AW: Restoring detached HEADs after Git operations

[Patrick Lehmann <Patrick.Lehmann@plc2.de>]

Hello Lars,

for your questions:
> If there are multiple branches with the same hash then your script would pick the first one. Can you imagine a situation where this would be a problem?

I can't think of a good solution to resolve it automatically. Maybe a script could print that there are multiple possibilities and it choose the first branch in the list.


> Plus, you are looking only at local branches. Wouldn't it make sense to look at remote branches, too?

This is also related to restoring tags. If we go this way, we should have this priority list:
- local branches
- remote branches
- tags


> Submodule processing is already quite slow if you have many of them. I wonder how much this approach would affect the performance.

Yes. It takes a few seconds to iterate all the submodules. It could be improved if the processing wouldn't be based on slow Bash scripts spawning lot's of sub-shells to execute multiple Git commands.



Is there a way to avoid detached DEADs at the beginning?
Many submodules are attached to a reference and get detached to a hash of the same reference. It would be better, if they never get detached when the current and new hash are the same.


Kind regards
    Patrick

________________________________________
Von: git-owner@vger.kernel.org [git-owner@vger.kernel.org]" im Auftrag von "Lars Schneider [larsxschneider@gmail.com]
Gesendet: Montag, 19. Juni 2017 11:30
Bis: Patrick Lehmann
Cc: Git Mailinglist; Stefan Beller
Betreff: Re: Restoring detached HEADs after Git operations

> On 19 Jun 2017, at 10:46, Patrick Lehmann <Patrick.Lehmann@plc2.de> wrote:
>
> Hello,
>
> I wrote a Bash script to recover branch names after Git operations have create detached HEADs in a Git repository containing lots of Git submodules. The script works recursively.

I did run into this situation myself and therefore
I understand your motivation. I've CC'ed Stefan as
he is a Submodule expert!


> I would like to see:
> a) that script or algorithm being integrated into Git by default
> b) that as a default behavior for all Git operations creating detached HEADs
>
> That's the command:
> --------------------------------
> git submodule foreach --recursive  'HEAD=$(git branch --list | head -n 1); if [[ "$HEAD" == *HEAD* ]]; then REF=$(git rev-parse HEAD); FOUND=0; for Branch in $(git branch --list | grep "^  " | sed -e "s/  //" ); do if [[ "$(git rev-parse "$Branch")" == $REF ]]; then echo -e "  \e[36mCheckout $Branch...\e[0m"; git checkout $Branch; FOUND=1; break; fi done; if [[ $FOUND -eq 0 ]]; then echo -e "  \e[31mNo matching branch found.\e[0m"; fi else echo -e "  \e[36mNothing to do.\e[0m"; fi'
> --------------------------------
>
> How does it work:
> 1. It uses git submodule foreach to dive into each Git submodule and execute a series of Bash commands.
> 2. It's reading the list of branches and checks if the submodule is in detached mode. The first line contains the string HEAD.
> 3. Retrieve the hash of the detached HEAD
> 4. Iterate all local branches and get their hashes
> 5. Compare the branch hashes with the detached HEAD's hash. If it matches do a checkout.

If there are multiple branches with the same hash then
your script would pick the first one. Can you imagine a
situation where this would be a problem?

Plus, you are looking only at local branches. Wouldn't it
make sense to look at remote branches, too?


> 6. Report if no branch name was found or if a HEAD was not in detached mode.
>
> The Bash code with line breaks and indentation:
> --------------------------------
> HEAD=$(git branch --list | head -n 1)
> if [[ "$HEAD" == *HEAD* ]]; then
>  REF=$(git rev-parse HEAD)
>  FOUND=0
>  for Branch in $(git branch --list | grep "^  " | sed -e "s/  //" ); do

There is a convenient "git for-each-ref" function to iterate over
branches in scripts. See here an example:
https://github.com/larsxschneider/scotty/blob/master/admin/oss-fork.sh#L88


>    if [[ "$(git rev-parse "$Branch")" == $REF ]]; then
>      echo -e "  \e[36mCheckout $Branch...\e[0m"
>      git checkout $Branch
>      FOUND=1
>      break
>    fi
>  done
>  if [[ $FOUND -eq 0 ]]; then
>    echo -e "  \e[31mNo matching branch found.\e[0m"
>  fi
> else
>  echo -e "  \e[36mNothing to do.\e[0m"
> fi
> --------------------------------
>
> Are their any chances to get it integrated into Git?
>
> I tried to register that code as a Git alias, but git config complains about quote problem not showing where. It neither specifies if it's a single or double quote problem. Any advice on how to register that piece of code as an alias?

Try to escape ". See here for an example:
https://github.com/Autodesk/enterprise-config-for-git/blob/master/config.include#L76-L94


> If wished, I think I could expand the script to also recover hash values to Git tags if no branch was found.

It would be indeed nice to see the tagged version on my prompt.

--

Submodule processing is already quite slow if you have many of them.
I wonder how much this approach would affect the performance.

- Lars