update-ref: add support for 'symref-create' command

Add 'symref-create' command to the '--stdin' mode 'git-update-ref' to
allow creation of symbolic refs in a transaction. The 'symref-create'
command takes in a <new-target>, which the created <ref> will point to.

Also, support the 'core.prefersymlinkrefs' config, wherein if the config
is set and the filesystem supports symlinks, we create the symbolic ref
as a symlink. We fallback to creating a regular symref if creating the
symlink is unsuccessful.

Helped-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-update-ref.txt

@@ -65,6 +65,7 @@ performs all modifications together.  Specify commands of the form:
 	create SP <ref> SP <new-oid> LF
 	delete SP <ref> [SP <old-oid>] LF
 	verify SP <ref> [SP <old-oid>] LF
+	symref-create SP <ref> SP <new-target> LF
 	symref-delete SP <ref> [SP <old-target>] LF
 	symref-verify SP <ref> [SP <old-target>] LF
 	option SP <opt> LF
@@ -88,6 +89,7 @@ quoting:
 	create SP <ref> NUL <new-oid> NUL
 	delete SP <ref> NUL [<old-oid>] NUL
 	verify SP <ref> NUL [<old-oid>] NUL
+	symref-create SP <ref> NUL <new-target> NUL
 	symref-delete SP <ref> [NUL <old-target>] NUL
 	symref-verify SP <ref> [NUL <old-target>] NUL
 	option SP <opt> NUL
@@ -121,6 +123,10 @@ verify::
 	Verify <ref> against <old-oid> but do not change it.  If
 	<old-oid> is zero or missing, the ref must not exist.
 
+symref-create:
+	Create symbolic ref <ref> with <new-target> after verifying
+	it does not exist.
+
 symref-delete::
 	Delete <ref> after verifying it exists with <old-target>, if given.
 

builtin/clone.c

@@ -547,7 +547,7 @@ static void write_remote_refs(const struct ref *local_refs)
 		if (!r->peer_ref)
 			continue;
 		if (ref_transaction_create(t, r->peer_ref->name, &r->old_oid,
-					   0, NULL, &err))
+					   NULL, 0, NULL, &err))
 			die("%s", err.buf);
 	}
 

builtin/update-ref.c

@@ -257,7 +257,7 @@ static void parse_cmd_create(struct ref_transaction *transaction,
 	if (*next != line_termination)
 		die("create %s: extra input: %s", refname, next);
 
-	if (ref_transaction_create(transaction, refname, &new_oid,
+	if (ref_transaction_create(transaction, refname, &new_oid, NULL,
 				   update_flags | create_reflog_flag,
 				   msg, &err))
 		die("%s", err.buf);
@@ -267,6 +267,35 @@ static void parse_cmd_create(struct ref_transaction *transaction,
 	strbuf_release(&err);
 }
 
+
+static void parse_cmd_symref_create(struct ref_transaction *transaction,
+				    const char *next, const char *end)
+{
+	struct strbuf err = STRBUF_INIT;
+	char *refname, *new_target;
+
+	refname = parse_refname(&next);
+	if (!refname)
+		die("symref-create: missing <ref>");
+
+	new_target = parse_next_refname(&next);
+	if (!new_target)
+		die("symref-create %s: missing <new-target>", refname);
+
+	if (*next != line_termination)
+		die("symref-create %s: extra input: %s", refname, next);
+
+	if (ref_transaction_create(transaction, refname, NULL, new_target,
+				   update_flags | create_reflog_flag,
+				   msg, &err))
+		die("%s", err.buf);
+
+	update_flags = default_flags;
+	free(refname);
+	free(new_target);
+	strbuf_release(&err);
+}
+
 static void parse_cmd_delete(struct ref_transaction *transaction,
 			     const char *next, const char *end)
 {
@@ -473,6 +502,7 @@ static const struct parse_cmd {
 	{ "create",        parse_cmd_create,        2, UPDATE_REFS_OPEN },
 	{ "delete",        parse_cmd_delete,        2, UPDATE_REFS_OPEN },
 	{ "verify",        parse_cmd_verify,        2, UPDATE_REFS_OPEN },
+	{ "symref-create", parse_cmd_symref_create, 2, UPDATE_REFS_OPEN },
 	{ "symref-delete", parse_cmd_symref_delete, 2, UPDATE_REFS_OPEN },
 	{ "symref-verify", parse_cmd_symref_verify, 2, UPDATE_REFS_OPEN },
 	{ "option",        parse_cmd_option,        1, UPDATE_REFS_OPEN },

