checkout: refactor die_if_checked_out() caller

There is a bit dense logic to make a call to "die_if_checked_out()"
while trying to check out a branch.  Extract it into a helper
function and give it a bit of comment to describe what is going on.

The most important part of the refactoring is the separation of the
guarding logic before making the call to die_if_checked_out() into
the caller specific part (e.g., the logic that decides that the
caller is trying to check out an existing branch) and the bypass due
to the "--ignore-other-worktrees" option.  The latter will be common
no matter how the current or future callers decides they need this
protection.

Signed-off-by: Junio C Hamano <[email protected]>

Author: gitster

builtin/checkout.c

@@ -1516,6 +1516,26 @@ static void die_if_some_operation_in_progress(void)
 	wt_status_state_free_buffers(&state);
 }
 
+/*
+ * die if attempting to checkout an existing branch that is in use
+ * in another worktree, unless ignore-other-wortrees option is given.
+ * The check is bypassed when the branch is already the current one,
+ * as it will not make things any worse.
+ */
+static void die_if_switching_to_a_branch_in_use(struct checkout_opts *opts,
+						const char *full_ref)
+{
+	int flags;
+	char *head_ref;
+
+	if (opts->ignore_other_worktrees)
+		return;
+	head_ref = resolve_refdup("HEAD", 0, NULL, &flags);
+	if (head_ref && (!(flags & REF_ISSYMREF) || strcmp(head_ref, full_ref)))
+		die_if_checked_out(full_ref, 1);
+	free(head_ref);
+}
+
 static int checkout_branch(struct checkout_opts *opts,
 			   struct branch_info *new_branch_info)
 {
@@ -1576,15 +1596,9 @@ static int checkout_branch(struct checkout_opts *opts,
 	if (!opts->can_switch_when_in_progress)
 		die_if_some_operation_in_progress();
 
-	if (new_branch_info->path && !opts->force_detach && !opts->new_branch &&
-	    !opts->ignore_other_worktrees) {
-		int flag;
-		char *head_ref = resolve_refdup("HEAD", 0, NULL, &flag);
-		if (head_ref &&
-		    (!(flag & REF_ISSYMREF) || strcmp(head_ref, new_branch_info->path)))
-			die_if_checked_out(new_branch_info->path, 1);
-		free(head_ref);
-	}
+	/* "git checkout <branch>" */
+	if (new_branch_info->path && !opts->force_detach && !opts->new_branch)
+		die_if_switching_to_a_branch_in_use(opts, new_branch_info->path);
 
 	if (!new_branch_info->commit && opts->new_branch) {
 		struct object_id rev;