Teach build.py to cross-compile go-based packages.

build/build.py

Index: build/build.py
diff --git a/build/build.py b/build/build.py
index cb280ab64ad71267c0f068e8838b42c30cb61c93..ebc02e9df8c904b0d809445bd0eefd6fe0acecf9 100755
--- a/build/build.py
+++ b/build/build.py
@@ -7,15 +7,18 @@
 invokes CIPD client to package and upload chunks of it to the CIPD repository as
 individual packages.
 
-See build/packages/*.yaml for definition of packages.
+See build/packages/*.yaml for definition of packages and README.md for more
+details.
 """
 
 import argparse
+import contextlib
 import glob
+import hashlib
 import json
 import os
 import platform
-import shutil
+import socket
 import subprocess
 import sys
 import tempfile
@@ -42,6 +45,56 @@ class UploadException(Exception):
   """Raised on errors during package upload step."""
 
 
+class PackageDef(object):
+  """Represents parsed package *.yaml file."""
+
+  def __init__(self, path, pkg_def):
+    self.path = path
+    self.pkg_def = pkg_def
+
+  @property
+  def name(self):
+    """Returns name of YAML file (without the directory path and extension)."""
+    return os.path.splitext(os.path.basename(self.path))[0]
+
+  @property
+  def uses_python_env(self):
+    """Returns True if 'uses_python_env' in the YAML file is set."""
+    return bool(self.pkg_def.get('uses_python_env'))
+
+  @property
+  def go_packages(self):
+    """Returns a list of Go packages that must be installed for this package."""
+    return self.pkg_def.get('go_packages') or []
+
+  def should_build(self, builder):
+    """Returns True if package should be built in the current environment.
+
+    Takes into account 'builders' and 'supports_cross_compilation' properties of
+    the package definition file.
+    """
+    # If '--builder' is not specified, ignore 'builders' property. Otherwise, if
+    # 'builders' YAML attribute it not empty, verify --builder is listed there.
+    builders = self.pkg_def.get('builders')
+    if builder and builders and builder not in builders:
+      return False
+
+    # If cross-compiling, pick only packages that support cross-compilation.
+    if is_cross_compiling():
+      return bool(self.pkg_def.get('supports_cross_compilation'))
+
+    return True
+
+
+def is_cross_compiling():
+  """Returns True if using GOOS or GOARCH env vars.
+
+  We also check at the start of the script that if one of them is used, then
+  the other is specified as well.
+  """
+  return bool(os.environ.get('GOOS')) or bool(os.environ.get('GOARCH'))
+
+
 def run_python(script, args):
   """Invokes a python script.
 
@@ -53,11 +106,11 @@ def run_python(script, args):
       args=['python', '-u', script] + list(args), executable=sys.executable)
 
 
-def run_cipd(go_workspace, cmd, args):
+def run_cipd(cipd_exe, cmd, args):
   """Invokes CIPD, parsing -json-output result.
 
   Args:
-    go_workspace: path to 'infra/go' or 'infra_internal/go'.
+    cipd_exe: path to cipd client binary to run.
     cmd: cipd subcommand to run.
     args: list of command line arguments to pass to the subcommand.
 
@@ -69,10 +122,7 @@ def run_cipd(go_workspace, cmd, args):
     fd, temp_file = tempfile.mkstemp(suffix='.json', prefix='cipd_%s' % cmd)
     os.close(fd)
 
-    cmd_line = [
-      os.path.join(go_workspace, 'bin', 'cipd' + EXE_SUFFIX),
-      cmd, '-json-output', temp_file,
-    ] + list(args)
+    cmd_line = [cipd_exe, cmd, '-json-output', temp_file] + list(args)
 
     print 'Running %s' % ' '.join(cmd_line)
     exit_code = subprocess.call(args=cmd_line, executable=cmd_line[0])
@@ -101,20 +151,32 @@ def print_title(title):
   print '-' * 80
 
