branch: add branch_checked_out() helper

The validate_new_branchname() method contains a check to see if a branch
is checked out in any non-bare worktree. This is intended to prevent a
force push that will mess up an existing checkout. This helper is not
suitable to performing just that check, because the method will die()
when the branch is checked out instead of returning an error code.

Create a new branch_checked_out() helper that performs the most basic
form of this check. To ensure we can call branch_checked_out() in a loop
with good performance, do a single preparation step that iterates over
all worktrees and stores their current HEAD branches in a strmap. The
branch_checked_out() helper can then discover these branches using a
hash lookup.

This helper is currently missing some key functionality. Namely: it
doesn't look for active rebases or bisects which mean that the branch is
"checked out" even though HEAD doesn't point to that ref. This
functionality will be added in a coming change.

We could use branch_checked_out() in validate_new_branchname(), but this
missing functionality would be a regression. However, we have no tests
that cover this case!

Add a new test script that will be expanded with these cross-worktree
ref updates. The current tests would still pass if we refactored
validate_new_branchname() to use this version of branch_checked_out().
The next change will fix that functionality and add the proper test
coverage.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

branch.c

@@ -10,6 +10,7 @@
 #include "worktree.h"
 #include "submodule-config.h"
 #include "run-command.h"
+#include "strmap.h"
 
 struct tracking {
 	struct refspec_item spec;
@@ -346,6 +347,41 @@ int validate_branchname(const char *name, struct strbuf *ref)
 	return ref_exists(ref->buf);
 }
 
+static int initialized_checked_out_branches;
+static struct strmap current_checked_out_branches = STRMAP_INIT;
+
+static void prepare_checked_out_branches(void)
+{
+	int i = 0;
+	struct worktree **worktrees;
+
+	if (initialized_checked_out_branches)
+		return;
+	initialized_checked_out_branches = 1;
+
+	worktrees = get_worktrees();
+
+	while (worktrees[i]) {
+		struct worktree *wt = worktrees[i++];
+
+		if (wt->is_bare)
+			continue;
+
+		if (wt->head_ref)
+			strmap_put(&current_checked_out_branches,
+				   wt->head_ref,
+				   xstrdup(wt->path));
+	}
+
+	free_worktrees(worktrees);
+}
+
+const char *branch_checked_out(const char *refname)
+{
+	prepare_checked_out_branches();
+	return strmap_get(&current_checked_out_branches, refname);
+}
+
 /*
  * Check if a branch 'name' can be created as a new branch; die otherwise.
  * 'force' can be used when it is OK for the named branch already exists.

branch.h

@@ -100,6 +100,13 @@ void create_branches_recursively(struct repository *r, const char *name,
 				 const char *tracking_name, int force,
 				 int reflog, int quiet, enum branch_track track,
 				 int dry_run);
+
+/*
+ * If the branch at 'refname' is currently checked out in a worktree,
+ * then return the path to that worktree.
+ */
+const char *branch_checked_out(const char *refname);
+
 /*
  * Check if 'name' can be a valid name for a branch; die otherwise.
  * Return 1 if the named branch already exists; return 0 otherwise.

t/t2407-worktree-heads.sh

@@ -0,0 +1,29 @@
+#!/bin/sh
+
+test_description='test operations trying to overwrite refs at worktree HEAD'
+
+TEST_PASSES_SANITIZE_LEAK=true
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	test_commit init &&
+	git branch -f fake-1 &&
+	git branch -f fake-2 &&
+
+	for i in 1 2 3 4
+	do
+		test_commit $i &&
+		git branch wt-$i &&
+		git worktree add wt-$i wt-$i || return 1
+	done
+'
+
+test_expect_success 'refuse to overwrite: checked out in worktree' '
+	for i in 1 2 3 4
+	do
+		test_must_fail git branch -f wt-$i HEAD 2>err
+		grep "cannot force update the branch" err || return 1
+	done
+'
+
+test_done