Re: [PATCH 1/1] read-cache: update index format default to v4

[Derrick Stolee <stolee@gmail.com>]

On 9/25/2018 3:06 AM, Patrick Steinhardt wrote:
> On Mon, Sep 24, 2018 at 11:32:23PM +0200, SZEDER Gábor wrote:
>> On Mon, Sep 24, 2018 at 02:15:30PM -0700, Derrick Stolee via GitGitGadget wrote:
>>> From: Derrick Stolee <dstolee@microsoft.com>
>>>
>>> The index v4 format has been available since 2012 with 9d22778
>>> "reach-cache.c: write prefix-compressed names in the index". Since
>>> the format has been stable for so long, almost all versions of Git
>>> in use today understand version 4, removing one barrier to upgrade
>>> -- that someone may want to downgrade and needs a working repo.
>> What about alternative implementations, like JGit, libgit2, etc.?
> Speaking of libgit2, we are able to read and write index v4 since
> commit c1b370e93

This is a good point, Szeder.

Patrick: I'm glad LibGit2 is up-to-date with index formats.

Unfortunately, taking a look (for the first time) at the JGit code 
reveals that they don't appear to have v4 support. In 
org.eclipse.jgit/src/org/eclipse/jgit/dircache/DirCache.java, the 
DirCache.readFrom() method: lines 488-494, I see the following snippet:

                 final int ver = NB.decodeInt32(hdr, 4);
                 boolean extended = false;
                 if (ver == 3)
                         extended = true;
                 else if (ver != 2)
                         throw new 
CorruptObjectException(MessageFormat.format(
JGitText.get().unknownDIRCVersion, Integer.valueOf(ver)));

It looks like this will immediately throw with versions other than 2 or 3.

I'm adding Jonathan Nieder to CC so he can check with JGit people about 
the impact of this change.

Thanks,

-Stolee