refs.c

@@ -1302,15 +1302,18 @@ int ref_transaction_update(struct ref_transaction *transaction,
 int ref_transaction_create(struct ref_transaction *transaction,
 			   const char *refname,
 			   const struct object_id *new_oid,
+			   const char *new_target,
 			   unsigned int flags, const char *msg,
 			   struct strbuf *err)
 {
-	if (!new_oid || is_null_oid(new_oid)) {
-		strbuf_addf(err, "'%s' has a null OID", refname);
+	if (new_oid && new_target)
+		BUG("create called with both new_oid and new_target set");
+	if ((!new_oid || is_null_oid(new_oid)) && !new_target) {
+		strbuf_addf(err, "'%s' has neither a valid OID nor a target", refname);
 		return 1;
 	}
 	return ref_transaction_update(transaction, refname, new_oid,
-				      null_oid(), NULL, NULL, flags,
+				      null_oid(), new_target, NULL, flags,
 				      msg, err);
 }
 

refs.h

@@ -753,6 +753,7 @@ int ref_transaction_update(struct ref_transaction *transaction,
 int ref_transaction_create(struct ref_transaction *transaction,
 			   const char *refname,
 			   const struct object_id *new_oid,
+			   const char *new_target,
 			   unsigned int flags, const char *msg,
 			   struct strbuf *err);
 

t/t0600-reffiles-backend.sh

@@ -472,4 +472,36 @@ test_expect_success POSIXPERM 'git reflog expire honors core.sharedRepository' '
 	esac
 '
 
+test_expect_success SYMLINKS 'symref transaction supports symlinks' '
+	test_when_finished "git symbolic-ref -d TEST_SYMREF_HEAD" &&
+	git update-ref refs/heads/new @ &&
+	test_config core.prefersymlinkrefs true &&
+	cat >stdin <<-EOF &&
+	start
+	symref-create TEST_SYMREF_HEAD refs/heads/new
+	prepare
+	commit
+	EOF
+	git update-ref --no-deref --stdin <stdin &&
+	test_path_is_symlink .git/TEST_SYMREF_HEAD &&
+	test "$(test_readlink .git/TEST_SYMREF_HEAD)" = refs/heads/new
+'
+
+test_expect_success 'symref transaction supports false symlink config' '
+	test_when_finished "git symbolic-ref -d TEST_SYMREF_HEAD" &&
+	git update-ref refs/heads/new @ &&
+	test_config core.prefersymlinkrefs false &&
+	cat >stdin <<-EOF &&
+	start
+	symref-create TEST_SYMREF_HEAD refs/heads/new
+	prepare
+	commit
+	EOF
+	git update-ref --no-deref --stdin <stdin &&
+	test_path_is_file .git/TEST_SYMREF_HEAD &&
+	git symbolic-ref TEST_SYMREF_HEAD >actual &&
+	echo refs/heads/new >expect &&
+	test_cmp expect actual
+'
+
 test_done

t/t1400-update-ref.sh

@@ -1797,6 +1797,71 @@ do
 		git update-ref --stdin $type --no-deref <stdin
 	'
 
