net/base/sdch_filter.h

   1 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 // SdchFilter applies open_vcdiff content decoding to a datastream.
   6 // This decoding uses a pre-cached dictionary of text fragments to decode
   7 // (expand) the stream back to its original contents.
   8 //
   9 // This SdchFilter internally uses open_vcdiff/vcdec library to do decoding.
  10 //
  11 // SdchFilter is also a subclass of Filter. See the latter's header file
  12 // filter.h for sample usage.
  13
  14 #ifndef NET_BASE_SDCH_FILTER_H_
  15 #define NET_BASE_SDCH_FILTER_H_
  16
  17 #include <string>
  18
  19 #include "base/memory/scoped_ptr.h"
  20 #include "net/base/filter.h"
  21 #include "net/base/net_export.h"
  22 #include "net/base/sdch_manager.h"
  23
  24 namespace open_vcdiff {
  25 class VCDiffStreamingDecoder;
  26 }
  27
  28 namespace net {
  29
  30 class NET_EXPORT_PRIVATE SdchFilter : public Filter {
  31  public:
  32   virtual ~SdchFilter();
  33
  34   // Initializes filter decoding mode and internal control blocks.
  35   bool InitDecoding(Filter::FilterType filter_type);
  36
  37   // Decode the pre-filter data and writes the output into |dest_buffer|
  38   // The function returns FilterStatus. See filter.h for its description.
  39   //
  40   // Upon entry, *dest_len is the total size (in number of chars) of the
  41   // destination buffer. Upon exit, *dest_len is the actual number of chars
  42   // written into the destination buffer.
  43   virtual FilterStatus ReadFilteredData(char* dest_buffer,
  44                                         int* dest_len) OVERRIDE;
  45
  46  private:
  47   // Internal status.  Once we enter an error state, we stop processing data.
  48   enum DecodingStatus {
  49     DECODING_UNINITIALIZED,
  50     WAITING_FOR_DICTIONARY_SELECTION,
  51     DECODING_IN_PROGRESS,
  52     DECODING_ERROR,
  53     META_REFRESH_RECOVERY,  // Decoding error being handled by a meta-refresh.
  54     PASS_THROUGH,  // Non-sdch content being passed without alteration.
  55   };
  56
  57   // Only to be instantiated by Filter::Factory.
  58   explicit SdchFilter(const FilterContext& filter_context);
  59   friend class Filter;
  60
  61   // Identify the suggested dictionary, and initialize underlying decompressor.
  62   Filter::FilterStatus InitializeDictionary();
  63
  64   // Move data that was internally buffered (after decompression) to the
  65   // specified dest_buffer.
  66   int OutputBufferExcess(char* const dest_buffer, size_t available_space);
  67
  68   // Context data from the owner of this filter.
  69   const FilterContext& filter_context_;
  70
  71   // Tracks the status of decoding.
  72   // This variable is initialized by InitDecoding and updated only by
  73   // ReadFilteredData.
  74   DecodingStatus decoding_status_;
  75
  76   // The underlying decoder that processes data.
  77   // This data structure is initialized by InitDecoding and updated in
  78   // ReadFilteredData.
  79   scoped_ptr<open_vcdiff::VCDiffStreamingDecoder> vcdiff_streaming_decoder_;
  80
  81   // In case we need to assemble the hash piecemeal, we have a place to store
  82   // a part of the hash until we "get all 8 bytes plus a null."
  83   std::string dictionary_hash_;
  84
  85   // After assembling an entire dictionary hash (the first 9 bytes of the
  86   // sdch payload, we check to see if it is plausible, meaning it has a null
  87   // termination, and has 8 characters that are possible in a net-safe base64
  88   // encoding.  If the hash is not plausible, then the payload is probably not
  89   // an SDCH encoded bundle, and various error recovery strategies can be
  90   // attempted.
  91   bool dictionary_hash_is_plausible_;
  92
  93   // We hold an in-memory copy of the dictionary during the entire decoding, as
  94   // it is used directly by the VC-DIFF decoding system.
  95   // That char* data is part of the dictionary_ we hold a reference to.
  96   scoped_refptr<SdchManager::Dictionary> dictionary_;
  97
  98   // The decoder may demand a larger output buffer than the target of
  99   // ReadFilteredData so we buffer the excess output between calls.
 100   std::string dest_buffer_excess_;
 101   // To avoid moving strings around too much, we save the index into
 102   // dest_buffer_excess_ that has the next byte to output.
 103   size_t dest_buffer_excess_index_;
 104
 105   // To get stats on activities, we keep track of source and target bytes.
 106   // Visit about:histograms/Sdch to see histogram data.
 107   size_t source_bytes_;
 108   size_t output_bytes_;
 109
 110   // Error recovery in content type may add an sdch filter type, in which case
 111   // we should gracefully perform pass through if the format is incorrect, or
 112   // an applicable dictionary can't be found.
 113   bool possible_pass_through_;
 114
 115   // The URL that is currently being filtered.
 116   // This is used to restrict use of a dictionary to a specific URL or path.
 117   GURL url_;
 118
 119   // To facilitate error recovery, allow filter to know if content is text/html
 120   // by checking within this mime type (we may do a meta-refresh via html).
 121   std::string mime_type_;
 122
 123   DISALLOW_COPY_AND_ASSIGN(SdchFilter);
 124 };
 125
 126 }  // namespace net
 127
 128 #endif  // NET_BASE_SDCH_FILTER_H_