Add a few more values for receive.denyCurrentBranch

For a long time, this developer thought that Git's insistence that
pushing into the current branch is evil was completely merited.

Just for fun, the original patch tried to show people that Git is right
there, and that it causes more trouble than it does good when Git allows
you to try to update the working tree for fast-forwards, or to detach the
HEAD, depending on some config settings.

Surprisingly, the opposite was shown.

So here is the support for two new options you can give the config
variable receive.denyCurrentBranch:

'updateInstead':
	Try to merge the working tree with the new tip of the branch
	(which can lead to really horrible merge conflicts).

'detachInstead':
	Detach the HEAD, thereby avoiding a disagreement between the
	HEAD and the index (as well as the working tree), possibly
	leaving the local user wondering how on earth her HEAD became
	so detached.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Documentation/config.txt

@@ -1638,6 +1638,11 @@ receive.denyCurrentBranch::
 	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 "refuse".
++
+There are two more options that are meant for Git experts: "updateInstead"
+which will run `read-tree -u -m HEAD` and "detachInstead" which will detach
+the HEAD so it does not need to change.  Both options come with their own
+set of possible *complications*, but can be appropriate in rare workflows.
 
 receive.denyNonFastForwards::
 	If set to true, git-receive-pack will deny a ref update which is

builtin/receive-pack.c

@@ -19,7 +19,9 @@ enum deny_action {
 	DENY_UNCONFIGURED,
 	DENY_IGNORE,
 	DENY_WARN,
-	DENY_REFUSE
+	DENY_REFUSE,
+	DENY_UPDATE_INSTEAD,
+	DENY_DETACH_INSTEAD,
 };
 
 static int deny_deletes;
@@ -87,7 +89,12 @@ static int receive_pack_config(const char *var, const char *value, void *cb)
 	}
 
 	if (!strcmp(var, "receive.denycurrentbranch")) {
-		deny_current_branch = parse_deny_action(var, value);
+		if (value && !strcasecmp(value, "updateinstead"))
+			deny_current_branch = DENY_UPDATE_INSTEAD;
+		else if (value && !strcasecmp(value, "detachinstead"))
+			deny_current_branch = DENY_DETACH_INSTEAD;
+		else
+			deny_current_branch = parse_deny_action(var, value);
 		return 0;
 	}
 
@@ -391,6 +398,44 @@ static void refuse_unconfigured_deny_delete_current(void)
 		rp_error("%s", refuse_unconfigured_deny_delete_current_msg[i]);
 }
 
+static void merge_worktree(unsigned char *sha1)
+{
+	const char *update_refresh[] = {
+		"update-index", "--refresh", NULL
+	};
+	const char *read_tree[] = {
+		"read-tree", "-u", "-m", sha1_to_hex(sha1), NULL
+	};
+	struct child_process child;
+	struct strbuf git_env = STRBUF_INIT;
+	const char *env[2];
+
+	if (is_bare_repository())
+		die ("denyCurrentBranch = updateInstead needs a worktree");
+
+	strbuf_addf(&git_env, "GIT_DIR=%s", absolute_path(get_git_dir()));
+	env[0] = git_env.buf;
+	env[1] = NULL;
+
+	memset(&child, 0, sizeof(child));
+	child.argv = update_refresh;
+	child.env = env;
+	child.dir = git_work_tree_cfg ? git_work_tree_cfg : "..";
+	child.stdout_to_stderr = 1;
+	child.git_cmd = 1;
+	if (run_command(&child))
+		die ("Could not refresh the index");
+
+	child.argv = read_tree;
+	child.no_stdin = 1;
+	child.no_stdout = 1;
+	child.stdout_to_stderr = 0;
+	if (run_command(&child))
+		die ("Could not merge working tree with new HEAD.  Good luck.");
+
+	strbuf_release(&git_env);
+}
+
 static const char *update(struct command *cmd)
 {
 	const char *name = cmd->ref_name;
@@ -422,6 +467,13 @@ static const char *update(struct command *cmd)
 			if (deny_current_branch == DENY_UNCONFIGURED)
 				refuse_unconfigured_deny();
 			return "branch is currently checked out";
+		case DENY_UPDATE_INSTEAD:
+			merge_worktree(new_sha1);
+			break;
+		case DENY_DETACH_INSTEAD:
+			update_ref("push into current branch (detach)", "HEAD",
+				old_sha1, NULL, REF_NODEREF, DIE_ON_ERR);
+			break;
 		}
 	}
 
@@ -450,6 +502,8 @@ static const char *update(struct command *cmd)
 					refuse_unconfigured_deny_delete_current();
 				rp_error("refusing to delete the current branch: %s", name);
 				return "deletion of the current branch prohibited";
+			default:
+				die ("Invalid denyDeleteCurrent setting");
 			}
 		}
 	}

t/t5516-fetch-push.sh

@@ -979,4 +979,40 @@ test_expect_success 'push --porcelain --dry-run rejected' '
 	test_cmp .git/foo .git/bar
 '
 
+test_expect_success 'receive.denyCurrentBranch = updateInstead' '
+	git push testrepo master &&
+	(cd testrepo &&
+		git reset --hard &&
+		git config receive.denyCurrentBranch updateInstead
+	) &&
+	test_commit third path2 &&
+	git push testrepo master &&
+	test $(git rev-parse HEAD) = $(cd testrepo && git rev-parse HEAD) &&
+	test third = "$(cat testrepo/path2)" &&
+	(cd testrepo &&
+		git update-index --refresh &&
+		git diff-files --quiet &&
+		git diff-index --cached HEAD --
+	)
+'
+
+test_expect_success 'receive.denyCurrentBranch = detachInstead' '
+	(cd testrepo &&
+		git reset --hard &&
+		git config receive.denyCurrentBranch detachInstead
+	) &&
+	OLDHEAD=$(cd testrepo && git rev-parse HEAD) &&
+	test_commit fourth path2 &&
+	test fourth = "$(cat path2)" &&
+	git push testrepo master &&
+	test $OLDHEAD = $(cd testrepo && git rev-parse HEAD) &&
+	test fourth != "$(cat testrepo/path2)" &&
+	(cd testrepo &&
+		test_must_fail git symbolic-ref HEAD &&
+		git update-index --refresh &&
+		git diff-files --quiet &&
+		git diff-index --cached HEAD --
+	)
+'
+
 test_done