Re: Question about .git/objects/info/alternates

From: Junio C Hamano <gitster@pobox.com>
Date: Mar 24, 2010, 19:58 UTC
Message-ID: <7vljdhh4po.fsf@alter.siamese.dyndns.org>
In-Reply-To: <a038bef51003241153g33445607qb3ab750e08b0584@mail.gmail.com>

Chris Packham <judge.packham@gmail.com> writes:

Show 6 quoted lines

> git repack -a did the correct thing.
>
> It occurs to me that the UI around alternates is a bit lacking i.e.
> there isn't a git command to display the alternates in use or to add
> them to an existing repository (or at least I couldn't find one
> skimming the docs or googling).

That's an understatement.  Thanks for starting the effort.

I will likely to have quite a few style issues with the script implementation, and am undecided if this should be a new command or an option to some existing command, but it's time we have a management facility for alternates.

Show 12 quoted lines

> diff --git a/Makefile b/Makefile
> index 3a6c6ea..1a7b084 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -334,6 +334,7 @@ TEST_PROGRAMS_NEED_X =
>  unexport CDPATH
>
>  SCRIPT_SH += git-am.sh
> +SCRIPT_SH += git-alternates.sh
>  SCRIPT_SH += git-bisect.sh
>  SCRIPT_SH += git-difftool--helper.sh
>  SCRIPT_SH += git-filter-branch.sh

You probably need one entry in the command-list.txt to classify where in the main manual page git(1) for this command to appear. I would suggest imitating "git config".

Show 17 quoted lines

> diff --git a/git-alternates.sh b/git-alternates.sh
> new file mode 100755
> index 0000000..74ec707
> --- /dev/null
> +++ b/git-alternates.sh
> @@ -0,0 +1,159 @@
> +#!/bin/sh
> ...
> +#
> +# Runs through the alternates file calling the callback function $1
> +# with the name of the alternate as the first argument to the callback
> +# any additional arguments are passed to the callback function.
> +#
> +walk_alternates()
> +{
> +    local alternates=$GIT_DIR/objects/info/alternates
> +    local callback=$1

We want to use this on platforms with ksh and dash, so please refrain from bash-ism features. "local" does not mix well with "#!/bin/sh".

> +    shift
> +
> +    if [ -e $alternates ]; then

We tend to avoid '[' and write it like this:

	if test -f "$alternates"
        then
        	...

Also notice that an indent is one tab and one tabstop is 8 places, and alternates need to be quoted in case $GIT_DIR has IFS whitespace in it.

> +        while read line
> +        do
> +            $callback $line $*

As the path to an alternate object store can contain IFS whitespace, line needs to be quoted. Also you call walk_alternates with things like "$dir" that could also be a path with IFS whitespace, it needs to be quoted properly. I suspect callback is only your own shell function, so it does not have to be quoted, but it is Ok to quote it for consistency. I.e.

	"$callback" "$line" "$@"

You are loooooose in quoting throughout your script, so I won't bother to point all of them out in the rest of the message. You also are loose in checking error returns (what you failed to write $newalternates file, for example), which needs to be fixed before the final version.

> +        done < $alternates

This is correct per POSIX even if $alternates itself has IFS whitespace in it, but newer bash on some platforms have an obnoxious "safety feature" (see 3fa7c3d work around an obnoxious bash "safety feature" on OpenBSD, 2010-01-26) that can cause it to barf on this. It is unfortunate but we need to quote it like this:

	done <"$alternates"

Also notice that there is no SP after < or > redirection (it is just a style thing).

Show 12 quoted lines

> +# Walk function to display one alternate object store and, if the user
> +# has specified -r, recursively call show_alternates on the git
> +# repository that the object store belongs to.
> +#
> +show_alternates_walk()
> +{
> +    say "Object store $1"
> +    say "    referenced via $GIT_DIR"
> +
> +    local new_git_dir=${line%%/objects}
> +    if [ "$recursive" == "true" -a "$GIT_DIR" != "$new_git_dir" ]
> +    then

	if test "$recursive" = true && test "$GIT_DIR" != "$new_git_dir"
        then
        	...

> +        GIT_DIR=$new_git_dir show_alternates

This is probably depending on a bug in bash and is not portable. See 76c9c0d (rebase -i: Export GIT_AUTHOR_* variables explicitly, 2010-01-22)

Show 5 quoted lines

> +add_alternate()
> +{
> +    if test ! -d $dir; then
> +        die "fatal: $dir is not a directory"
> +    fi

This will not work with relative alternates. In two repositories A and B, where B borrows from A:

	A/.git/objects/
        B/.git/objects/info/alternates

the alternates file in B can point at A's .git/objects, relative to its own .git/objects/.

Show 9 quoted lines

> +    walk_alternates check_current_alternate_walk $dir
> +
> +    # At this point we know that $dir is a directory that exists
> +    # and that its not already being used as an alternate. We could
> +    # go further and verify that $dir has valid objects.
> +
> +    # if we're still going we can safely add the alternate
> +    touch $GIT_DIR/objects/info/alternates
> +    echo "$(readlink -f $dir)" >> $GIT_DIR/objects/info/alternates

What is this touch for?

Is readlink(1) portable enough across platforms? A more fundamental question is if resolving symbolic link like this behind user is a sensible thing to do, especially as you are ...

> +    say "$dir added as an alternate"

... lying to the user here, if $dir indeed is a symbolic link.

Show 6 quoted lines

> +rewrite_alternates()
> +{
> +    if test "$1" != "$2"; then
> +        echo $2 >> $3
> +    fi
> +}

That's a misleading name for this helper function.

Show 15 quoted lines

> +del_alternate()
> +{
> +    if test ! $force = "true"; then
> +        say "Not forced, use"
> +        say "   'git repack -a' to fetch missing objects, then "
> +        say "   '$dashless -f -d $dir' to remove the alternate"
> +        die
> +    fi
> +
> +    local alternates=$GIT_DIR/objects/info/alternates
> +
> +    new_alts_file=$(mktemp $alternates-XXXXXX)
> +    touch $new_alts_file
> +
> +    walk_alternates rewrite_alternates $dir $new_alts_file

I think this is a more expensive way to say:

	grep -v -F "$dir" <"$alternates" >"$new_alternates"

Show 6 quoted lines

> +    mv $new_alts_file $alternates
> +
> +    # save the git from repeatedly reading a 0 length file
> +    if test $(stat -c "%s" $alternates) -eq 0; then
> +        rm $alternates
> +    fi

Good point, but it would be better to do something like this:

	grep -v -F "$dir" <"$alternates" >"$new_alternates"
	if test -s "$new_alternates"
        then
        	mv "$new_alternates" "$alternates"
	else
        	rm -f "$alternates"
	fi

without making it less portable by using "stat".