 
-def build_go(go_workspace, packages):
-  """Bootstraps go environment and rebuilds (and installs) Go packages.
+def print_go_step_title(title):
+  """Same as 'print_title', but also appends values of GOOS and GOARCH."""
+  if is_cross_compiling():
+    title += '\n' + '-' * 80
+    title += '\n  GOOS=%s' % os.environ['GOOS']
+    title += '\n  GOARCH=%s' % os.environ['GOARCH']
+    if 'GOARM' in os.environ:
+      title += '\n  GOARM=%s' % os.environ['GOARM']
+  print_title(title)
 
-  Compiles and installs packages into default GOBIN, which is <path>/go/bin/
-  (it is setup by go/env.py and depends on what workspace is used).
+
+@contextlib.contextmanager
+def hacked_workspace(go_workspace, goos=None, goarch=None):
+  """Symlinks Go workspace into new root, modifies os.environ.
+
+  Go toolset embeds absolute paths to *.go files into the executable. Use
+  symlink with stable path to make executables independent of checkout path.
 
   Args:
     go_workspace: path to 'infra/go' or 'infra_internal/go'.
-    packages: list of packages to build (can include '...' patterns).
-  """
-  print_title('Compiling Go code: %s' % ', '.join(packages))
+    goos: if set, overrides GOOS environment variable (removes it if '').
+    goarch: if set, overrides GOARCH environment variable (removes it if '').
 
-  # Go toolchain embeds absolute paths to *.go files into the executable. Use
-  # symlink with stable path to make executables independent of checkout path.
+  Yields:
+    Path where go_workspace is symlinked to.
+  """
   new_root = None
   new_workspace = go_workspace
   if sys.platform != 'win32':
@@ -127,49 +189,222 @@ def build_go(go_workspace, packages):
     assert not rel.startswith('..'), rel
     new_workspace = os.path.join(new_root, rel)
 
-  # Remove any stale binaries and libraries.
-  shutil.rmtree(os.path.join(new_workspace, 'bin'), ignore_errors=True)
-  shutil.rmtree(os.path.join(new_workspace, 'pkg'), ignore_errors=True)
+  orig_environ = os.environ.copy()
+
+  if goos is not None:
+    if goos == '':
+      os.environ.pop('GOOS', None)
+    else:
+      os.environ['GOOS'] = goos
+  if goarch is not None:
+    if goarch == '':
+      os.environ.pop('GOARCH', None)
+    else:
+      os.environ['GOARCH'] = goarch
+
+  # Make sure we build ARMv6 code even if the host is ARMv7. See the comment in
+  # get_host_package_vars for reasons why. Also explicitly set GOARM to 6 when
+  # cross-compiling (it should be '6' in this case by default anyway).
+  plat = platform.machine().lower()
+  if plat.startswith('arm') or os.environ.get('GOARCH') == 'arm':
+    os.environ['GOARM'] = '6'
+  else:
+    os.environ.pop('GOARM', None)
 
