getenv may return None, so assert it isn't None for CC and srctree
environmental variables required for the script.
Disable an optional warning related to Popen.

Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/perf/util/setup.py | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/tools/perf/util/setup.py b/tools/perf/util/setup.py
index 649550e9b7aa..dd289d15acfd 100644
--- a/tools/perf/util/setup.py
+++ b/tools/perf/util/setup.py
@@ -3,6 +3,7 @@ from subprocess import Popen, PIPE
 from re import sub
 
 cc = getenv("CC")
+assert cc, "Environment variable CC not set"
 
 # Check if CC has options, as is the case in yocto, where it uses CC="cc --sysroot..."
 cc_tokens = cc.split()
@@ -12,8 +13,13 @@ if len(cc_tokens) > 1:
 else:
     cc_options = ""
 
+# ignore optional stderr could be None as it is set to PIPE to avoid that.
+# mypy: disable-error-code="union-attr"
 cc_is_clang = b"clang version" in Popen([cc, "-v"], stderr=PIPE).stderr.readline()
-src_feature_tests  = getenv('srctree') + '/tools/build/feature'
+
+srctree = getenv('srctree')
+assert srctree, "Environment variable srctree, for the Linux sources, not set"
+src_feature_tests  = f'{srctree}/tools/build/feature'
 
 def clang_has_option(option):
     cc_output = Popen([cc, cc_options + option, path.join(src_feature_tests, "test-hello.c") ], stderr=PIPE).stderr.readlines()
@@ -71,7 +77,7 @@ else:
 # The python headers have mixed code with declarations (decls after asserts, for instance)
 cflags += [ "-Wno-declaration-after-statement" ]
 
-src_perf  = getenv('srctree') + '/tools/perf'
+src_perf  = f'{srctree}/tools/perf'
 build_lib = getenv('PYTHON_EXTBUILD_LIB')
 build_tmp = getenv('PYTHON_EXTBUILD_TMP')
 
-- 
2.47.0.163.g1226f6d8fa-goog