[build] Allow gitian-build.py to run on macOS

Co-authored-by: Andrew#128 <andrew-128-github@protonmail.com>

Summary
-------

As a follow-up to !1248, it turns out with some minor modifications,
it's possible to build using gitian on Docker for macOS!  This may save
time for people that run macOS natively (such as me).

In short: This MR allows the `--docker` options to work on macOS without failing.

In a future MR I will also compose some documentation about how to get
it working on macOS 100%.

For now it also needs you to use a fork of gitian-builder from here:

    https://github.com/cculianu/gitian-builder.git
    BRANCH: macos_support_no_debian_cache

There is a PR for this change that I submitted to the main
gitian-builder repo: devrandom/gitian-builder#252

Test Plan
---------

- Run gitian builder on Linux as normal to ensure that nothing broke.

**Optional:**

- If you are on a **macOS** host, get docker installed and running, check-out
  this branch, and also check-out: https://github.com/cculianu/gitian-builder.git
  branch: `macos_support_no_debian_cache` into your `gitian-builder`
  directory.
- Put the modified script from this MR on the top-level, next to your
  `bitcoin-cash-node` directory.
- Run: `./gitian-build.py --setup --docker`
- Run: `./gitian-build.py --docker -b -D -n -c -o lwm "satoshi" master`
- After a great deal of time waiting for it to rebuild the universe, you
  should have some static binaries built!

Author: cculianu

contrib/gitian-build.py

@@ -15,25 +15,34 @@
     BOLD = ('\033[0m', '\033[1m')
     RED = ('\033[0m', '\033[0;31m')
 
+is_mac = sys.platform == 'darwin'
+
 
 def setup():
     global args, workdir
-    programs = ['ruby', 'git', 'apt-cacher-ng', 'make', 'wget']
-    if args.kvm:
-        programs += ['python-vm-builder', 'qemu-kvm', 'qemu-utils']
-    elif args.docker:
-        dockers = ['docker.io', 'docker-ce']
-        for i in dockers:
-            return_code = subprocess.call(
-                ['sudo', 'apt-get', 'install', '-qq', i])
-            if return_code == 0:
-                break
-        if return_code != 0:
-            print('Cannot find any way to install docker', file=sys.stderr)
-            exit(1)
+    if not is_mac:
+        programs = ['ruby', 'git', 'apt-cacher-ng', 'make', 'wget']
+        if args.kvm:
+            programs += ['python-vm-builder', 'qemu-kvm', 'qemu-utils']
+        elif args.docker:
+            dockers = ['docker.io', 'docker-ce']
+            for i in dockers:
+                return_code = subprocess.call(
+                    ['sudo', 'apt-get', 'install', '-qq', i])
+                if return_code == 0:
+                    break
+            if return_code != 0:
+                print('Cannot find any way to install docker', file=sys.stderr)
+                exit(1)
+        else:
+            programs += ['lxc', 'debootstrap']
+        subprocess.check_call(['sudo', 'apt-get', 'install', '-qq'] + programs)
     else:
-        programs += ['lxc', 'debootstrap']
-    subprocess.check_call(['sudo', 'apt-get', 'install', '-qq'] + programs)
+        if args.docker:
+            print("Warning: macOS support is experimental and requires docker be already installed", file=sys.stderr)
+        else:
+            sys.exit("macOS support only works in --docker mode")
+
     if not os.path.isdir('gitian-builder'):
         subprocess.check_call(
             ['git', 'clone', 'https://github.com/devrandom/gitian-builder.git'])
@@ -240,7 +249,7 @@ def main():
     args = parser.parse_args()
     workdir = os.getcwd()
 
-    args.is_bionic = b'bionic' in subprocess.check_output(
+    args.is_bionic = not is_mac and b'bionic' in subprocess.check_output(
         ['lsb_release', '-cs'])
 
     if args.kvm and args.docker:

doc/gitian-building.md

@@ -17,9 +17,9 @@ More independent Gitian builders are needed, which is why this guide exists.
 It is preferred you follow these steps yourself instead of using someone else's
 VM image to avoid 'contaminating' the build.
 
-If you are running Debian or Ubuntu, see
-[Running Gitian with Docker on Ubuntu/Debian](./gitian-building/gitian-building-docker.md)
-for a relitively straightforward setup and build process.
+If you are running Debian, Ubuntu or Mac, see
+[Running Gitian with Docker](./gitian-building/gitian-building-docker.md)
+for a relatively straightforward setup and build process.
 
 The instructions below use the automated script [gitian-build.py](../contrib/gitian-build.py)
 which only works in Debian/Ubuntu. For manual steps and instructions for fully

doc/gitian-building/gitian-building-docker.md

@@ -1,28 +1,34 @@
-# Running Gitian with Docker on Ubuntu/Debian
-This is a streamlined guide for running Gitian builds with Docker on Ubuntu or
-Debian hardware.  The following steps have been tested on Ubuntu 18.04.05.
-Similar, if not the same steps should work on other versions of Ubuntu and
-Debian.
+# Running Gitian with Docker
+This is a streamlined guide for running Gitian builds with Docker on Ubuntu,
+Debian or Mac hardware.
 
 # Setup
 Ensure you have Docker installed.  See https://docs.docker.com/get-docker/ for
 installation instructions.
 
-Make sure to go through the Linux post-install walkthrough, especially the
+If you're using Linux, make sure to go through the Linux post-install
+walkthrough, especially the
 [Manage Docker as a non-root user](https://docs.docker.com/engine/install/linux-postinstall/#manage-docker-as-a-non-root-user)
 section, to avoid having to use `sudo` all the time.
 
-Prepare a workspace directory (e.g. `~/bchn-gitian`) and `cd` into it.  You'll
+## Prepare a build workspace
+Create a workspace directory (e.g. `~/bchn-gitian`) and `cd` into it.  You'll
 only need to run through this setup once so long as you retain the workspace.
 
-```bash
-# Install dependencies
-sudo apt install curl git
+## Install dependencies
+You'll need `git` and `curl` installed.  If on Linux, just do `sudo apt install
+curl git`.
 
+## Gitian setup
+```bash
 # Fetch the `gitian-build.py` script
 git clone https://gitlab.com/bitcoin-cash-node/bitcoin-cash-node.git
 cp bitcoin-cash-node/contrib/gitian-build.py .
 
+# If you are on a MacOS host, you will need the MacOS-capable fork
+# of gitian-builder, if on Linux, you can skip this step.
+git clone -b macos_support_no_debian_cache https://github.com/cculianu/gitian-builder.git  
+
 # Run the initial Gitian setup
 ./gitian-build.py --docker --setup