merge: allow fast-forward when merging a tracked tag

Long time ago at fab47d05 ("merge: force edit and no-ff mode when
merging a tag object", 2011-11-07), "git merge" was made to always
create a merge commit when merging a tag, even when the side branch
being merged is a descendant of the current branch.

This default is good for merges made by upstream maintainers to
integrate work signed by downstream contributors, but will leave
pointless no-ff merges when downstream contributors pull a newer
release tag to make their long-running topic branches catch up with
the upstream.  When there is no local work left on the topic, such a
merge should simply fast-forward to the commit pointed at by the
release tag.

Update the default (again) for "git merge" that merges a tag object
to (1) --no-ff (i.e. create a merge commit even when side branch
fast forwards) if the tag being merged is not at its expected place
in refs/tags/ hierarchy and (2) --ff (i.e. allow fast-forward update
when able) otherwise.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/merge-options.txt

@@ -35,7 +35,8 @@ set to `no` at the beginning of them.
 --no-ff::
 	Create a merge commit even when the merge resolves as a
 	fast-forward.  This is the default behaviour when merging an
-	annotated (and possibly signed) tag.
+	annotated (and possibly signed) tag that is not stored in
+	its natural place in 'refs/tags/' hierarchy.
 
 --ff-only::
 	Refuse to merge and exit with a non-zero status unless the

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 +1126,43 @@ static struct commit_list *collect_parents(struct commit *head_commit,
 	return remoteheads;
 }
 
+static int merging_a_throwaway_tag(struct commit *commit)
+{
+	char *tag_ref;
+	struct object_id oid;
+	int is_throwaway_tag = 0;
+
+	/* Are we merging a tag? */
+	if (!merge_remote_util(commit) ||
+	    !merge_remote_util(commit)->obj ||
+	    merge_remote_util(commit)->obj->type != OBJ_TAG)
+		return is_throwaway_tag;
+
+	/*
+	 * 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.
+	 *
+	 * 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.
+	 */
+	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))
+		is_throwaway_tag = 0;
+	else
+		is_throwaway_tag = 1;
+	free(tag_ref);
+	return is_throwaway_tag;
+}
+
 int cmd_merge(int argc, const char **argv, const char *prefix)
 {
 	struct object_id result_tree, stash, head_oid;
@@ -1322,10 +1360,7 @@ 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;
 	}
 

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'\''

t/t7600-merge.sh

@@ -700,6 +700,42 @@ test_expect_success 'merge --no-ff --edit' '
 	test_cmp expected actual
 '
 
+test_expect_success 'merge annotated/signed tag w/o tracking' '
+	test_when_finished "rm -rf dst; git tag -d anno1" &&
+	git tag -a -m "anno c1" anno1 c1 &&
+	git init dst &&
+	git rev-parse c1 >dst/expect &&
+	(
+		# c0 fast-forwards to c1 but because this repository
+		# is not a "downstream" whose refs/tags follows along
+		# tag from the "upstream", this pull defaults to --no-ff
+		cd dst &&
+		git pull .. c0 &&
+		git pull .. anno1 &&
+		git rev-parse HEAD^2 >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'merge annotated/signed tag w/ tracking' '
+	test_when_finished "rm -rf dst; git tag -d anno1" &&
+	git tag -a -m "anno c1" anno1 c1 &&
+	git init dst &&
+	git rev-parse c1 >dst/expect &&
+	(
+		# c0 fast-forwards to c1 and because this repository
+		# is a "downstream" whose refs/tags follows along
+		# tag from the "upstream", this pull defaults to --ff
+		cd dst &&
+		git remote add origin .. &&
+		git pull origin c0 &&
+		git fetch origin &&
+		git merge anno1 &&
+		git rev-parse HEAD >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_expect_success GPG 'merge --ff-only tag' '
 	git reset --hard c0 &&
 	git commit --allow-empty -m "A newer commit" &&
@@ -718,7 +754,7 @@ 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