Re: suboptimal behavior of fast-import in some cases with "from"

[Mike Hommey <mh@glandium.org>]

On Mon, Jul 06, 2015 at 03:54:35PM -0700, Junio C Hamano wrote:
> Mike Hommey <mh@glandium.org> writes:
> 
> > One of the first things parse_from does is unconditionally throw away
> > the tree for the given branch, and then the "from" tree is loaded. So
> > when the "from" commit is the current head of the branch, that make
> > fast-import do more work than necessary.
> 
> If it is very common that the next commit the input stream wants to
> create is often on top of the commit that was just created, and if
> it is very common that the input stream producer knows what the
> commit object name of the commit that was just created, then
> optimising for that case does not sound too bad.  It really depends
> on two factors:
> 
>  - How likely is it that other people make the same mistake?

Looks like the answer is: very. Assuming my quick glance at the code is
not mistaken, the same mistake is made in at least git-p4.py (yes, the
one that comes with git), felipec's git-remote-hg, and hg-fast-export,
and that's 100% of the sample I looked at.

I won't claim to know what fast-import is doing, not having looked at
more than the parse_from* functions and the commit message for 4cabf858,
but it seems plausible this also skips making tree deltas for those
trees.

>  - How bad the damage to parse_from() would be if we wanted to
>    optimize for this case?

I /think/ it would look like this (untested), which doesn't seem too
damaging:

diff --git a/fast-import.c b/fast-import.c
index e78ca10..cb232e0 100644
--- a/fast-import.c
+++ b/fast-import.c
@@ -2588,14 +2588,12 @@ static int parse_from(struct branch *b)
 {
        const char *from;
        struct branch *s;
+       unsigned char sha1[20];
 
        if (!skip_prefix(command_buf.buf, "from ", &from))
                return 0;
 
-       if (b->branch_tree.tree) {
-               release_tree_content_recursive(b->branch_tree.tree);
-               b->branch_tree.tree = NULL;
-       }
+       hashcpy(sha1, b->branch_tree.versions[1].sha1);
 
        s = lookup_branch(from);
        if (b == s)
@@ -2626,6 +2624,11 @@ static int parse_from(struct branch *b)
        else
                die("Invalid ref name or SHA1 expression: %s", from);
 
+       if (b->branch_tree.tree && hashcmp(sha1, b->branch_tree.versions[1].sha1)) {
+               release_tree_content_recursive(b->branch_tree.tree);
+               b->branch_tree.tree = NULL;
+       }
+
        read_next_command();
        return 1;
 }

Mike