Copy over runfiles library from Bazel (#13)

This uses `root_symlinks` to preserve the fixed runfiles path at which
the library must be available and the private `skip_conflict_checking`
attribute to preserve backwards compatibility by not enabling strict
runfiles path conflict checking.

.bazelci/presubmit.yml

@@ -8,7 +8,7 @@ matrix:
     - windows
   bazel:
     - 6.5.0
-    - 7.3.2
+    - 7.4.0
 
 tasks:
   test_module_bzlmod:

WORKSPACE

@@ -0,0 +1,7 @@
+workspace(name = "rules_shell")
+
+load("//shell:repositories.bzl", "rules_shell_dependencies", "rules_shell_toolchains")
+
+rules_shell_dependencies()
+
+rules_shell_toolchains()

shell/private/root_symlinks.bzl

@@ -0,0 +1,54 @@
+# Copyright 2024 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Helper rule to preserve the legacy runfiles path for the runfiles lib."""
+
+# Requires the private skip_conflict_checking parameter on ctx.runfiles, which
+# is only available as of Bazel 7.4.0. We only use it when the native shell
+# rules are not available.
+ROOT_SYMLINKS_SUPPORTED = not hasattr(native, "sh_binary")
+
+def _single_file_or_fail(target):
+    files = target[DefaultInfo].files.to_list()
+    if len(files) != 1:
+        fail("Expected exactly one file in {}, got {}".format(target.label, files))
+    return files[0]
+
+def _root_symlinks_impl(ctx):
+    runfiles = ctx.runfiles(
+        root_symlinks = {
+            path: _single_file_or_fail(target)
+            for target, path in ctx.attr.root_symlinks.items()
+        },
+        # Adding root symlinks from Starlark usually enables conflict checking,
+        # but that would break backwards compatibility as it affects all
+        # runfiles, not just the symlinks.
+        skip_conflict_checking = True,
+    )
+    return [
+        DefaultInfo(
+            files = depset(),
+            runfiles = runfiles,
+        ),
+    ]
+
+root_symlinks = rule(
+    implementation = _root_symlinks_impl,
+    attrs = {
+        "root_symlinks": attr.label_keyed_string_dict(
+            allow_files = True,
+            mandatory = True,
+        ),
+    },
+)

shell/runfiles/BUILD

@@ -0,0 +1,22 @@
+load("//shell:sh_library.bzl", "sh_library")
+load("//shell/private:root_symlinks.bzl", "ROOT_SYMLINKS_SUPPORTED", "root_symlinks")
+
+alias(
+    name = "runfiles",
+    actual = ":runfiles_impl" if ROOT_SYMLINKS_SUPPORTED else "@bazel_tools//tools/bash/runfiles",
+    visibility = ["//visibility:public"],
+)
+
+sh_library(
+    name = "runfiles_impl",
+    data = [":runfiles_at_legacy_location"],
+    tags = ["manual"],
+)
+
+root_symlinks(
+    name = "runfiles_at_legacy_location",
+    root_symlinks = {
+        "runfiles.bash": "bazel_tools/tools/bash/runfiles/runfiles.bash",
+    },
+    tags = ["manual"],
+)