receive-pack: detect push to current branch of non-bare repo

Pushing into the currently checked out branch of a non-bare
repository can be dangerous; the HEAD then loses sync with
the index and working tree, and it looks in the receiving
repo as if the pushed changes have been reverted in the
index (since they were never there in the first place).

This patch adds a safety valve that checks for this
condition and either generates a warning or denies the
update. We trigger the check only on a non-bare repository,
since a bare repo does not have a working tree (and in fact,
pushing to the HEAD branch is a common workflow for
publishing repositories).

The behavior is configurable via receive.denyCurrentBranch,
defaulting to "warn" so as not to break existing setups
(though it may, after a deprecation period, switch to
"refuse" by default). For users who know what they are doing
and want to silence the warning (e.g., because they have a
post-receive hook that reconciles the HEAD and working
tree), they can turn off the warning by setting it to false
or "ignore".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1198,6 +1198,15 @@ receive.denyNonFastForwards::
 	even if that push is forced. This configuration variable is
 	set when initializing a shared repository.
 
+receive.denyCurrentBranch::
+	If set to true or "refuse", receive-pack will deny a ref update
+	to the currently checked out branch of a non-bare repository.
+	Such a push is potentially dangerous because it brings the HEAD
+	out of sync with the index and working tree. If set to "warn",
+	print a warning of such a push to stderr, but allow the push to
+	proceed. If set to false or "ignore", allow such pushes with no
+	message. Defaults to "warn".
+
 transfer.unpackLimit::
 	When `fetch.unpackLimit` or `receive.unpackLimit` are
 	not set, the value of this variable is used instead.

builtin-receive-pack.c

@@ -11,8 +11,15 @@
 
 static const char receive_pack_usage[] = "git-receive-pack <git-dir>";
 
+enum deny_action {
+	DENY_IGNORE,
+	DENY_WARN,
+	DENY_REFUSE,
+};
+
 static int deny_deletes = 0;
 static int deny_non_fast_forwards = 0;
+static enum deny_action deny_current_branch = DENY_WARN;
 static int receive_fsck_objects;
 static int receive_unpack_limit = -1;
 static int transfer_unpack_limit = -1;
@@ -22,6 +29,21 @@ static int report_status;
 static char capabilities[] = " report-status delete-refs ";
 static int capabilities_sent;
 
+static enum deny_action parse_deny_action(const char *var, const char *value)
+{
+	if (value) {
+		if (!strcasecmp(value, "ignore"))
+			return DENY_IGNORE;
+		if (!strcasecmp(value, "warn"))
+			return DENY_WARN;
+		if (!strcasecmp(value, "refuse"))
+			return DENY_REFUSE;
+	}
+	if (git_config_bool(var, value))
+		return DENY_REFUSE;
+	return DENY_IGNORE;
+}
+
 static int receive_pack_config(const char *var, const char *value, void *cb)
 {
 	if (strcmp(var, "receive.denydeletes") == 0) {
@@ -49,6 +71,11 @@ static int receive_pack_config(const char *var, const char *value, void *cb)
 		return 0;
 	}
 
+	if (!strcmp(var, "receive.denycurrentbranch")) {
+		deny_current_branch = parse_deny_action(var, value);
+		return 0;
+	}
+
 	return git_default_config(var, value, cb);
 }
 
@@ -173,6 +200,20 @@ static int run_update_hook(struct command *cmd)
 	return hook_status(run_command(&proc), update_hook);
 }
 
+static int is_ref_checked_out(const char *ref)
+{
+	unsigned char sha1[20];
+	const char *head;
+
+	if (is_bare_repository())
+		return 0;
+
+	head = resolve_ref("HEAD", sha1, 0, NULL);
+	if (!head)
+		return 0;
+	return !strcmp(head, ref);
+}
+
 static const char *update(struct command *cmd)
 {
 	const char *name = cmd->ref_name;
@@ -186,6 +227,24 @@ static const char *update(struct command *cmd)
 		return "funny refname";
 	}
 
+	switch (deny_current_branch) {
+	case DENY_IGNORE:
+		break;
+	case DENY_WARN:
+		if (!is_ref_checked_out(name))
+			break;
+		warning("updating the currently checked out branch; this may"
+			" cause confusion,\n"
+			"as the index and working tree do not reflect changes"
+			" that are now in HEAD.");
+		break;
+	case DENY_REFUSE:
+		if (!is_ref_checked_out(name))
+			break;
+		error("refusing to update checked out branch: %s", name);
+		return "branch is currently checked out";
+	}
+
 	if (!is_null_sha1(new_sha1) && !has_sha1_file(new_sha1)) {
 		error("unpack should have generated %s, "
 		      "but I can't find it!", sha1_to_hex(new_sha1));

t/t5516-fetch-push.sh

@@ -486,4 +486,41 @@ test_expect_success 'allow deleting an invalid remote ref' '
 
 '
 
+test_expect_success 'warn on push to HEAD of non-bare repository' '
+	mk_test heads/master
+	(cd testrepo &&
+		git checkout master &&
+		git config receive.denyCurrentBranch warn) &&
+	git push testrepo master 2>stderr &&
+	grep "warning.*this may cause confusion" stderr
+'
+
+test_expect_success 'deny push to HEAD of non-bare repository' '
+	mk_test heads/master
+	(cd testrepo &&
+		git checkout master &&
+		git config receive.denyCurrentBranch true) &&
+	test_must_fail git push testrepo master
+'
+
+test_expect_success 'allow push to HEAD of bare repository (bare)' '
+	mk_test heads/master
+	(cd testrepo &&
+		git checkout master &&
+		git config receive.denyCurrentBranch true &&
+		git config core.bare true) &&
+	git push testrepo master 2>stderr &&
+	! grep "warning.*this may cause confusion" stderr
+'
+
+test_expect_success 'allow push to HEAD of non-bare repository (config)' '
+	mk_test heads/master
+	(cd testrepo &&
+		git checkout master &&
+		git config receive.denyCurrentBranch false
+	) &&
+	git push testrepo master 2>stderr &&
+	! grep "warning.*this may cause confusion" stderr
+'
+
 test_done