-  # Recompile ('-a').
   try:
+    yield new_workspace
+  finally:
+    # Apparently 'os.environ = orig_environ' doesn't actually modify process
+    # environment, only modifications of os.environ object itself do.
+    for k, v in orig_environ.iteritems():
+      os.environ[k] = v
+    for k in os.environ.keys():
+      if k not in orig_environ:
+        os.environ.pop(k)
+    if new_root:
+      os.remove(new_root)
+
+
+def bootstrap_go_toolset(go_workspace):
+  """Makes sure go toolset is installed and returns its 'go env' environment.
+
+  Used to verify that our platform detection in get_host_package_vars() matches
+  the Go toolset being used.
+  """
+  with hacked_workspace(go_workspace) as new_workspace:
+    print_go_step_title('Making sure Go toolset is installed')
+    # env.py does the actual job of bootstrapping if the toolset is missing.
+    output = subprocess.check_output(
+        args=[
+          'python', '-u', os.path.join(new_workspace, 'env.py'),
+          'go', 'env',
+        ],
+        executable=sys.executable)
+    # See https://github.com/golang/go/blob/master/src/cmd/go/env.go for format
+    # of the output.
+    print 'Go environ:'
+    print output.strip()
+    env = {}
+    for line in output.splitlines():
+      k, _, v = line.lstrip('set ').partition('=')
+      if v.startswith('"') and v.endswith('"'):
+        v = v.strip('"')
+      env[k] = v
+    return env
+
+
+def run_go_clean(go_workspace, packages, goos=None, goarch=None):
+  """Removes object files and executables left from building given packages.
+
+  Transitively cleans all dependencies (including stdlib!) and removes
+  executables from GOBIN.
+
+  Args:
+    go_workspace: path to 'infra/go' or 'infra_internal/go'.
+    packages: list of go packages to clean (can include '...' patterns).
+    goos: if set, overrides GOOS environment variable (removes it if '').
+    goarch: if set, overrides GOARCH environment variable (removes it if '').
+  """
+  with hacked_workspace(go_workspace, goos, goarch) as new_workspace:
+    print_go_step_title('Cleaning:\n  %s' % '\n  '.join(packages))
     subprocess.check_call(
         args=[
           'python', '-u', os.path.join(new_workspace, 'env.py'),
-          'go', 'install', '-a', '-v',
+          'go', 'clean', '-i', '-r',
         ] + list(packages),
         executable=sys.executable,
         stderr=subprocess.STDOUT)
-  finally:
-    if new_root:
-      os.remove(new_root)
+    # Above command is either silent (without '-x') or too verbose (with '-x').
+    # Prefer silent version, but add a note that it's alright.
+    print 'Done.'
+
 
+def run_go_install(
+    go_workspace, packages, rebuild=False, goos=None, goarch=None):
+  """Builds (and installs) Go packages into GOBIN via 'go install ...'.
 
-def enumerate_packages_to_build(package_def_dir, package_def_files=None):
-  """Returns a list of absolute paths to files in build/packages/*.yaml.
+  Compiles and installs packages into default GOBIN, which is <go_workspace>/bin
+  (it is setup by go/env.py).
 
   Args:
+    go_workspace: path to 'infra/go' or 'infra_internal/go'.
+    packages: list of go packages to build (can include '...' patterns).
+    rebuild: if True, will forcefully rebuild all dependences.
+    goos: if set, overrides GOOS environment variable (removes it if '').
+    goarch: if set, overrides GOARCH environment variable (removes it if '').
+  """
+  rebuild_opt = ['-a'] if rebuild else []
+  title = 'Rebuilding' if rebuild else 'Building'
+  with hacked_workspace(go_workspace, goos, goarch) as new_workspace:
+    print_go_step_title('%s:\n  %s' % (title, '\n  '.join(packages)))
+    subprocess.check_call(
+        args=[
+          'python', '-u', os.path.join(new_workspace, 'env.py'),
+          'go', 'install', '-v',
+        ] + rebuild_opt + list(packages),
+        executable=sys.executable,
+        stderr=subprocess.STDOUT)
+
+
+def run_go_build(
+    go_workspace, package, output, rebuild=False, goos=None, goarch=None):
+  """Builds single Go package.
+
+  Args:
+    go_workspace: path to 'infra/go' or 'infra_internal/go'.
+    package: go package to build.
+    output: where to put the resulting binary.
+    rebuild: if True, will forcefully rebuild all dependences.
+    goos: if set, overrides GOOS environment variable (removes it if '').
+    goarch: if set, overrides GOARCH environment variable (removes it if '').
+  """
+  rebuild_opt = ['-a'] if rebuild else []
+  title = 'Rebuilding' if rebuild else 'Building'
+  with hacked_workspace(go_workspace, goos, goarch) as new_workspace:
+    print_go_step_title('%s %s' % (title, package))
+    subprocess.check_call(
+        args=[
+          'python', '-u', os.path.join(new_workspace, 'env.py'),
+          'go', 'build',
+        ] + rebuild_opt + ['-v', '-o', output, package],
+        executable=sys.executable,
+        stderr=subprocess.STDOUT)
+
+
+def build_go_code(go_workspace, pkg_defs):
+  """Builds and installs all Go packages used by the given PackageDefs.
+
+  Understands GOOS and GOARCH and uses slightly different build strategy when
+  cross-compiling. In the end <go_workspace>/bin will have all built binaries,
+  and only them (regardless of whether we are cross-compiling or not).
+
+  Args:
+    go_workspace: path to 'infra/go' or 'infra_internal/go'.
+    pkg_defs: list of PackageDef objects that define what to build.
+  """
+  # Grab a set of all go packages we need to build and install into GOBIN.
+  to_install = []
+  for p in pkg_defs:
+    to_install.extend(p.go_packages)
+  to_install = sorted(set(to_install))
+  if not to_install:
+    return
+
+  # Make sure there are no stale files in the workspace.
+  run_go_clean(go_workspace, to_install)
+
+  if not is_cross_compiling():
+    # If not cross-compiling, build all Go code in a single "go install" step,
+    # it's faster that way. We can't do that when cross-compiling, since
+    # 'go install' isn't supposed to be used for cross-compilation and the
+    # toolset actively complains with "go install: cannot install cross-compiled
+    # binaries when GOBIN is set".
+    run_go_install(go_workspace, to_install)
+  else:
+    # Prebuild stdlib once. 'go build' calls below are discarding build results,
+    # so it's better to install as much shared stuff as possible beforehand.
+    run_go_install(go_workspace, ['std'])
+
+    # Build packages one by one and put the resulting binaries into GOBIN, as if
+    # they were installed there. It's where the rest of the build.py code
+    # expects them to be (see also 'root' property in package definition YAMLs).
+    go_bin = os.path.join(go_workspace, 'bin')
+    exe_suffix = get_target_package_vars()['exe_suffix']
+    for pkg in to_install:
+      name = pkg[pkg.rfind('/')+1:]
+      run_go_build(go_workspace, pkg, os.path.join(go_bin, name + exe_suffix))
+
+
+def enumerate_packages(py_venv, package_def_dir, package_def_files):
+  """Returns a list PackageDef instances for files in build/packages/*.yaml.
+
+  Args:
+    py_env: path to python ENV where to look for YAML parser.
     package_def_dir: path to build/packages dir to search for *.yaml.
     package_def_files: optional list of filenames to limit results to.
 
