Re: My Git Dev Blog - Week 9

From: Atharva Raykar <raykar.ath@gmail.com>
Date: Jul 19, 2021, 08:48 UTC
Message-ID: <6A97A724-4B80-4DDC-B5D2-83F8AD0B27EF@gmail.com>
In-Reply-To: <a8122438-8d8b-9e7d-d66a-dc6eff7a0817@gmail.com>

On 19-Jul-2021, at 08:28, Kaartic Sivaraam <kaartic.sivaraam@gmail.com> wrote:

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> 
> Hi Atharva and all,
> 
> On 19/07/21 12:49 am, Kaartic Sivaraam wrote:
>> On 18/07/21 5:29 pm, Atharva Raykar wrote:
>>> Passing the superprefix explicitly
>> My gut instinct tells me we could get away without having to go this way but
>> I haven't yet been able to figure out how. How 'prefix', 'super-prefix' and
>> 'recursive-prefix' is still puzzling me. In case anyone has knowledge about
>> this please chime in and enlighten us. It would be very helpful.
>> 
> 
> I just noticed I was short and unhelpful here. To expand on what information would be
> helpful on this respect, here's a snippet of a private e-mail I sent which I've
> tweaked a little to make it relevant for the list audience:
> 
> -- 8< --
>>> ...
>>> 
> 
> I took a look at this. I'm not able to find the exact issue but my gut instinct tells me
> it has something to do with how prefix, super-prefix and recursive-prefix
> are handled before and after the conversion. To be more specific, I'm having doubts
> about whether the following code snippet[4] in shell ...
> 
>    prefix=$(git submodule--helper relative-path "$prefix$sm_path/" "$wt_prefix")
>    wt_prefix=
>    sanitize_submodule_env
>    cd "$sm_path" &&
>    eval cmd_update
> 
> ... has been properly converted to C. Particularly, the fact that recursive calls to
> 'cmd_update' is made in the existing shell implementation in case '--recusive' is
> passed needs to be observed well. This would subsequently result in multiple calls to
> 'cmd_init' with different 'prefix' and 'wt_prefix' values. So, that needs to be
> observed well too.

The whole deal with prefixes confused me initially a lot as well, so I'll try to explain what's going on there first, and then show why I had trouble with it.

The terminology of 'recursive-prefix' was something I borrowed from the update-clone helper[5]. Just to give you a summary of which shell variable maps to which C variable, along with my understanding of what they represent:

| shell      | C                | description                               |
|------------+------------------+-------------------------------------------|
| $prefix    | recursive_prefix | this represents the superproject prefix,  |
|            |                  | ie, the path of the superproject relative |
|            |                  | to the current subproject.                |
|            |                  |                                           |
| $wt_prefix | prefix           | this is the path into the working tree,   |
|            |                  | ie, the path of the directory from the    |
|            |                  | root of the git directory's worktree      |

The display paths that are generated for the error messages require one of these to be non-NULL, but not both[6].

I feel confident the values of the various prefixes are handled properly during the recursion because I setup a new worktree on based master as my "control", and a combination of running with GIT_TRACE2 and printing the values showed me that both of them handle the prefixes identically.

The issue with the subprocess approach[7] seems to be not with the prefixes, but rather with the configuration not being properly read when it is set in the same command invocation.

[5] https://github.com/git/git/blob/75ae10bc75336db031ee58d13c5037b929235912/git-submodule.sh#L536-L550 [6] https://github.com/git/git/blob/75ae10bc75336db031ee58d13c5037b929235912/builtin/submodule--helper.c#L258-L260 [7] https://github.com/tfidfwastaken/git/commits/submodule-helper-update-1a

Show 19 quoted lines

> I have not been able to get a complete picture of this myself. I'll try to take better
> look later and see if I could get anything.
> 
> Note that this could also be me just not being able to get a proper picture of it.
> Just wanted to share it in case others are able to get any ideas or able to think of a
> possible cause.
> 
> [4]: https://github.com/git/git/blob/abb21c7263616f01c5e950861a29279ab21cb02f/git-submodule.sh#L654-L658
> -- >8 --
> 
> I hope that gives a better idea about what kind of information would be helpful.
> Kindly let me know if it doesn't.
> 
> Atharva,
> 
> In the meanwhile, I think you could continue polishing your attempted solution
> and preparing to send it to the list. I'll try to dig more on my gut instinct
> and let you know if I get anything. If you have some free time left after
> polishing your series, you could try digging into it too :)

Sure. Thanks for the time and effort you are putting into this!

> -- 
> Sivaraam