fix(tar): append slash to top-level directory mtree entries

bsdtar's mtree format has a quirk wherein entries without "/" in their
first word are treated as "relative" entries, and "relative" directories
will cause tar to "change directory" into the declared directory entry.
If such a directory is followed by a "relative" entry, then the file
will be created within the directory, instead of at top-level as
expected. To mitigate, we append a slash to top-level directory entries.

Fixes bazel-contrib#851.

BUILD.bazel

@@ -5,6 +5,7 @@ load("@bazel_skylib//rules:copy_file.bzl", "copy_file")
 load("@bazel_skylib//rules:write_file.bzl", "write_file")
 load("@buildifier_prebuilt//:rules.bzl", "buildifier")
 load("//lib:diff_test.bzl", "diff_test")
+load("//lib:tar.bzl", "mtree_spec")
 load("//lib:testing.bzl", "assert_contains")
 load("//lib:utils.bzl", "is_bazel_7_or_greater")
 load("//lib:write_source_files.bzl", "write_source_files")
@@ -136,3 +137,31 @@ bzl_library(
     visibility = ["//visibility:public"],
     deps = ["@bazel_gazelle//:deps"],
 )
+
+# Test case for mtree_spec: Ensure that multiple entries at the root directory are handled correctly (bug #851)
+# See lib/tests/tar/BUILD.bazel for why this is here.
+write_file(
+    name = "tar_test13_main",
+    out = "13project/__main__.py",
+    content = ["__main__.py"],
+)
+
+write_file(
+    name = "tar_test13_bin",
+    out = "13project_bin",
+    content = ["project_bin"],
+)
+
+mtree_spec(
+    name = "tar_test13_mtree",
+    srcs = [
+        ":tar_test13_main",
+        ":tar_test13_bin",
+    ]
+)
+
+diff_test(
+    name = "tar_test13",
+    file1 = "tar_test13_mtree",
+    file2 = "//lib/tests/tar:expected13.mtree",
+)

lib/private/modify_mtree.awk

@@ -7,6 +7,17 @@
         } else if (index($1, strip_prefix) == 1) {
             # this line starts with the strip_prefix
             sub("^" strip_prefix "/", "");
+
+            # NOTE: The mtree format treats file paths without slashes as "relative" entries.
+            #       If a relative entry is a directory, then it will "change directory" to that
+            #       directory, and any subsequent "relative" entries will be created inside that
+            #       directory. This causes issues when there is a top-level directory that is
+            #       followed by a top-level file, as the file will be created inside the directory.
+            #       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 "/";
+            }
         } else {
             # this line declares some path under a parent directory, which will be discarded
             next;

lib/private/tar.bzl

@@ -195,6 +195,16 @@ def _expand(file, expander, transform = to_repository_relative_path):
         segments = path.split("/")
         for i in range(1, len(segments)):
             parent = "/".join(segments[:i])
+
+            # NOTE: The mtree format treats file paths without slashes as "relative" entries.
+            #       If a relative entry is a directory, then it will "change directory" to that
+            #       directory, and any subsequent "relative" entries will be created inside that
+            #       directory. This causes issues when there is a top-level directory that is
+            #       followed by a top-level file, as the file will be created inside the directory.
+            #       To avoid this, we append a slash to the directory path to make it a "full" entry.
+            if i == 1:
+                parent += "/"
+
             lines.append(_mtree_line(parent, "dir"))
 
         lines.append(_mtree_line(_vis_encode(path), "file", content = _vis_encode(e.path)))

lib/tests/tar/BUILD.bazel

@@ -358,3 +358,42 @@ diff_test(
     file1 = "modified2.mtree",
     file2 = "expected2.mtree",
 )
+
+# Case 13: Ensure that multiple entries at the root directory are handled correctly (bug #851)
+# NOTE: The mtree_spec part of this test is placed at the root BUILD.bazel because
+#       that's the only way to ensure that the mtree_spec is processed before the tar rule.
+exports_files(["expected13.mtree"])
+
+# Case 14: Ensure mtree_mutate correctly handles prefix stripping for top-level directories (bug #851)
+
+write_file(
+    name = "test14_main",
+    out = "14project/__main__.py",
+    content = ["__main__.py"],
+)
+
+write_file(
+    name = "test14_bin",
+    out = "14project_bin",
+    content = ["project_bin"],
+)
+
+mtree_spec(
+    name = "mtree14",
+    srcs = [
+        ":test14_main",
+        ":test14_bin",
+    ],
+)
+
+mtree_mutate(
+    name = "strip_prefix14",
+    mtree = "mtree14",
+    strip_prefix = "lib/tests/tar",
+)
+
+diff_test(
+    name = "test14",
+    file1 = ":strip_prefix14",
+    file2 = "expected14.mtree",
+)

lib/tests/tar/expected13.mtree

@@ -0,0 +1,3 @@
+13project/ uid=0 gid=0 time=1672560000 mode=0755 type=dir
+13project/__main__.py uid=0 gid=0 time=1672560000 mode=0755 type=file content=bazel-out/k8-opt/bin/13project/__main__.py
+13project_bin uid=0 gid=0 time=1672560000 mode=0755 type=file content=bazel-out/k8-opt/bin/13project_bin

lib/tests/tar/expected14.mtree

@@ -0,0 +1,3 @@
+14project/ uid=0 gid=0 time=1672560000 mode=0755 type=dir
+14project/__main__.py uid=0 gid=0 time=1672560000 mode=0755 type=file content=bazel-out/k8-opt/bin/lib/tests/tar/14project/__main__.py
+14project_bin uid=0 gid=0 time=1672560000 mode=0755 type=file content=bazel-out/k8-opt/bin/lib/tests/tar/14project_bin