Add support for building Go for Android.

go/mobile_env.py

+#!/usr/bin/env python
+# Copyright 2014 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"""Like env.py, but for building go-on-mobile.
+
+It wraps env.py, runs `gomobile init`, and adds mobile-specific env vars.
+"""
+
+import os
+import pipes
+import platform
+import subprocess
+import sys
+
+
+ANDROID_NDK_PATH = os.path.join(
+    os.path.abspath(os.path.dirname(__file__)), '.vendor', 'pkg', 'gomobile',
+    'android-ndk-r12b')

[iannucci, 2016/08/10 01:40:36]

is this path variable? Is there a way we could extract it from the gomobile tool
and then record it to a file inside of .vendor?

[bpastene, 2016/08/10 22:56:06]

It would probably change as frequently as chromium changes its ndk (once a
year).

The command `gomobile version` should return the relevant information, but it's
complaining that "binary is out of date" for strange reasons, so I just went
with hard coding the path there.

+
+# Used for mapping GOARCH values to NDK toolsets.
+ANDROID_TOOLSETS = {
+  'arm': 'arm',
+  'arm64': 'aarch64',
+  '386': 'x86',
+  'amd64': 'x86_64',
+}
+
+
+def _get_android_env(env):
+  """Adds GO* and C* environment variables needed for building for android."""
+  if platform.system() != 'Linux':
+    raise Exception(
+        'Only Linux hosts supported for android cross-compiling')
+
+  arch = env.get('GOARCH')

[iannucci, 2016/08/10 01:40:35]

does env.py set this? If not, this would be a very atypical thing to have in an
environment.

[bpastene, 2016/08/10 22:56:06]

I'd say GOARCH is a very typical thing to have in your environment when
cross-compiling. It's explicitly set in the one place in our infra that
cross-compiles go:
https://chromium.googlesource.com/infra/infra/+/master/recipes/recipes/infra_...

+  if arch not in ANDROID_TOOLSETS:
+    raise Exception(
+        'Specified arch not currently supported on android: %s' % arch)
+
+  toolset = ANDROID_TOOLSETS[arch]
+
+  # Needed when cross-compiling uses of cgo.
+  env['CGO_ENABLED'] = '1'
+  env['CC'] = os.path.join(
+      ANDROID_NDK_PATH, arch, 'bin', '%s-linux-androideabi-clang' % toolset)
+  env['CXX'] = os.path.join(
+      ANDROID_NDK_PATH, arch, 'bin', '%s-linux-androideabi-clang++' % toolset)
+
+  # Compiler/linker needs access to android system headers in the NDK.
+  sysroot_path = os.path.join(ANDROID_NDK_PATH, arch, 'sysroot')
+  env['CGO_CFLAGS'] = '-I %s/usr/include --sysroot %s' % (
+      sysroot_path, sysroot_path)
+  env['CGO_LDFLAGS'] = '-L %s/usr/lib --sysroot %s' % (
+      sysroot_path, sysroot_path)
+
+
+# Run `gomobile init` to fetch the android NDK.
+if not os.path.exists(ANDROID_NDK_PATH):

[iannucci, 2016/08/10 01:40:35]

This should be in a `main` function with the `if __name__ == '__main__'` dance
at the bottom. I see that env.py doesn't do this for some reason, but it's
kind-of dangerous/annoying. Right now if you import this python file from
another one, this code will run, which is pretty icky.

[Vadim Sh., 2016/08/10 01:51:01]

env.py does "assert __name__ == '__main__'" on top :) 

[bpastene, 2016/08/10 22:56:06]

Added it here anyway.

+  cmd = [sys.executable, 'env.py', 'gomobile', 'init']
+  subprocess.check_call(cmd)
+
+# Keep track of any changed env vars for printing to stdout later.
+old = os.environ.copy()
+new = os.environ.copy()
+if 'android' in old.get('GOOS', ''):
+  _get_android_env(new)
+
+if len(sys.argv) == 1:
+  cmd = [sys.executable, 'env.py']
+  print subprocess.check_output(cmd, env=new).strip()

[iannucci, 2016/08/10 01:40:35]

we should set cwd == our directory here.

[bpastene, 2016/08/10 22:56:06]

Done.

+  for key, value in sorted(new.iteritems()):
+    if old.get(key) != value:
+      print 'export %s=%s' % (key, pipes.quote(value))
+else:
+  cmd = [sys.executable, 'env.py']
+  cmd.extend(sys.argv[1:])
+  sys.exit(subprocess.call(cmd, env=new))