golang/xenlight: do not hard code libxl dir in gengotypes.py

Currently, in order to 'import idl' in gengotypes.py, we derive the path
of the libxl source directory from the XEN_ROOT environment variable, and
append that to sys.path so python can see idl.py. Since the the recent move of
libxl to tools/libs/light, this hard coding breaks the build.

Instead, check for the environment variable LIBXL_SRC_DIR, but move this
check to a try-except block (with empty except). This simply makes the
real error more visible, and does not strictly require that
LIBXL_SRC_DIR is used. Finally, update the Makefile to set LIBXL_SRC_DIR
rather than XEN_ROOT when calling gengotypes.py.

Signed-off-by: Nick Rosbrook <rosbrookn@ainfosec.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>

tools/golang/xenlight/Makefile

@@ -16,7 +16,7 @@ all: build
 GOXL_GEN_FILES = types.gen.go helpers.gen.go
 
 %.gen.go: gengotypes.py $(LIBXL_SRC_DIR)/libxl_types.idl $(LIBXL_SRC_DIR)/idl.py
-	XEN_ROOT=$(XEN_ROOT) $(PYTHON) gengotypes.py $(LIBXL_SRC_DIR)/libxl_types.idl
+	LIBXL_SRC_DIR=$(LIBXL_SRC_DIR) $(PYTHON) gengotypes.py $(LIBXL_SRC_DIR)/libxl_types.idl
 
 # Go will do its own dependency checking, and not actuall go through
 # with the build if none of the input files have changed.

tools/golang/xenlight/gengotypes.py

@@ -3,7 +3,14 @@
 import os
 import sys
 
-sys.path.append('{0}/tools/libxl'.format(os.environ['XEN_ROOT']))
+try:
+    sys.path.append(os.environ['LIBXL_SRC_DIR'])
+except:
+    # If we get here, then we expect the 'import idl'
+    # expression to fail. That error is more informative,
+    # so let it happen.
+    pass
+
 import idl
 
 # Go versions of some builtin types.