   Returns:
-    List of absolute paths to *.yaml files under packages_dir.
+    List of PackageDef instances parsed from *.yaml files under packages_dir.
   """
-  # All existing package by default.
-  if not package_def_files:
-    return sorted(glob.glob(os.path.join(package_def_dir, '*.yaml')))
   paths = []
-  for name in package_def_files:
-    abs_path = os.path.join(package_def_dir, name)
-    if not os.path.isfile(abs_path):
-      raise BuildException('No such package definition file: %s' % name)
-    paths.append(abs_path)
-  return sorted(paths)
+  if not package_def_files:
+    # All existing package by default.
+    paths = glob.glob(os.path.join(package_def_dir, '*.yaml'))
+  else:
+    # Otherwise pick only the ones in 'package_def_files' list.
+    for name in package_def_files:
+      abs_path = os.path.abspath(os.path.join(package_def_dir, name))
+      if not os.path.isfile(abs_path):
+        raise BuildException('No such package definition file: %s' % name)
+      paths.append(abs_path)
+  return [PackageDef(p, read_yaml(py_venv, p)) for p in sorted(paths)]
 
 
 def read_yaml(py_venv, path):
   """Returns content of YAML file as python dict."""
   # YAML lib is in venv, not activated here. Go through hoops.
+  # TODO(vadimsh): Doesn't work on ARM, since we have no working infra_python
+  # venv there. Replace this hack with vendored pure-python PyYAML.
   oneliner = (
       'import json, sys, yaml; '
       'json.dump(yaml.safe_load(sys.stdin), sys.stdout)')
@@ -193,21 +428,72 @@ def read_yaml(py_venv, path):
   return json.loads(out)
 
 
-def should_process_on_builder(pkg_def_file, py_venv, builder):
-  """Returns True if package should be processed by current CI builder."""
-  if not builder:
-    return True
-  builders = read_yaml(py_venv, pkg_def_file).get('builders')
-  return not builders or builder in builders
-
-
 def get_package_vars():
-  """Returns a dict with variables that describe the current environment.
+  """Returns a dict with variables that describe the package target environment.
 
   Variables can be referenced in the package definition YAML as
   ${variable_name}. It allows to reuse exact same definition file for similar
   packages (e.g. packages with same cross platform binary, but for different
   platforms).
+
+  If running in cross-compilation mode, uses GOOS and GOARCH to figure out the
+  target platform instead of examining the host environment.
+  """
+  if is_cross_compiling():
+    return get_target_package_vars()
+  return get_host_package_vars()
+
+
+def get_target_package_vars():
+  """Returns a dict with variables that describe cross-compilation target env.
+
+  Examines os.environ for GOOS, GOARCH and GOARM.
+
+  The returned dict contains only 'platform' and 'exe_suffix' entries.
+  """
+  assert is_cross_compiling()
+  goos = os.environ['GOOS']
+  goarch = os.environ['GOARCH']
+
+  if goarch not in ('386', 'amd64', 'arm'):
+    raise BuildException('Unsupported GOARCH %s' % goarch)
+
+  # There are many ARMs, pick the concrete instruction set. 'v6' is the default,
+  # don't try to support other variants for now.
+  #
+  # See https://golang.org/doc/install/source#environment.
+  if goarch == 'arm':
+    goarm = os.environ.get('GOARM', '6')
+    if goarm != '6':
+      raise BuildException('Unsupported GOARM value %s' % goarm)
+    arch = 'armv6l'
+  else:
+    arch = goarch
+
+  # We use 'mac' instead of 'darwin'.
+  if goos == 'darwin':
+    goos = 'mac'
+
+  return {
+    'exe_suffix': '.exe' if goos == 'windows' else '',
+    'platform': '%s-%s' % (goos, arch),
+  }
+
+
+def get_host_package_vars():
+  """Returns a dict with variables that describe the current host environment.
+
+  The returned platform may not match the machine environment exactly, but it is
+  compatible with it.
+
+  For example, on ARMv7 machines we claim that we are in fact running ARMv6
+  (which is subset of ARMv7), since we don't really care about v7 over v6
+  difference and want to reduce the variability in supported architectures
+  instead.
+
+  Similarly, if running on 64-bit Linux with 32-bit user space (based on python
+  interpreter bitness), we claim that machine is 32-bit, since most 32-bit Linux
+  Chrome Infra bots are in fact running 64-bit kernels with 32-bit userlands.
   """
   # linux, mac or windows.
   platform_variant = {
@@ -240,10 +526,20 @@ def get_package_vars():
     'i686': '386',
     'x86': '386',
     'x86_64': 'amd64',
+    'armv6l': 'armv6l',
+    'armv7l': 'armv6l', # we prefer to use older instruction set for builds
   }.get(platform.machine().lower())
   if not platform_arch:
     raise ValueError('Unknown machine arch: %s' % platform.machine())
 
+  # Most 32-bit Linux Chrome Infra bots are in fact running 64-bit kernel with
+  # 32-bit userland. Detect this case (based on bitness of the python
+  # interpreter) and report the bot as '386'.
+  if (platform_variant == 'linux' and
+      platform_arch == 'amd64' and
+      sys.maxsize == (2 ** 31) - 1):
+    platform_arch = '386'
+
   return {
     # e.g. '.exe' or ''.
     'exe_suffix': EXE_SUFFIX,
@@ -256,12 +552,12 @@ def get_package_vars():
   }
 
 
-def build_pkg(go_workspace, pkg_def_file, out_file, package_vars):
+def build_pkg(cipd_exe, pkg_def, out_file, package_vars):
   """Invokes CIPD client to build a package.
 
   Args:
-    go_workspace: path to 'infra/go' or 'infra_internal/go'.
-    pkg_def_file: path to *.yaml file with package definition.
+    cipd_exe: path to cipd client binary to use.
+    pkg_def: instance of PackageDef representing this package.
     out_file: where to store the built package.
     package_vars: dict with variables to pass as -pkg-var to cipd.
 
