Re: Reading commit objects

From: Shawn Pearce <spearce@spearce.org>
Date: May 22, 2013, 05:59 UTC
Message-ID: <CAJo=hJtqACW+CR5FkmDfwyK1Wg3Kcppy6DbW7P=On_qJyvsYvQ@mail.gmail.com>
In-Reply-To: <CABx5MBSnpZTthOHECqkbpdbFfkb4e_uSo-rh4owBc8B_oSKjJQ@mail.gmail.com>

On Tue, May 21, 2013 at 3:18 PM, Chico Sokol <chico.sokol@gmail.com> wrote:

Show 18 quoted lines

> Ok, we discovered that the commit object actually contains the tree
> object's sha1, by reading its contents with python zlib library.
>
> So the bug must be with our java code (we're building a java lib).
>
> Is there any non-standard issue in git's zlib compression? We're
> decompressing its contents with java default zlib api, so it should
> work normally, here's our code, that's printing that wrong output:
>
> import java.io.File;
> import java.io.FileInputStream;
> import java.util.zip.InflaterInputStream;
> import org.apache.commons.io.IOUtils;
> ...
> File obj = new File(".git/objects/25/0f67ef017fcb97b5371a302526872cfcadad21");
> InflaterInputStream inflaterInputStream = new InflaterInputStream(new
> FileInputStream(obj));
> System.out.println(IOUtils.readLines(inflaterInputStream));

...

Show 8 quoted lines

>>> Currently, we're trying to parse commit objects. After decompressing
>>> the contents of a commit object file we got the following output:
>>>
>>> commit 191
>>> author Francisco Sokol <chico.sokol@gmail.com> 1369140112 -0300
>>> committer Francisco Sokol <chico.sokol@gmail.com> 1369140112 -0300
>>>
>>> first commit

Your code is broken. IOUtils is probably corrupting what you get back. After inflating the stream you should see the object type ("commit"), space, its length in bytes as a base 10 string, and then a NUL ('\0'). Following that is the tree line, and parent(s) if any. I wonder if IOUtils discarded the remainder of the line after the NUL and did not consider the tree line.

And you wonder why JGit code is confusing. We can't rely on "standard Java APIs" to do the right thing, because commonly used libraries have made assumptions that disagree with the way Git works.