[PATCH v4] test_dir_is_empty: fix edge cases with newlines and hyphens

[William Chargin <wchargin@gmail.com>]

While the `test_dir_is_empty` function appears correct in most normal
use cases, it can improperly pass if a directory contains a filename
with a newline, and can improperly fail if an empty directory looks like
an argument to `ls`. This patch changes the implementation to check that
the output of `ls -a` has at most two lines (for `.` and `..`), which
should be better behaved, and adds the `--` delimiter before the
directory name when invoking `ls`.

The newly added unit test fails before this change and passes after it.

Signed-off-by: William Chargin <wchargin@gmail.com>
---
This patch depends on "t: factor out FUNNYNAMES as shared lazy prereq"
(2018-08-06), which is now in master.

I originally wrote this patch for the standalone Sharness library, but
that library advises that such patches be sent to the Git mailing list
first.

Tested on GNU/Linux (Mint 18.2) and macOS (10.13).

 t/t0000-basic.sh        | 43 +++++++++++++++++++++++++++++++++++++++++
 t/test-lib-functions.sh |  2 +-
 2 files changed, 44 insertions(+), 1 deletion(-)

diff --git a/t/t0000-basic.sh b/t/t0000-basic.sh
index 850f651e4e..a5c57c6aa5 100755
--- a/t/t0000-basic.sh
+++ b/t/t0000-basic.sh
@@ -821,6 +821,49 @@ test_expect_success 'tests clean up even on failures' "
 	EOF
 "
 
+test_expect_success FUNNYNAMES \
+	'test_dir_is_empty behaves even in pathological cases' "
+	run_sub_test_lib_test \
+		dir-empty 'behavior of test_dir_is_empty' <<-\\EOF &&
+	test_expect_success 'should pass with actually empty directory' '
+		mkdir empty_dir &&
+		test_dir_is_empty empty_dir
+	'
+	test_expect_success 'should fail with a normal filename' '
+		mkdir nonempty_dir &&
+		>nonempty_dir/some_file &&
+		! test_dir_is_empty nonempty_dir
+	'
+	test_expect_success 'should fail with dot-newline-dot filename' '
+		mkdir pathological_dir &&
+		>\"pathological_dir/.
+	.\" &&
+		! test_dir_is_empty pathological_dir
+	'
+	test_expect_success 'should pass with an empty directory \"-l\"' '
+		mkdir -- -l &&
+		test_dir_is_empty -l &&
+		rmdir -- -l
+	'
+	test_expect_success 'should pass with an empty directory \"--wat\"' '
+		mkdir -- --wat &&
+		test_dir_is_empty --wat &&
+		rmdir -- --wat
+	'
+	test_done
+	EOF
+	check_sub_test_lib_test dir-empty <<-\\EOF
+	> ok 1 - should pass with actually empty directory
+	> ok 2 - should fail with a normal filename
+	> ok 3 - should fail with dot-newline-dot filename
+	> ok 4 - should pass with an empty directory \"-l\"
+	> ok 5 - should pass with an empty directory \"--wat\"
+	> # passed all 5 test(s)
+	> 1..5
+	EOF
+"
+
+
 ################################################################
 # Basics of the basics
 
diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index 4207af4077..3df6b8027f 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -576,7 +576,7 @@ test_path_exists () {
 # Check if the directory exists and is empty as expected, barf otherwise.
 test_dir_is_empty () {
 	test_path_is_dir "$1" &&
-	if test -n "$(ls -a1 "$1" | egrep -v '^\.\.?$')"
+	if test "$(ls -a1 -- "$1" | wc -l)" -gt 2
 	then
 		echo "Directory '$1' is not empty, it contains:"
 		ls -la "$1"
-- 
2.18.0.549.gd66323a05