fix: correctly find runfiles root for symlinks (#2665)

`$maybe_runfiles_root` doesn't seem to be a real variable. Based on the
presence of the `while` loop, it seems that this code wants to try
resolving the symlink one level at a time (`readlink`, not `realpath`)
until it can find runfiles?

---------

Co-authored-by: Ignas Anikevicius <[email protected]>

Author: lpulley

CHANGELOG.md

@@ -57,6 +57,7 @@ Unreleased changes template.
 
 {#v0-0-0-fixed}
 ### Fixed
+* (runfiles) ({obj}`--bootstrap_impl=script`) Follow symlinks when searching for runfiles.
 * Do not try to run `chmod` when downloading non-windows hermetic toolchain
   repositories on Windows. Fixes
   [#2660](https://github.com/bazel-contrib/rules_python/issues/2660).

python/private/stage1_bootstrap_template.sh

@@ -81,8 +81,7 @@ else
       if [[ ! -L "$stub_filename" ]]; then
         break
       fi
-      target=$(realpath $maybe_runfiles_root)
-      stub_filename="$target"
+      stub_filename=$(readlink $stub_filename)
     done
     echo >&2 "Unable to find runfiles directory for $1"
     exit 1

tests/bootstrap_impls/BUILD.bazel

@@ -70,6 +70,13 @@ sh_py_run_test(
     venvs_use_declare_symlink = "no",
 )
 
+sh_py_run_test(
+    name = "run_binary_find_runfiles_test",
+    py_src = "bin.py",
+    sh_src = "run_binary_find_runfiles_test.sh",
+    target_compatible_with = SUPPORTS_BOOTSTRAP_SCRIPT,
+)
+
 sh_py_run_test(
     name = "run_binary_bootstrap_script_zip_yes_test",
     bootstrap_impl = "script",
@@ -88,6 +95,14 @@ sh_py_run_test(
     target_compatible_with = SUPPORTS_BOOTSTRAP_SCRIPT,
 )
 
+sh_py_run_test(
+    name = "run_binary_bootstrap_script_find_runfiles_test",
+    bootstrap_impl = "script",
+    py_src = "bin.py",
+    sh_src = "run_binary_find_runfiles_test.sh",
+    target_compatible_with = SUPPORTS_BOOTSTRAP_SCRIPT,
+)
+
 py_reconfig_test(
     name = "sys_path_order_bootstrap_script_test",
     srcs = ["sys_path_order_test.py"],

tests/bootstrap_impls/run_binary_find_runfiles_test.sh

@@ -0,0 +1,59 @@
+# Copyright 2023 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.
+
+# --- begin runfiles.bash initialization v3 ---
+# Copy-pasted from the Bazel Bash runfiles library v3.
+set -uo pipefail; set +e; f=bazel_tools/tools/bash/runfiles/runfiles.bash
+source "${RUNFILES_DIR:-/dev/null}/$f" 2>/dev/null || \
+  source "$(grep -sm1 "^$f " "${RUNFILES_MANIFEST_FILE:-/dev/null}" | cut -f2- -d' ')" 2>/dev/null || \
+  source "$0.runfiles/$f" 2>/dev/null || \
+  source "$(grep -sm1 "^$f " "$0.runfiles_manifest" | cut -f2- -d' ')" 2>/dev/null || \
+  source "$(grep -sm1 "^$f " "$0.exe.runfiles_manifest" | cut -f2- -d' ')" 2>/dev/null || \
+  { echo>&2 "ERROR: cannot find $f"; exit 1; }; f=; set -e
+# --- end runfiles.bash initialization v3 ---
+set +e
+
+bin=$(rlocation $BIN_RLOCATION)
+if [[ -z "$bin" ]]; then
+  echo "Unable to locate test binary: $BIN_RLOCATION"
+  exit 1
+fi
+
+bin_link_layer_1=$TEST_TMPDIR/link1
+ln -s "$bin" "$bin_link_layer_1"
+bin_link_layer_2=$TEST_TMPDIR/link2
+ln -s "$bin_link_layer_1" "$bin_link_layer_2"
+
+result=$(RUNFILES_DIR='' RUNFILES_MANIFEST_FILE='' $bin)
+result_link_layer_1=$(RUNFILES_DIR='' RUNFILES_MANIFEST_FILE='' $bin_link_layer_1)
+result_link_layer_2=$(RUNFILES_DIR='' RUNFILES_MANIFEST_FILE='' $bin_link_layer_2)
+
+if [[ "$result" != "$result_link_layer_1" ]]; then
+    echo "Output from test does not match output when invoked via a link;"
+    echo "Output from test:"
+    echo "$result"
+    echo "Output when invoked via a link:"
+    echo "$result_link_layer_1"
+    exit 1
+fi
+if [[ "$result" != "$result_link_layer_2" ]]; then
+    echo "Output from test does not match output when invoked via a link to a link;"
+    echo "Output from test:"
+    echo "$result"
+    echo "Output when invoked via a link to a link:"
+    echo "$result_link_layer_2"
+    exit 1
+fi
+
+exit 0