Re: Restoring detached HEADs after Git operations

From: Junio C Hamano <gitster@pobox.com>
Date: Jun 19, 2017, 17:55 UTC
Message-ID: <xmqqefufuakk.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <CAGZ79kY0gwk7KRY2iAVTXPBjPzx+mkciVWRR2z2cDgiBjQ2uuw@mail.gmail.com>

Stefan Beller <sbeller@google.com> writes:

Show 24 quoted lines

> On Mon, Jun 19, 2017 at 2:52 AM, Patrick Lehmann
> <Patrick.Lehmann@plc2.de> wrote:
>> 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
>
> For remote branches you would create a local branch of the same name
> (if such a branch would not exist, possibly setting it up to track that remote
> branch)?
>
>> - tags
>
> as said in the other email and similar to remote branches, we'd not want to have
> HEAD pointing to them directly but somehow have a local branch.

Let's step back a bit. We detach the HEAD for a good reason, no? Why is it a good idea to move them back on to a branch picked among multiple ones that all happen to be pointing at the same commit?

The user may build on a history of a submodule, and then may push the result out to a particular branch at the other side; that is when being on a named branch in the submodule becomes useful, but even then I do not think randomly picking one branch and be on it is a good thing to do.

I would understand the workflow would go more like so:

 - You do something at the superproject (e.g. create a new branch X
   from an existing commit and check it out), which results in
   submodules' HEADs getting detached at the commits bound to the
   superproject's tree.

 - Because you want to make changes to both submodules and the
   superproject in a consistent way, you'd want to commit changes to
   all of these repositories and the push the result out in an
   atomic way.

 - Hence you tell "Hey, Git, I want all the submodules that I
   modified to be on branch X" from the superproject.

   - This may succeed in a submodule where X is a new name, or the
     current tip of branch X is an ancestor of the detached HEAD.

   - This may fail in a submodule where there is branch X that does
     not want to move to the detached HEAD's state.  In this latter
     case, the user needs to deal with the situation (perhaps the
     old X is expendable; perhaps the HEAD's commit may need to be
     merged to old X; perhaps there are other cases).

though.