@@ -271,18 +567,18 @@ def build_pkg(go_workspace, pkg_def_file, out_file, package_vars):
   Raises:
     BuildException on error.
   """
-  print_title('Building: %s' % os.path.basename(pkg_def_file))
+  print_title('Building: %s' % os.path.basename(out_file))
 
   # Make sure not stale output remains.
   if os.path.isfile(out_file):
     os.remove(out_file)
 
   # Build the package.
-  args = ['-pkg-def', pkg_def_file]
+  args = ['-pkg-def', pkg_def.path]
   for k, v in sorted(package_vars.items()):
     args.extend(['-pkg-var', '%s:%s' % (k, v)])
   args.extend(['-out', out_file])
-  exit_code, json_output = run_cipd(go_workspace, 'pkg-build', args)
+  exit_code, json_output = run_cipd(cipd_exe, 'pkg-build', args)
   if exit_code:
     print
     print >> sys.stderr, 'FAILED! ' * 10
@@ -294,11 +590,11 @@ def build_pkg(go_workspace, pkg_def_file, out_file, package_vars):
   return info
 
 
-def upload_pkg(go_workspace, pkg_file, service_url, tags, service_account):
+def upload_pkg(cipd_exe, pkg_file, service_url, tags, service_account):
   """Uploads existing *.cipd file to the storage and tags it.
 
   Args:
-    go_workspace: path to 'infra/go' or 'infra_internal/go'.
+    cipd_exe: path to cipd client binary to use.
     pkg_file: path to *.cipd file to upload.
     service_url: URL of a package repository service.
     tags: a list of tags to attach to uploaded package instance.
@@ -319,7 +615,7 @@ def upload_pkg(go_workspace, pkg_file, service_url, tags, service_account):
   if service_account:
     args.extend(['-service-account-json', service_account])
   args.append(pkg_file)
-  exit_code, json_output = run_cipd(go_workspace, 'pkg-register', args)
+  exit_code, json_output = run_cipd(cipd_exe, 'pkg-register', args)
   if exit_code:
     print
     print >> sys.stderr, 'FAILED! ' * 10
@@ -329,6 +625,79 @@ def upload_pkg(go_workspace, pkg_file, service_url, tags, service_account):
   return info
 
 
+def build_cipd_client(go_workspace, out_dir):
+  """Builds cipd client binary for the host platform.
+
+  Ignores GOOS and GOARCH env vars. Puts the client binary into
+  '<out_dir>/.cipd_client/cipd_<digest>'.
+
+  This binary is used by build.py itself and later by test_packages.py.
+
+  Args:
+    go_workspace: path to Go workspace root (contains 'env.py', 'src', etc).
+    out_dir: build output directory, will be used to store the binary.
+
+  Returns:
+    Path to the built binary.
+  """
+  # To avoid rebuilding cipd client all the time, we cache it in out/*, using
+  # a combination of DEPS+deps.lock+bootstrap.py as a cache key (they define
+  # exact set of sources used to build the cipd binary).
+  #
+  # We can't just use the client in infra.git/cipd/* because it is built by this
+  # script itself: it introduced bootstrap dependency cycle in case we need to
+  # add a new platform or if we wipe cipd backend storage.
+  seed_paths = [
+    os.path.join(ROOT, 'DEPS'),
+    os.path.join(ROOT, 'go', 'deps.lock'),
+    os.path.join(ROOT, 'go', 'bootstrap.py'),
+  ]
+  digest = hashlib.sha1()
+  for p in seed_paths:
+    with open(p, 'rb') as f:
+      digest.update(f.read())
+  cache_key = digest.hexdigest()[:20]
+
+  # Already have it?
+  cipd_out_dir = os.path.join(out_dir, '.cipd_client')
+  cipd_exe = os.path.join(cipd_out_dir, 'cipd_%s%s' % (cache_key, EXE_SUFFIX))
+  if os.path.exists(cipd_exe):
+    return cipd_exe
+
+  # Nuke all previous copies, make sure out_dir exists.
+  if os.path.exists(cipd_out_dir):
+    for p in glob.glob(os.path.join(cipd_out_dir, 'cipd_*')):
+      os.remove(p)
+  else:
+    os.makedirs(cipd_out_dir)
+
+  # Build cipd client binary for the host platform.
+  run_go_build(
+      go_workspace,
+      package='github.com/luci/luci-go/client/cmd/cipd',
+      output=cipd_exe,
+      rebuild=True,
+      goos='',
+      goarch='')
+
+  return cipd_exe
+
+
+def get_build_out_file(package_out_dir, pkg_def):
+  """Returns a path where to put built *.cipd package file.
+
+  Args:
+    package_out_dir: root directory where to put *.cipd files.
+    pkg_def: instance of PackageDef being built.
+  """
+  # When cross-compiling, append a suffix to package file name to indicate that
+  # it's for foreign platform.
+  sfx = ''
+  if is_cross_compiling():
+    sfx = '+' + get_target_package_vars()['platform']
+  return os.path.join(package_out_dir, pkg_def.name + sfx + '.cipd')
+
+
 def run(
     py_venv,
     go_workspace,
@@ -343,7 +712,7 @@ def run(
     tags,
     service_account_json,
     json_output):
-  """Rebuild python and Go universes and CIPD packages.
+  """Rebuilds python and Go universes and CIPD packages.
 
