Revert of [NaCl SDK] Remove create_nmf dependency on NACL_SDK_ROOT env var

native_client_sdk/src/tools/create_nmf.py

Index: native_client_sdk/src/tools/create_nmf.py
diff --git a/native_client_sdk/src/tools/create_nmf.py b/native_client_sdk/src/tools/create_nmf.py
index dc7e552a615cfac4705d00180f69bfa658fa01e2..de72408c4df141bbcac49f8255d724aa5f18beae 100755
--- a/native_client_sdk/src/tools/create_nmf.py
+++ b/native_client_sdk/src/tools/create_nmf.py
@@ -466,16 +466,15 @@
 
 
 def GetSDKRoot():
-  """Returns the root directory of the NaCl SDK.
-  """
-  # This script should be installed in NACL_SDK_ROOT/tools. Assert that
-  # the 'toolchain' folder exists within this directory in case, for
-  # example, this script is moved to a different location.
-  # During the Chrome build this script is sometimes run outside of
-  # of an SDK but in these cases it should always be run with --objdump=
-  # and --no-default-libpath which avoids the need to call this function.
-  sdk_root = os.path.dirname(SCRIPT_DIR)
-  assert(os.path.exists(os.path.join(sdk_root, 'toolchain')))
+  """Determine current NACL_SDK_ROOT, either via the environment variable
+  itself, or by attempting to derive it from the location of this script.
+  """
+  sdk_root = os.environ.get('NACL_SDK_ROOT')
+  if not sdk_root:
+    sdk_root = os.path.dirname(SCRIPT_DIR)
+    if not os.path.exists(os.path.join(sdk_root, 'toolchain')):
+      return None
+
   return sdk_root
 
 
@@ -483,8 +482,12 @@
   """Derive path to objdump executable to use for determining shared
   object dependencies.
   """
+  sdk_root = GetSDKRoot()
+  if not sdk_root:
+    return None
+
   osname = getos.GetPlatform()
-  toolchain = os.path.join(GetSDKRoot(), 'toolchain', '%s_x86_glibc' % osname)
+  toolchain = os.path.join(sdk_root, 'toolchain', '%s_x86_glibc' % osname)
   objdump = os.path.join(toolchain, 'bin', 'x86_64-nacl-objdump')
   if osname == 'win':
     objdump += '.exe'
@@ -505,6 +508,10 @@
   """
   assert(config in ('Debug', 'Release'))
   sdk_root = GetSDKRoot()
+  if not sdk_root:
+    # TOOD(sbc): output a warning here?  We would also need to suppress
+    # the warning when run from the chromium build.
+    return []
 
   osname = getos.GetPlatform()
   libpath = [
@@ -573,8 +580,8 @@
                       help='Rename FOO as BAR',
                       action='append', default=[], metavar='FOO,BAR')
   parser.add_argument('-x', '--extra-files',
-                      help='Add extra key:file tuple to the "files"'
-                           ' section of the .nmf',
+                      help=('Add extra key:file tuple to the "files"' +
+                            ' section of the .nmf'),
                       action='append', default=[], metavar='FILE')
   parser.add_argument('-O', '--pnacl-optlevel',
                       help='Set the optimization level to N in PNaCl manifests',
@@ -662,7 +669,7 @@
                  pnacl_debug_optlevel=pnacl_debug_optlevel,
                  nmf_root=nmf_root)
 
-  if options.output is None:
+  if not options.output:
     sys.stdout.write(nmf.GetJson())
   else:
     with open(options.output, 'w') as output: