fix(tar): handle strip_prefix of root directory in mtree_mutate (#995)

* tar: Add test case for root directory strip_prefix (#978)

* Use space delimiter to skip entry if missing path after strip_prefix

lib/private/modify_mtree.awk

@@ -16,7 +16,13 @@
             #       To avoid this, we append a slash to the directory path to make it a "full" entry.
             components = split($1, _, "/");
             if ($0 ~ /type=dir/ && components == 1) {
-                $1 = $1 "/";
+                if ($0 !~ /^ /) {
+                    $1 = $1 "/";
+                }
+                else {
+                    # this line is the root directory and only contains orphaned keywords, which will be discarded
+                    next;
+                }
             }
         } else {
             # this line declares some path under a parent directory, which will be discarded

lib/tests/tar/BUILD.bazel

@@ -465,3 +465,41 @@ assert_unused_listing(
         "lib/tests/tar/unused/space in name.txt",
     ],
 )
+
+#############
+# Example 16: Ensure that root directories are properly handled with strip_prefix (bug #978)
+# Don't assume that files or directories exist after stripping the prefix. See (bug #851)
+
+_SRCS16 = [
+    "src_file",
+    ":fixture1",
+]
+
+mtree_spec(
+    name = "mtree16",
+    srcs = _SRCS16,
+)
+
+mtree_mutate(
+    name = "strip_prefix16",
+    mtree = ":mtree16",
+    strip_prefix = "lib",
+)
+
+tar(
+    name = "tar_strip_prefix16",
+    srcs = _SRCS16,
+    out = "16.tar",
+    mtree = "strip_prefix16",
+)
+
+assert_tar_listing(
+    name = "test_strip_prefix16",
+    actual = "tar_strip_prefix16",
+    expected = [
+        "drwxr-xr-x  0 0      0           0 Jan  1  2023 tests/",
+        "drwxr-xr-x  0 0      0           0 Jan  1  2023 tests/tar/",
+        "-rwxr-xr-x  0 0      0          21 Jan  1  2023 tests/tar/src_file",
+        "-rwxr-xr-x  0 0      0           7 Jan  1  2023 tests/tar/generated.txt",
+    ],
+)