   Args:
     py_venv: path to 'infra/ENV' or 'infra_internal/ENV'.
@@ -365,26 +734,39 @@ def run(
   """
   assert build or upload, 'Both build and upload are False, nothing to do'
 
-  # Remove stale output so that test_packages.py do not test old files when
-  # invoked without arguments.
-  if build:
-    for path in glob.glob(os.path.join(package_out_dir, '*.cipd')):
-      os.remove(path)
-
-  packages_to_build = [
-    p for p in enumerate_packages_to_build(package_def_dir, package_def_files)
-    if should_process_on_builder(p, py_venv, builder)
-  ]
+  # We need both GOOS and GOARCH or none.
+  if is_cross_compiling():
+    if not os.environ.get('GOOS') or not os.environ.get('GOARCH'):
+      print >> sys.stderr, (
+          'When cross-compiling both GOOS and GOARCH environment variables '
+          'must be set.')
+      return 1
+    if os.environ.get('GOARM', '6') != '6':
+      print >> sys.stderr, 'Only GOARM=6 is supported for now.'
+      return 1
+
+  # Append tags related to the build host. They are especially important when
+  # cross-compiling: cross-compiled packages can be identified by comparing the
+  # platform in the package name with value of 'build_host_platform' tag.
+  tags = list(tags)
+  host_vars = get_host_package_vars()
+  tags.append('build_host_hostname:' + socket.gethostname().split('.')[0])
+  tags.append('build_host_platform:' + host_vars['platform'])
+  tags.append('build_host_os_ver:' + host_vars['os_ver'])
+
+  all_packages = enumerate_packages(py_venv, package_def_dir, package_def_files)
+  packages_to_build = [p for p in all_packages if p.should_build(builder)]
 
   print_title('Overview')
-  print 'Service URL: %s' % service_url
-  print
+  if upload:
+    print 'Service URL: %s' % service_url
+    print
   if builder:
     print 'Package definition files to process on %s:' % builder
   else:
     print 'Package definition files to process:'
-  for pkg_def_file in packages_to_build:
-    print '  %s' % os.path.basename(pkg_def_file)
+  for pkg_def in packages_to_build:
+    print '  %s' % pkg_def.name
   if not packages_to_build:
     print '  <none>'
   print
@@ -392,7 +774,7 @@ def run(
   package_vars = get_package_vars()
   for k, v in sorted(package_vars.items()):
     print '  %s = %s' % (k, v)
-  if tags:
+  if upload and tags:
     print
     print 'Tags to attach to uploaded packages:'
     for tag in sorted(tags):
@@ -402,32 +784,62 @@ def run(
     print 'Nothing to do.'
     return 0
 
