Date:   Wed, 14 Feb 2018 10:12:44 -0800
From:   Junio C Hamano <gitster@...ox.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Mauro Carvalho Chehab <mchehab@....samsung.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Linux-Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Git Mailing List <git@...r.kernel.org>
Subject: Re: linux-next: unnecessary merge in the v4l-dvb tree

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> On Tue, Feb 13, 2018 at 9:18 AM, Junio C Hamano <gitster@...ox.com> wrote:
>>
>> That makes me wonder if another heuristic I floated earlier is more
>> appropriate.  When merging a tag object T, if refs/tags/T exists and
>> it is that tag object, then an updated "merge" would default to "--ff";
>> otherwise, it would keep the current default of creating a merge even
>> when we could fast-forward, in order to record that tag T in the
>> resulting history.
>
> Oooh. Yes, that sounds like the right thing to do.
>
> So the "no fast-forward" logic would trigger only if the name we used
> for merging is one of the temporary ones (ie .git/{FETCH,MERGE}_HEAD),
> not if the mentioned tag is already a normal tag reference.
>
> Then it's very explicitly about "don't lose the signing information".
>
> I'd still have to teach people to use "--only-ff" if they don't do the
> "fetch and merge" model but literally just do  "git pull upstream
> vX.Y", but at least the case Mauro describes would automatically just
> DTRT.
>
> Me likey.

The implementation cannot exactly be "did the user give FETCH_HEAD
or v4.16-rc1 from the command line?", because we'd want to catch it
when Mauro says "git fetch linus && git merge v4.16-rc1" and behave
identically as "git pull linus v4.16-rc1" (and the latter internally
gets turned into "git merge FETCH_HEAD").

So, instead, we read the "tag" line from the tag object to learn the
tagname T, see if refs/tags/T exists and points at that object, to
see if we are Mauro who follows your tags, or if we are you who
fetch and merge contributors' "for-linus" signed tag (which I am
assuming you won't contaminate your refs/tags/ hierarchy with).

There are a few fallouts in the testsuite if we go this route.  I am
not quite decided if I like the approach.

 builtin/merge.c          | 42 ++++++++++++++++++++++++++++++++++++++----
 t/t6200-fmt-merge-msg.sh |  2 +-
 t/t7600-merge.sh         |  2 +-
 3 files changed, 40 insertions(+), 6 deletions(-)

diff --git a/builtin/merge.c b/builtin/merge.c
index 30264cfd7c..45c7916505 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -33,6 +33,7 @@
 #include "sequencer.h"
 #include "string-list.h"
 #include "packfile.h"
+#include "tag.h"
 
 #define DEFAULT_TWOHEAD (1<<0)
 #define DEFAULT_OCTOPUS (1<<1)
@@ -1125,6 +1126,42 @@ static struct commit_list *collect_parents(struct commit *head_commit,
 	return remoteheads;
 }
 
+static int merging_a_throwaway_tag(struct commit *commit)
+{
+	const char *tag_ref;
+	struct object_id oid;
+
+	/* Are we merging a tag? */
+	if (!merge_remote_util(commit) ||
+	    !merge_remote_util(commit)->obj ||
+	    merge_remote_util(commit)->obj->type != OBJ_TAG)
+		return 0;
+
+	/*
+	 * Now we know we are merging a tag object.  Are we downstream
+	 * and following the tags from upstream?  If so, we must have
+	 * the tag object pointed at by "refs/tags/$T" where $T is the
+	 * tagname recorded in the tag object.  We want to allow such
+	 * a "just to catch up" merge to fast-forward.
+	 */
+	tag_ref = xstrfmt("refs/tags/%s",
+			  ((struct tag *)merge_remote_util(commit)->obj)->tag);
+
+	if (!read_ref(tag_ref, &oid) &&
+	    !oidcmp(&oid, &merge_remote_util(commit)->obj->oid))
+		return 0;
+
+	/*
+	 * Otherwise, we are playing an integrator's role, making a
+	 * merge with a throw-away tag from a contributor with
+	 * something like "git pull $contributor $signed_tag".
+	 * We want to forbid such a merge from fast-forwarding
+	 * by default; otherwise we would not keep the signature
+	 * anywhere.
+	 */
+	return 1;
+}
+
 int cmd_merge(int argc, const char **argv, const char *prefix)
 {
 	struct object_id result_tree, stash, head_oid;
@@ -1322,10 +1359,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 			    oid_to_hex(&commit->object.oid));
 		setenv(buf.buf, merge_remote_util(commit)->name, 1);
 		strbuf_reset(&buf);
-		if (fast_forward != FF_ONLY &&
-		    merge_remote_util(commit) &&
-		    merge_remote_util(commit)->obj &&
-		    merge_remote_util(commit)->obj->type == OBJ_TAG)
+		if (fast_forward != FF_ONLY && merging_a_throwaway_tag(commit))
 			fast_forward = FF_NO;
 	}
 
diff --git a/t/t6200-fmt-merge-msg.sh b/t/t6200-fmt-merge-msg.sh
index 2e2fb0e957..a54a52aaa4 100755
--- a/t/t6200-fmt-merge-msg.sh
+++ b/t/t6200-fmt-merge-msg.sh
@@ -512,7 +512,7 @@ test_expect_success 'merge-msg with "merging" an annotated tag' '
 
 	test_when_finished "git reset --hard" &&
 	annote=$(git rev-parse annote) &&
-	git merge --no-commit $annote &&
+	git merge --no-commit --no-ff $annote &&
 	{
 		cat <<-EOF
 		Merge tag '\''$annote'\''
diff --git a/t/t7600-merge.sh b/t/t7600-merge.sh
index dfde6a675a..28a1c43ca7 100755
--- a/t/t7600-merge.sh
+++ b/t/t7600-merge.sh
@@ -718,7 +718,7 @@ test_expect_success GPG 'merge --no-edit tag should skip editor' '
 	git tag -f -s -m "A newer commit" signed &&
 	git reset --hard c0 &&
 
-	EDITOR=false git merge --no-edit signed &&
+	EDITOR=false git merge --no-edit --no-ff signed &&
 	git rev-parse signed^0 >expect &&
 	git rev-parse HEAD^2 >actual &&
 	test_cmp expect actual

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