thodg/libgit2

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    Commit

  • Hash : c51065e3
    Author : Roberto Tyley
    Date : 2011-10-24T14:39:03 

    Tolerate zlib deflation with window size < 32Kb

libgit2 currently identifies loose objects as corrupt if they've been
deflated using a window size less than 32Kb, because the
is_zlib_compressed_data() function doesn't recognise the header
byte as a zlib header. This patch makes the method tolerant of
all valid window sizes (15-bit to 8-bit) - but doesn't sacrifice
it's accuracy in distingushing the standard loose-object format
from the experimental (now abandoned) format. It's based on a patch
which has been merged into C-Git master branch:

https://github.com/git/git/commit/7f684a2aff636f44a506

On memory constrained systems zlib may use a much smaller window
size - working on Agit, I found that Android uses a 4KB window;
giving a header byte of 0x48, not 0x78. Consequently all loose
objects generated by the Android platform appear 'corrupt' :(

It might appear that this patch changes isStandardFormat() to the
point where it could incorrectly identify the experimental format as
the standard one, but the two criteria (bitmask & checksum) can only
give a false result for an experimental object where both of the
following are true:

1) object size is exactly 8 bytes when uncompressed (bitmask)
2) [single-byte in-pack git type&size header] * 256
   + [1st byte of the following zlib header] % 31 = 0 (checksum)

As it happens, for all possible combinations of valid object type
(1-4) and window bits (0-7), the only time when the checksum will be
divisible by 31 is for 0x1838 - ie object type *1*, a Commit - which,
due the fields all Commit objects must contain, could never be as
small as 8 bytes in size.

Given this, the combination of the two criteria (bitmask & checksum)
always correctly determines the buffer format, and is more tolerant
than the previous version.

References:

Android uses a 4KB window for deflation:
http://android.git.kernel.org/?p=platform/libcore.git;a=blob;f=luni/src/main/native/java_util_zip_Deflater.cpp;h=c0b2feff196e63a7b85d97cf9ae5bb258

Code snippet searching for false positives with the zlib checksum:
https://gist.github.com/1118177

Change-Id: Ifd84cd2bd6b46f087c9984fb4cbd8309f483dec0

  • README.md

  • libgit2 - the Git linkable library

    libgit2 is a portable, pure C implementation of the Git core methods provided as a re-entrant linkable library with a solid API, allowing you to write native speed custom Git applications in any language with bindings.

    libgit2 is licensed under a very permissive license (GPLv2 with a special Linking Exception). This basically means that you can link it (unmodified) with any kind of software without having to release its source code.

    What It Can Do

    libgit2 is already very usable.

    • SHA conversions, formatting and shortening
    • abstracked ODB backend system
    • commit, tag, tree and blob parsing, editing, and write-back
    • tree traversal
    • revision walking
    • index file (staging area) manipulation
    • reference management (including packed references)
    • config file management
    • high level repository management
    • thread safety and reentrancy
    • descriptive and detailed error messages
    • …and more (over 175 different API calls)

    Building libgit2 - Using CMake

    libgit2 builds cleanly on most platforms without any external dependencies. Under Unix-like systems, like Linux, * BSD and Mac OS X, libgit2 expects pthreads to be available; they should be installed by default on all systems. Under Windows, libgit2 uses the native Windows API for threading.

    The libgit2 library is built using CMake 2.6+ (http://www.cmake.org) on all platforms.

    On most systems you can build the library using the following commands

    $ mkdir build && cd build
$ cmake ..
$ cmake --build .

    Alternatively you can point the CMake GUI tool to the CMakeLists.txt file and generate platform specific build project or IDE workspace.

    To install the library you can specify the install prefix by setting:

    $ cmake .. -DCMAKE_INSTALL_PREFIX=/install/prefix
$ cmake --build . --target install

    If you want to build a universal binary for Mac OS X, CMake sets it all up for you if you use -DCMAKE_OSX_ARCHITECTURES="i386;x86_64" when configuring.

    For more advanced use or questions about CMake please read http://www.cmake.org/Wiki/CMake_FAQ.

    The following CMake variables are declared:

    • INSTALL_BIN: Where to install binaries to.
    • INSTALL_LIB: Where to install libraries to.
    • INSTALL_INC: Where to install headers to.
    • BUILD_SHARED_LIBS: Build libgit2 as a Shared Library (defaults to ON)
    • BUILD_TESTS: Build the libgit2 test suite (defaults to ON)
    • THREADSAFE: Build libgit2 with threading support (defaults to OFF)

    Language Bindings

    Here are the bindings to libgit2 that are currently available:

    If you start another language binding to libgit2, please let us know so we can add it to the list.

    How Can I Contribute

    Fork libgit2/libgit2 on GitHub, add your improvement, push it to a branch in your fork named for the topic, send a pull request.

    You can also file bugs or feature requests under the libgit2 project on GitHub, or join us on the mailing list by sending an email to:

    libgit2@librelist.com

    License

    libgit2 is under GPL2 with linking exemption. This means you can link to the library with any program, commercial, open source or other. However, you cannot modify libgit2 and distribute it without supplying the source.

    See the COPYING file for the full license text.

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