cp: fix some cases with infinite recursion

As noted in the PR, cp -R has some surprising behavior.  Typically, when
you `cp -R foo bar` where both foo and bar exist, foo is cleanly copied
to foo/bar.  When you `cp -R foo foo` (where foo clearly exists), cp(1)
goes a little off the rails as it creates foo/foo, then discovers that
and creates foo/foo/foo, so on and so forth, until it eventually fails.

POSIX doesn't seem to disallow this behavior, but it isn't very useful.
GNU cp(1) will detect the recursion and squash it, but emit a message in
the process that it has done so.

This change seemingly follows the GNU behavior, but it currently doesn't
warn about the situation -- the author feels that the final product is
about what one might expect from doing this and thus, doesn't need a
warning.  The author doesn't feel strongly about this.

PR:		235438
Reviewed by:	bapt
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D33944

bin/cp/tests/cp_test.sh

@@ -57,8 +57,85 @@ chrdev_body()
 	check_size trunc 0
 }
 
+atf_test_case matching_srctgt
+matching_srctgt_body()
+{
+
+	# PR235438: `cp -R foo foo` would previously infinitely recurse and
+	# eventually error out.
+	mkdir foo
+	echo "qux" > foo/bar
+	cp foo/bar foo/zoo
+
+	atf_check cp -R foo foo
+	atf_check -o inline:"qux\n" cat foo/foo/bar
+	atf_check -o inline:"qux\n" cat foo/foo/zoo
+	atf_check -e not-empty -s not-exit:0 stat foo/foo/foo
+}
+
+atf_test_case matching_srctgt_contained
+matching_srctgt_contained_body()
+{
+
+	# Let's do the same thing, except we'll try to recursively copy foo into
+	# one of its subdirectories.
+	mkdir foo
+	echo "qux" > foo/bar
+	mkdir foo/loo
+	mkdir foo/moo
+	mkdir foo/roo
+	cp foo/bar foo/zoo
+
+	atf_check cp -R foo foo/moo
+	atf_check -o inline:"qux\n" cat foo/moo/foo/bar
+	atf_check -o inline:"qux\n" cat foo/moo/foo/zoo
+	atf_check -e not-empty -s not-exit:0 stat foo/moo/foo/moo
+}
+
+atf_test_case matching_srctgt_link
+matching_srctgt_link_body()
+{
+
+	mkdir foo
+	echo "qux" > foo/bar
+	cp foo/bar foo/zoo
+
+	atf_check ln -s foo roo
+	atf_check cp -RH roo foo
+	atf_check -o inline:"qux\n" cat foo/roo/bar
+	atf_check -o inline:"qux\n" cat foo/roo/zoo
+}
+
+atf_test_case matching_srctgt_nonexistent
+matching_srctgt_nonexistent_body()
+{
+
+	# We'll copy foo to a nonexistent subdirectory; ideally, we would
+	# skip just the directory and end up with a layout like;
+	#
+	# foo/
+	#     bar
+	#     dne/
+	#         bar
+	#         zoo
+	#     zoo
+	#
+	mkdir foo
+	echo "qux" > foo/bar
+	cp foo/bar foo/zoo
+
+	atf_check cp -R foo foo/dne
+	atf_check -o inline:"qux\n" cat foo/dne/bar
+	atf_check -o inline:"qux\n" cat foo/dne/zoo
+	atf_check -e not-empty -s not-exit:0 stat foo/dne/foo
+}
+
 atf_init_test_cases()
 {
 	atf_add_test_case basic
 	atf_add_test_case chrdev
+	atf_add_test_case matching_srctgt
+	atf_add_test_case matching_srctgt_contained
+	atf_add_test_case matching_srctgt_link
+	atf_add_test_case matching_srctgt_nonexistent
 }