+	test_expect_success "stdin $type symref-create fails with too many arguments" '
+		format_command $type "symref-create refs/heads/symref" "$a" "$a" >stdin &&
+		test_must_fail git update-ref --stdin $type --no-deref <stdin 2>err &&
+		if test "$type" = "-z"
+		then
+			grep "fatal: unknown command: $a" err
+		else
+			grep "fatal: symref-create refs/heads/symref: extra input:  $a" err
+		fi
+	'
+
+	test_expect_success "stdin $type symref-create fails with no target" '
+		format_command $type "symref-create refs/heads/symref" >stdin &&
+		test_must_fail git update-ref --stdin $type --no-deref <stdin
+	'
+
+	test_expect_success "stdin $type symref-create fails with empty target" '
+		format_command $type "symref-create refs/heads/symref" "" >stdin &&
+		test_must_fail git update-ref --stdin $type --no-deref <stdin
+	'
+
+	test_expect_success "stdin $type symref-create works" '
+		test_when_finished "git symbolic-ref -d refs/heads/symref" &&
+		format_command $type "symref-create refs/heads/symref" "$a" >stdin &&
+		git update-ref --stdin $type --no-deref <stdin &&
+		git symbolic-ref refs/heads/symref >expect &&
+		echo $a >actual &&
+		test_cmp expect actual
+	'
+
+	test_expect_success "stdin $type symref-create works with --no-deref" '
+		test_when_finished "git symbolic-ref -d refs/heads/symref" &&
+		format_command $type "symref-create refs/heads/symref" "$a" &&
+		git update-ref --stdin $type <stdin 2>err
+	'
+
+	test_expect_success "stdin $type create dangling symref ref works" '
+		test_when_finished "git symbolic-ref -d refs/heads/symref" &&
+		format_command $type "symref-create refs/heads/symref" "refs/heads/unkown" >stdin &&
+		git update-ref --stdin $type --no-deref <stdin &&
+		git symbolic-ref refs/heads/symref >expect &&
+		echo refs/heads/unkown >actual &&
+		test_cmp expect actual
+	'
+
+	test_expect_success "stdin $type symref-create does not create reflogs by default" '
+		test_when_finished "git symbolic-ref -d refs/symref" &&
+		format_command $type "symref-create refs/symref" "$a" >stdin &&
+		git update-ref --stdin $type --no-deref <stdin &&
+		git symbolic-ref refs/symref >expect &&
+		echo $a >actual &&
+		test_cmp expect actual &&
+		test_must_fail git reflog exists refs/symref
+	'
+
+	test_expect_success "stdin $type symref-create reflogs with --create-reflog" '
+		test_when_finished "git symbolic-ref -d refs/heads/symref" &&
+		format_command $type "symref-create refs/heads/symref" "$a" >stdin &&
+		git update-ref --create-reflog --stdin $type --no-deref <stdin &&
+		git symbolic-ref refs/heads/symref >expect &&
+		echo $a >actual &&
+		test_cmp expect actual &&
+		git reflog exists refs/heads/symref
+	'
+
 done
 
 test_done

t/t1416-ref-transaction-hooks.sh

@@ -189,15 +189,18 @@ test_expect_success 'hook gets all queued symref updates' '
 	prepared
 	ref:refs/heads/main $ZERO_OID refs/heads/symref
 	ref:refs/heads/main $ZERO_OID refs/heads/symrefd
+	$ZERO_OID ref:refs/heads/main refs/heads/symrefc
 	committed
 	ref:refs/heads/main $ZERO_OID refs/heads/symref
 	ref:refs/heads/main $ZERO_OID refs/heads/symrefd
+	$ZERO_OID ref:refs/heads/main refs/heads/symrefc
 	EOF
 
 	git update-ref --no-deref --stdin <<-EOF &&
 	start
 	symref-verify refs/heads/symref refs/heads/main
 	symref-delete refs/heads/symrefd refs/heads/main
+	symref-create refs/heads/symrefc refs/heads/main
 	prepare
 	commit
 	EOF

t/t5605-clone-local.sh

@@ -163,7 +163,7 @@ test_expect_success REFFILES 'local clone from repo with corrupt refs fails grac
 	echo a >corrupt/.git/refs/heads/topic &&
 
 	test_must_fail git clone corrupt working 2>err &&
-	grep "has a null OID" err
+	grep "has neither a valid OID nor a target" err
 '
 
 test_done