entries. For example, a sparse entry with a key of
  // "sparse entry name", and a signature of 0x052AF76, may have a child entry
  // named "Range_sparse entry name:052af76:4", which stores data in the range
  // 0x400000 to 0x4FFFFF.
  //
  // Each child entry keeps track of all the 1 KB blocks that have been written
  // to the entry, but being a regular entry, it will happily return zeros for any
  // read that spans data not written before. The actual sparse data is stored in
  // one of the data streams of the child entry (at index 1), while the control
  // information is stored in another stream (at index 2), both by parents and
  // the children.
  
  // This structure contains the control information for parent and child entries.
  // It is stored at offset 0 of the data stream with index 2.
  // It is possible to write to a child entry in a way that causes the last block
  // to be only partialy filled. In that case, last_block and last_block_len will
  // keep track of that block.
  struct SparseHeader {
    int64 signature;          // The parent and children signature.
    uint32 magic;             // Structure identifier (equal to kIndexMagic).
    int32 parent_key_len;     // Key length for the parent entry.
    int32 last_block;         // Index of the last written block.
    int32 last_block_len;     // Lenght of the last written block.
    int32 dummy[10];
  };
  
  // The SparseHeader will be followed by a bitmap, as described by this
  // structure.
  struct SparseData {
    SparseHeader header;
    uint32 bitmap[32];        // Bitmap representation of known children (if this
                              // is a parent entry), or used blocks (for child
                              // entries. The size is fixed for child entries but
                              // not for parents; it can be as small as 4 bytes
                              // and as large as 8 KB.
  };
  
  // The number of blocks stored by a child entry.
  const int kNumSparseBits = 1024;
  COMPILE_ASSERT(sizeof(SparseData) == sizeof(SparseHeader) + kNumSparseBits / 8,
                 Invalid_SparseData_bitmap);
  
  }  // namespace disk_cache
  
  #endif  // NET_DISK_CACHE_DISK_FORMAT_H_
  