Re: [PATCH v2 6/7] subtree: more robustly distinguish subtree and mainline commits

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

Hi,

On Tue, 6 Oct 2020, Tom Clarkson via GitGitGadget wrote:

> From: Tom Clarkson <tom@tqclarkson.com>
>
> Prevent a mainline commit without $dir being treated as a subtree
> commit and pulling in the entire mainline history. Any valid subtree
> commit will have only valid subtree commits as parents, which will be
> unchanged by check_parents.

I feel like this is only half the picture because I have a hard time
stitching these two sentences together.

After studying the code and your patch a bit, it appears to me that
`process_split_commit()` calls `check_parents()` first, which will call
`process_split_commit()` for all as yet unmapped parents. So basically, it
recurses until it found a commit all of whose parents are already mapped,
then permeates that information all the way back.

Doesn't this cause serious issues with stack overflows and all for long
commit histories?

> Signed-off-by: Tom Clarkson <tom@tqclarkson.com>
> ---
>  contrib/subtree/git-subtree.sh | 24 +++++++++++-------------
>  1 file changed, 11 insertions(+), 13 deletions(-)
>
> diff --git a/contrib/subtree/git-subtree.sh b/contrib/subtree/git-subtree.sh
> index e56621a986..fa6293b372 100755
> --- a/contrib/subtree/git-subtree.sh
> +++ b/contrib/subtree/git-subtree.sh
> @@ -224,8 +224,6 @@ cache_setup () {
>  	fi
>  	mkdir -p "$cachedir" ||
>  		die "Can't create new cachedir: $cachedir"
> -	mkdir -p "$cachedir/notree" ||
> -		die "Can't create new cachedir: $cachedir/notree"

It might make sense to talk about this a bit in the commit message.
Essentially, you are replacing the `notree/<rev>` files by mapping `<rev>`
to the empty string.

This makes me wonder, again, whether the file system layout of the cache
can hold up to the demands. If a main project were to merge a subtree
with, say, 10 million commits, wouldn't that mean that `git subtree` would
now fill one directory with 10 million files? I cannot imagine that this
performs well, still.

>  	debug "Using cachedir: $cachedir" >&2
>  }
>
> @@ -255,18 +253,11 @@ check_parents () {
>  	local indent=$(($2 + 1))
>  	for miss in $missed
>  	do
> -		if ! test -r "$cachedir/notree/$miss"
> -		then
> -			debug "  unprocessed parent commit: $miss ($indent)"
> -			process_split_commit "$miss" "" "$indent"
> -		fi
> +		debug "  unprocessed parent commit: $miss ($indent)"
> +		process_split_commit "$miss" "" "$indent"

That makes sense to me, as the `missed` variable only contains as yet
unmapped commits, therefore we do not have to have an equivalent `test -r`
check.

Ciao,
Dscho

>  	done
>  }
>
> -set_notree () {
> -	echo "1" > "$cachedir/notree/$1"
> -}
> -
>  cache_set () {
>  	oldrev="$1"
>  	newrev="$2"
> @@ -719,11 +710,18 @@ process_split_commit () {
>  	# vs. a mainline commit?  Does it matter?
>  	if test -z "$tree"
>  	then
> -		set_notree "$rev"
>  		if test -n "$newparents"
>  		then
> -			cache_set "$rev" "$rev"
> +			if test "$newparents" = "$parents"
> +			then
> +				# if all parents were subtrees, this can be a subtree commit
> +				cache_set "$rev" "$rev"
> +			else
> +				# a mainline commit with tree missing is equivalent to the initial commit
> +				cache_set "$rev" ""
> +			fi
>  		else
> +			# no parents with valid subtree mappings means a commit prior to subtree add
>  			cache_set "$rev" ""
>  		fi
>  		return
> --
> gitgitgadget
>
>