Re: [PATCH 7/7] xdiff: make diff3 the default conflictStyle

From: Elijah Newren <newren@gmail.com>
To: Felipe Contreras <felipe.contreras@gmail.com>
Cc: "Sergey Organov" <sorganov@gmail.com>,
	"Junio C Hamano" <gitster@pobox.com>,
	"Johannes Sixt" <j6t@kdbg.org>, "Jeff King" <peff@peff.net>,
	"David Aguilar" <davvid@gmail.com>,
	"Bagas Sanjaya" <bagasdotme@gmail.com>,
	"Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
	"Denton Liu" <liu.denton@gmail.com>,
	"Git Mailing List" <git@vger.kernel.org>,
	"Elijah Newren" <newren@gmail.com>
Subject: Re: [PATCH 7/7] xdiff: make diff3 the default conflictStyle
Date: Fri, 11 Jun 2021 12:02:35 -0700	[thread overview]
Message-ID: <20210611190235.1970106-1-newren@gmail.com> (raw)
In-Reply-To: <60c3a41bd25e3_8d0f2089e@natae.notmuch>


On Fri, Jun 11, 2021 at 10:57 AM Felipe Contreras <felipe.contreras@gmail.com> wrote:

> Elijah Newren wrote:
> > On Fri, Jun 11, 2021 at 8:32 AM Felipe Contreras <felipe.contreras@gmail.com>
> > wrote:
...
> > The alternative to the above two options was the
> > make-a-virtual-merge-base-by-merging-merge-bases strategy.  It apparently
> > was very successful.
> 
> OK. That makes sense.
> 
> > But it does mean that merge bases can have conflict markers in them.
> 
> But why? And even if they do, why do they have to be diff3 conflict
> markers?

This could be changed; I suspect it just was a natural consequence of how
the code was built.  (Recursive means there's not a separate code-path for
merging the merge-bases, so they get the same merge style by default.)

> This would be more human-friendly:
> 
>   <<<<<<< HEAD
>   D
>   ||||||| merged common ancestors
>   <<<<<<<<< Temporary merge branch 1
>   B
>   =========
>   A
>   >>>>>>>>> Temporary merge branch 2
>   =======
>   C
>   >>>>>>> C

I suspect that would be as easy as this (not compiled or tested):

diff --git a/ll-merge.c b/ll-merge.c
index 095a4d820e..bdd129cbd6 100644
--- a/ll-merge.c
+++ b/ll-merge.c
@@ -131,7 +131,7 @@ static int ll_xdl_merge(const struct ll_merge_driver *drv_unused,
 	xmp.level = XDL_MERGE_ZEALOUS;
 	xmp.favor = opts->variant;
 	xmp.xpp.flags = opts->xdl_opts;
-	if (git_xmerge_style >= 0)
+	if (git_xmerge_style >= 0 && !opts->virtual_ancestor)
 		xmp.style = git_xmerge_style;
 	if (marker_size > 0)
 		xmp.marker_size = marker_size;


> Or just put a stub conflict marker:
> 
>   <<<<<<< HEAD
>   D
>   ||||||| merged common ancestors
>   <<<<<<<<< Temporary merge >>>>>>>>>
>   =======
>   C
>   >>>>>>> C

I don't know what would be involved to do this one; I think it wouldn't
be too hard, but I don't think we'd want to pursue this option.

> Or just use the base of the virtual merge:
> 
>   <<<<<<< HEAD
>   D
>   ||||||| merged common ancestors
>   1
>   =======
>   C
>   >>>>>>> C

I think that implementing this choice would look like this (again, not
compiled or tested and I'm not familiar with xdiff so take it with a
big grain of salt):


diff --git a/ll-merge.c b/ll-merge.c
index 095a4d820e..dbc7f76951 100644
--- a/ll-merge.c
+++ b/ll-merge.c
@@ -130,6 +130,8 @@ static int ll_xdl_merge(const struct ll_merge_driver *drv_unused,
 	memset(&xmp, 0, sizeof(xmp));
 	xmp.level = XDL_MERGE_ZEALOUS;
 	xmp.favor = opts->variant;
+	if (git_xmerge_style >= 0 && opts->virtual_ancestor)
+		xmp.favor = XDL_MERGE_FAVOR_BASE;
 	xmp.xpp.flags = opts->xdl_opts;
 	if (git_xmerge_style >= 0)
 		xmp.style = git_xmerge_style;
diff --git a/xdiff/xdiff.h b/xdiff/xdiff.h
index 8629ae287c..b8d1a536c2 100644
--- a/xdiff/xdiff.h
+++ b/xdiff/xdiff.h
@@ -62,6 +62,7 @@ extern "C" {
 #define XDL_MERGE_FAVOR_OURS 1
 #define XDL_MERGE_FAVOR_THEIRS 2
 #define XDL_MERGE_FAVOR_UNION 3
+#define XDL_MERGE_FAVOR_BASE 4
 
 /* merge output styles */
 #define XDL_MERGE_DIFF3 1
diff --git a/xdiff/xmerge.c b/xdiff/xmerge.c
index 95871a0b6e..a8dc42595a 100644
--- a/xdiff/xmerge.c
+++ b/xdiff/xmerge.c
@@ -313,6 +313,9 @@ static int xdl_fill_merge_buffer(xdfenv_t *xe1, const char *name1,
 			if (m->mode & 2)
 				size += xdl_recs_copy(xe2, m->i2, m->chg2, 0, 0,
 						      dest ? dest + size : NULL);
+		} else if (m->mode == 4) {
+			size += xdl_orig_copy(xe1, m->i0, m->chg0, needs_cr, 0,
+					      dest ? dest + size : NULL);
 		} else
 			continue;
 		i = m->i1 + m->chg1;

> We don't have to use diff3 all the way.

Right, thus my mention in the other email to consider adding a
XDL_MERGE_FAVOR_BASE -- which you then also mention here in your third
option, and which I've now given at least a partial patch for.  Not
sure if it's a crazy idea or a great idea, since I don't do very many
criss-cross merges myself.



Hope that helps,
Elijah
