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

Chris Packham [off-list ref] writes:

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.

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".

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).

+# 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)

+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/.

+    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.

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

That's a misleading name for this helper function.

+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"

+    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".