On Thu, Jan 16, 2014 at 12:21:04PM -0800, Junio C Hamano wrote:
> "W. Trevor King" <wk...@tremily.us> writes:
> > @@ -155,13 +155,31 @@ it contains local modifications.
> >  
> >  update::
> >     Update the registered submodules, i.e. clone missing submodules and
> > -   checkout the commit specified in the index of the containing repository.
> > -   This will make the submodules HEAD be detached unless `--rebase` or
> > -   `--merge` is specified or the key `submodule.$name.update` is set to
> > -   `rebase`, `merge` or `none`. `none` can be overridden by specifying
> > -   `--checkout`. Setting the key `submodule.$name.update` to `!command`
> > -   will cause `command` to be run. `command` can be any arbitrary shell
> > -   command that takes a single argument, namely the sha1 to update to.
> > +   checkout the commit specified in the index of the containing
> > +   repository.  The update mode defaults to 'checkout', but be
> > +   configured with the 'submodule.<name>.update' setting or the
> > +   '--rebase', '--merge', or 'checkout' options.
> Not '--checkout'?

Oops, will fix in v5.

> > ++
> > +For updates that clone missing submodules, checkout-mode updates will
> > +create submodules with detached HEADs; all other modes will create
> > +submodules with a local branch named after 'submodule.<path>.branch'.
> > ++
> > +For updates that do not clone missing submodules, the submodule's HEAD
> That is, updates that update submodules that are already checked out?

It's submodules for which $sm_path/.git does not exist.

> > +is only touched when the remote reference does not match the
> > +submodule's HEAD (for none-mode updates, the submodule is never
> > +touched).  The remote reference is usually the gitlinked commit from
> > +the superproject's tree, but with '--remote' it is the upstream
> > +subproject's 'submodule.<name>.branch'.  This remote reference is
> > +integrated with the submodule's HEAD using the specified update mode.
> I think copying some motivation from the log message of 06b1abb5
> (submodule update: add --remote for submodule's upstream changes,
> 2012-12-19) would help the readers here.

I think a brief reference to --remote is best here, mentioning that it
has something to do with the upstream subproject.  More detail on when
you should use --remote should probably go under the docs for --remote

> A naïve expectation from a casual reader of the above would be "The
> superproject's tree ought to point at the same commit as the tip of
> the branch used in the submodule (modulo mirroring delays and
> somesuch),

What is the branch used in the submodule?  The remote subproject's
current submodule.<name>.branch?  The local submodule's
submodule.<name>.branch (or localBranch) branch?  The submodule's
current HEAD?

> if the repository of the superproject and submodules are maintained
> properly", which would lead to "when would any sane person need to
> use --remote in the first place???".

--remote is not for sane people (who will probably be pulling from
withing the submodule itself).  --remote is for folks who track too
many submodules to care about them as individuals, who just want to
blindly update to whatever the upstream subproject maintainer has in
submodule.<name>.branch.  For example, if you are a distribution with
a hundred submodules tracking all the projects you package, and want
to bump them all to a their current trunk tip in one fell swoop.

> If I am reading 06b1abb5 correctly, the primary motivation behind
> "--remote" seems to be that it is exactly to help the person who
> wants to update superproject to satisify the "... are maintained
> properly" part by fetching the latest in each of the submodules in
> his superproject in preparation to 'git add .' them.  I still do not
> think "--remote" was a better way than the "foreach", but that is a
> separate topic.

I agree now ;), the problems with:

  $ git submodule foreach 'git pull'


1. You may not be on the “right” local branch to begin with, and
2. You may not have out-of-tree submodule configs setting up the
   “right” upstream for that branch.

06b1abb did not address the former, and added a new .gitmodules-level
submodule.<name>.branch to help with the latter.  I now think all of
this would be easier if we had automatic submodule local-branch
checkouts (fixing problem 1) and synchronized out-of-tree submodule
configs (fixing problem 2).  Fixing problem 1 is all you need if you
aren't interested in sharing your out-of-tree configs.

I think 06b1abb was inspired by “we already have a way to integrate
changes in the gitlinked commit, we should reuse those to integrate
other changes”.  In hindsight, changes in the gitlinked commit are
really a checkout-time issue, while changes in other places (like the
remote subproject) are pull-time issues.  Mixing them together was
more confusing than it was worth.


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