[build-script] Introduce --dry-run mode

utils/SwiftBuildSupport.py

@@ -67,6 +67,7 @@ def _get_default_source_root():
 
 def print_with_argv0(message):
     print(sys.argv[0] + ": " + message)
+    sys.stdout.flush()
 
 
 def quote_shell_command(args):
@@ -82,6 +83,7 @@ def check_call(args, print_command=False, verbose=False, disable_sleep=False):
 
     if print_command:
         print(os.getcwd() + "$ " + quote_shell_command(args))
+        sys.stdout.flush()
     try:
         return subprocess.check_call(args)
     except subprocess.CalledProcessError as e:
@@ -101,6 +103,7 @@ def check_call(args, print_command=False, verbose=False, disable_sleep=False):
 def check_output(args, print_command=False, verbose=False):
     if print_command:
         print(os.getcwd() + "$ " + quote_shell_command(args))
+        sys.stdout.flush()
     try:
         return subprocess.check_output(args)
     except subprocess.CalledProcessError as e:

utils/build-script

@@ -57,6 +57,12 @@ def main_preset():
     parser = argparse.ArgumentParser(
         formatter_class=argparse.RawDescriptionHelpFormatter,
         description="""Builds Swift using a preset.""")
+    parser.add_argument(
+        "-n", "--dry-run",
+        help="print the commands that would be executed, but do not execute "
+             "them",
+        action="store_true",
+        default=False)
     parser.add_argument(
         "--preset-file",
         help="load presets from the specified file",
@@ -110,6 +116,8 @@ def main_preset():
         args.preset_substitutions, args.preset_file_names, args.preset)
 
     build_script_args = [sys.argv[0]]
+    if args.dry_run:
+        build_script_args += ["--dry-run"]
     build_script_args += preset_args
     if args.distcc:
         build_script_args += ["--distcc"]
@@ -306,6 +314,13 @@ It is a policy decision aimed at making the builds uniform across all
 environments and easily reproducible by engineers who are not familiar with the
 details of the setups of other systems or automated environments.""")
 
+    parser.add_argument(
+        "-n", "--dry-run",
+        help="print the commands that would be executed, but do not execute "
+             "them",
+        action="store_true",
+        default=False)
+
     targets_group = parser.add_argument_group(
         title="Host and cross-compilation targets")
     targets_group.add_argument(
@@ -968,6 +983,8 @@ details of the setups of other systems or automated environments.""")
         if '--check-args-only' in args.build_script_impl_args:
             return 0
 
+    shell.dry_run = args.dry_run
+
     # Prepare and validate toolchain
     toolchain = host_toolchain(xcrun_toolchain=args.darwin_xcrun_toolchain)
 
@@ -1419,6 +1436,9 @@ details of the setups of other systems or automated environments.""")
 
     build_script_impl_args += args.build_script_impl_args
 
+    if args.dry_run:
+        build_script_impl_args += ["--dry-run"]
+
     check_call([build_script_impl] + build_script_impl_args,
                disable_sleep=True)