Make --setup command independent

A user can run 'gitian-build.py --setup' ignoring signer
and version options.
Get rid of warning about macOS build during setup for free.

contrib/gitian-build.py

@@ -152,7 +152,7 @@ def verify():
 def main():
     global args, workdir
 
-    parser = argparse.ArgumentParser(usage='%(prog)s [options] signer version')
+    parser = argparse.ArgumentParser(description='Script for running full Gitian builds.')
     parser.add_argument('-c', '--commit', action='store_true', dest='commit', help='Indicate that the version argument is for a commit or branch')
     parser.add_argument('-p', '--pull', action='store_true', dest='pull', help='Indicate that the version argument is the number of a github repository pull request')
     parser.add_argument('-u', '--url', dest='url', default='https://github.com/bitcoin/bitcoin', help='Specify the URL of the repository. Default is %(default)s')
@@ -168,27 +168,17 @@ def main():
     parser.add_argument('-S', '--setup', action='store_true', dest='setup', help='Set up the Gitian building environment. Only works on Debian-based systems (Ubuntu, Debian)')
     parser.add_argument('-D', '--detach-sign', action='store_true', dest='detach_sign', help='Create the assert file for detached signing. Will not commit anything.')
     parser.add_argument('-n', '--no-commit', action='store_false', dest='commit_files', help='Do not commit anything to git')
-    parser.add_argument('signer', help='GPG signer to sign each build assert file')
+    parser.add_argument('signer', nargs='?', help='GPG signer to sign each build assert file')
-    parser.add_argument('version', help='Version number, commit, or branch to build. If building a commit or branch, the -c option must be specified')
+    parser.add_argument('version', nargs='?', help='Version number, commit, or branch to build. If building a commit or branch, the -c option must be specified')
 
     args = parser.parse_args()
     workdir = os.getcwd()
 
-    args.linux = 'l' in args.os
-    args.windows = 'w' in args.os
-    args.macos = 'm' in args.os
-
     args.is_bionic = b'bionic' in subprocess.check_output(['lsb_release', '-cs'])
 
-    if args.buildsign:
-        args.build = True
-        args.sign = True
-
     if args.kvm and args.docker:
         raise Exception('Error: cannot have both kvm and docker')
 
-    args.sign_prog = 'true' if args.detach_sign else 'gpg --detach-sign'
-
     # Ensure no more than one environment variable for gitian-builder (USE_LXC, USE_VBOX, USE_DOCKER) is set as they
     # can interfere (e.g., USE_LXC being set shadows USE_DOCKER; for details see gitian-builder/libexec/make-clean-vm).
     os.environ['USE_LXC'] = ''
@@ -203,19 +193,34 @@ def main():
         if 'LXC_GUEST_IP' not in os.environ.keys():
             os.environ['LXC_GUEST_IP'] = '10.0.3.5'
 
+    if args.setup:
+        setup()
+
+    if args.buildsign:
+        args.build = True
+        args.sign = True
+
+    if not args.build and not args.sign and not args.verify:
+        sys.exit(0)
+
+    args.linux = 'l' in args.os
+    args.windows = 'w' in args.os
+    args.macos = 'm' in args.os
+
     # Disable for MacOS if no SDK found
     if args.macos and not os.path.isfile('gitian-builder/inputs/MacOSX10.11.sdk.tar.gz'):
         print('Cannot build for MacOS, SDK does not exist. Will build for other OSes')
         args.macos = False
 
+    args.sign_prog = 'true' if args.detach_sign else 'gpg --detach-sign'
+
     script_name = os.path.basename(sys.argv[0])
-    # Signer and version shouldn't be empty
+    if not args.signer:
-    if args.signer == '':
+        print(script_name+': Missing signer')
-        print(script_name+': Missing signer.')
         print('Try '+script_name+' --help for more information')
         sys.exit(1)
-    if args.version == '':
+    if not args.version:
-        print(script_name+': Missing version.')
+        print(script_name+': Missing version')
         print('Try '+script_name+' --help for more information')
         sys.exit(1)
 
@@ -224,9 +229,6 @@ def main():
         raise Exception('Cannot have both commit and pull')
     args.commit = ('' if args.commit else 'v') + args.version
 
-    if args.setup:
-        setup()
-
     os.chdir('bitcoin')
     if args.pull:
         subprocess.check_call(['git', 'fetch', args.url, 'refs/pull/'+args.version+'/merge'])