+  # Remove old build artifacts to avoid stale files in case the script crashes
+  # for some reason.
+  if build:
+    print_title('Cleaning %s' % package_out_dir)
+    if not os.path.exists(package_out_dir):
+      os.makedirs(package_out_dir)
+    cleaned = False
+    for pkg_def in packages_to_build:
+      out_file = get_build_out_file(package_out_dir, pkg_def)
+      if os.path.exists(out_file):
+        print 'Removing stale %s' % os.path.basename(out_file)
+        os.remove(out_file)
+        cleaned = True
+    if not cleaned:
+      print 'Nothing to clean'
+
+  # Make sure we have a Go toolset and it matches the host platform we detected
+  # in get_host_package_vars(). Otherwise we may end up uploading wrong binaries
+  # under host platform CIPD package suffix. It's important on Linux with 64-bit
+  # kernel and 32-bit userland (we must use 32-bit Go in that case, even if
+  # 64-bit Go works too).
+  go_env = bootstrap_go_toolset(go_workspace)
+  expected_arch = host_vars['platform'].split('-')[1]
+  if go_env['GOHOSTARCH'] != expected_arch:
+    print >> sys.stderr, (
+        'Go toolset GOHOSTARCH (%s) doesn\'t match expected architecture (%s)' %
+        (go_env['GOHOSTARCH'], expected_arch))
+    return 1
+
+  # Build the cipd client needed later to build or upload packages.
+  cipd_exe = build_cipd_client(go_workspace, package_out_dir)
+
   # Build the world.
   if build:
-    build_callback()
+    build_callback(packages_to_build)
 
   # Package it.
   failed = []
   succeeded = []
-  for pkg_def_file in packages_to_build:
-    # path/name.yaml -> out/name.cipd.
-    name = os.path.splitext(os.path.basename(pkg_def_file))[0]
-    out_file = os.path.join(package_out_dir, name + '.cipd')
+  for pkg_def in packages_to_build:
+    out_file = get_build_out_file(package_out_dir, pkg_def)
     try:
       info = None
       if build:
-        info = build_pkg(go_workspace, pkg_def_file, out_file, package_vars)
+        info = build_pkg(cipd_exe, pkg_def, out_file, package_vars)
       if upload:
         info = upload_pkg(
-            go_workspace,
+            cipd_exe,
             out_file,
             service_url,
             tags,
             service_account_json)
       assert info is not None
-      succeeded.append({'pkg_def_name': name, 'info': info})
+      succeeded.append({'pkg_def_name': pkg_def.name, 'info': info})
     except (BuildException, UploadException) as e:
-      failed.append({'pkg_def_name': name, 'error': str(e)})
+      failed.append({'pkg_def_name': pkg_def.name, 'error': str(e)})
 
   print_title('Summary')
   for d in failed:
@@ -448,23 +860,24 @@ def run(
   return 1 if failed else 0
 
 
-def build_infra():
-  """Builds infra.git multiverse."""
-  # Python side.
-  print_title('Making sure python virtual environment is fresh')
-  run_python(
-      script=os.path.join(ROOT, 'bootstrap', 'bootstrap.py'),
-      args=[
-        '--deps_file',
-        os.path.join(ROOT, 'bootstrap', 'deps.pyl'),
-        os.path.join(ROOT, 'ENV'),
-      ])
-  # Go side.
-  build_go(os.path.join(ROOT, 'go'), [
-    'infra/...',
-    'github.com/luci/luci-go/client/...',
-    'github.com/luci/luci-go/tools/...',
-    ])
+def build_infra(pkg_defs):
+  """Builds infra.git multiverse.
+
+  Args:
+    pkg_defs: list of PackageDef instances for packages being built.
+  """
+  # Skip building python if not used or if cross-compiling.
+  if any(p.uses_python_env for p in pkg_defs) and not is_cross_compiling():
+    print_title('Making sure python virtual environment is fresh')
+    run_python(
+        script=os.path.join(ROOT, 'bootstrap', 'bootstrap.py'),
+        args=[
+          '--deps_file',
+          os.path.join(ROOT, 'bootstrap', 'deps.pyl'),
+          os.path.join(ROOT, 'ENV'),
+        ])
+  # Build all necessary go binaries.
+  build_go_code(os.path.join(ROOT, 'go'), pkg_defs)
 
 
 def main(