[build-script] Respect TOOLCHAINS environment variable (#14202) (#14285)

* Respect the TOOLCHAINS environment variable if set when deciding the darwin xcrun toolchain.

* Updated test/lit.cfg to respect the TOOLCHAINS environment variable when deciding the xcrun toolchain for Darwin platforms, rather than hardcoding the default.

* Fixed the default darwin xcrun toolchain logic.

(cherry picked from commit a436044)

Author: Rostepher

test/lit.cfg

@@ -152,10 +152,11 @@ if config.test_exec_root is None:
 # name: The name of this test suite.
 config.name = 'Swift(%s)' % config.variant_suffix[1:]
 
-# Tweak the environment appropriately for various platforms.
+# Respect the TOOLCHAINS environment variable when deciding the xcrun
+# toolchain for Darwin platforms.
 if platform.system() == 'Darwin':
-    # Prefer the latest version of the Xcode tools.
-    config.environment['TOOLCHAINS'] = 'default'
+    config.environment['TOOLCHAINS'] = \
+        os.environ.get('TOOLCHAINS', config.darwin_xcrun_toolchain)
 
 # testFormat: The test format to use to interpret tests.
 config.test_format = swift_test.SwiftTest(coverage_mode=config.coverage_mode)

utils/build-script

@@ -18,6 +18,7 @@ import platform
 import sys
 import time
 
+from build_swift import defaults
 from build_swift import driver_arguments
 
 from swift_build_support.swift_build_support import (
@@ -1041,6 +1042,13 @@ def main_normal():
     shell.dry_run = args.dry_run
 
     # Prepare and validate toolchain
+    if args.darwin_xcrun_toolchain is None:
+        xcrun_toolchain = os.environ.get('TOOLCHAINS',
+                                         defaults.DARWIN_XCRUN_TOOLCHAIN)
+
+        diagnostics.note('Using toolchain {}'.format(xcrun_toolchain))
+        args.darwin_xcrun_toolchain = xcrun_toolchain
+
     toolchain = host_toolchain(xcrun_toolchain=args.darwin_xcrun_toolchain)
     os.environ['TOOLCHAINS'] = args.darwin_xcrun_toolchain
 

utils/build_swift/driver_arguments.py

@@ -314,7 +314,6 @@ def create_argument_parser():
            help='the number of parallel build jobs to use')
 
     option('--darwin-xcrun-toolchain', store,
-           default=defaults.DARWIN_XCRUN_TOOLCHAIN,
            help='the name of the toolchain to use on Darwin')
     option('--cmake', store_path(executable=True),
            help='the path to a CMake executable that will be used to build '

utils/build_swift/tests/expected_options.py

@@ -107,8 +107,7 @@
         defaults.DARWIN_DEPLOYMENT_VERSION_TVOS,
     'darwin_deployment_version_watchos':
         defaults.DARWIN_DEPLOYMENT_VERSION_WATCHOS,
-    'darwin_xcrun_toolchain':
-        defaults.DARWIN_XCRUN_TOOLCHAIN,
+    'darwin_xcrun_toolchain': None,
     'distcc': False,
     'dry_run': False,
     'enable_asan': False,