Add support for Bazel's sharding env variables

Closes #2491

tests/ExtraTests/CMakeLists.txt

@@ -161,6 +161,18 @@ set_tests_properties(BazelEnv::TESTBRIDGE_TEST_ONLY
 )
 
 
+add_test(NAME BazelEnv::Sharding
+  COMMAND
+    "${PYTHON_EXECUTABLE}" "${CATCH_DIR}/tests/TestScripts/testBazelSharding.py"
+      $<TARGET_FILE:BazelReporterNoCatchConfig>
+      "${CMAKE_CURRENT_BINARY_DIR}"
+)
+set_tests_properties(BazelEnv::Sharding
+  PROPERTIES
+    LABELS "uses-python"
+)
+
+
 # The default handler on Windows leads to the just-in-time debugger firing,
 # which makes this test unsuitable for CI and headless runs, as it opens
 # up an interactive dialog.

tests/TestScripts/testBazelSharding.py

@@ -0,0 +1,75 @@
+#!/usr/bin/env python3
+
+#              Copyright Catch2 Authors
+# Distributed under the Boost Software License, Version 1.0.
+#   (See accompanying file LICENSE_1_0.txt or copy at
+#        https://www.boost.org/LICENSE_1_0.txt)
+
+# SPDX-License-Identifier: BSL-1.0
+
+import os
+import re
+import sys
+import subprocess
+
+"""
+Test that Catch2 recognizes the three sharding-related environment variables
+and responds accordingly (running only the selected shard, creating the
+response file, etc).
+
+Requires 2 arguments, path to Catch2 binary to run and the output directory
+for the output file.
+"""
+if len(sys.argv) != 3:
+    print("Wrong number of arguments: {}".format(len(sys.argv)))
+    print("Usage: {} test-bin-path output-dir".format(sys.argv[0]))
+    exit(1)
+
+
+bin_path = os.path.abspath(sys.argv[1])
+output_dir = os.path.abspath(sys.argv[2])
+info_file_path = os.path.join(output_dir, '{}.shard-support'.format(os.path.basename(bin_path)))
+
+# Ensure no file exists from previous test runs
+if os.path.isfile(info_file_path):
+    os.remove(info_file_path)
+
+print('bin path:', bin_path)
+print('shard confirmation path:', info_file_path)
+
+env = os.environ.copy()
+# We will run only one shard, and it should have the passing test.
+# This simplifies our work a bit, and if we have any regression in this
+# functionality we can make more complex tests later.
+env["BAZEL_TEST"] = "1"
+env["TEST_SHARD_INDEX"] = "0"
+env["TEST_TOTAL_SHARDS"] = "2"
+env["TEST_SHARD_STATUS_FILE"] = info_file_path
+
+
+try:
+    ret = subprocess.run(
+        bin_path,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+        check=True,
+        universal_newlines=True,
+        env=env
+    )
+    stdout = ret.stdout
+except subprocess.SubprocessError as ex:
+    print('Could not run "{}"'.format(bin_path))
+    print("Return code: {}".format(ex.returncode))
+    print("stdout: {}".format(ex.stdout))
+    print("stderr: {}".format(ex.stderr))
+    raise
+
+
+if not "All tests passed (1 assertion in 1 test case)" in stdout:
+    print("Did not find expected output in stdout.")
+    print("stdout:\n{}".format(stdout))
+    exit(1)
+
+if not os.path.isfile(info_file_path):
+    print("Catch2 did not create expected file at path '{}'".format(info_file_path))
+    exit(2)