simplecache

The simplecache package provides support for reading Chromium simple cache v9 (other versions untested).

Learn more: http://www.chromium.org/developers/design-documents/network-stack/disk-cache/very-simple-backend

See the example_test.go for an example of how to read an image from cache in testdata.

This project also includes a tool to read the cache from command line, read this README.

Short cache format

The simple cache contains different files:

Filename	Description
index	Fake index file
#####_0	Data stream file
#####_1	Data stream file
#####_s	Data stream file
index-dir/the-real-index	The real index file

Fake index file format (index)

offset	size	value	description
0	8	0x656e74657220796f	Magic
8	4	9	Version
12	12	0x0	Padding

Real index file format (the-real-index)

Overview:

• File header
• Last write reason (starting v7)
• Index table
• Last modified

File header

The index file header (struct indexHeader) is 36 bytes in size and consists of:

offset	size	value	description
0	4		Payload size
4	4		Payload CRC32
8	8	0x656e74657220796f	Magic
16	4	9	Version
20	8		Number of entries
28	8		Cache size

Index table

The index table is an array of entries. An entry (struct indexEntry) is 24 bytes in size and consists of:

offset	size	value	description
0	8		Hash
8	8		Last used
16	8		Size

Data stream file format (#####_0)

Overview:

• File header
• URL
• Data stream (stream 1)
• Stream EOF
• HTTP headers (stream 0)
• (optionally) the SHA256 of the URL
• Stream EOF

File header

The index file header (struct entryHeader) is 24 bytes in size and consists of:

offset	size	value	description
0	8	0xfcfb6d1ba7725c30	Magic
8	4	5	Version
12	4		URL length
16	4		URL hash
20	4	0x0	null byte padding

Stream EOF

The separator (struct entryEOF) contains information about the stream above. It is 24 bytes in size and consists of:

offset	size	value	description
0	8	0xf4fa6f45970d41d8	Magic
8	4		Flag
12	4		stream CRC32
16	4		stream size
20	4	0x0	null byte padding

Flag

value	description
0
1	the stream has CRC32
2	the URL has SHA256
3	1 + 2

Data stream file format (#####_1)

Overview:

• File header
• URL
• Data stream (stream 2)
• Stream EOF

Data stream file format (#####_s)

Overview:

• File header
• many of the following:
  • Range header
  • Range data stream

File header

The index file header (struct entryHeader) is 24 bytes in size and consists of:

offset	size	value	description
0	8	0xfcfb6d1ba7725c30	Magic
8	4	5	Version
12	4		URL length
16	4		URL MD5
20	4	0x0	null byte padding

Range header

The range header (struct sparseRangeHeader) contains information about the following range stream. It is 28 bytes in size and consists of:

offset	size	value	description
0	8	0xeb97bf016553676b	Magic
8	8		stream offset
16	8		stream size
24	4		stream CRC32