skip new signing test when we can't find apksigner

Also add some error handling to the find_apksigner() method.

fdroidserver/common.py

@@ -429,7 +429,11 @@ def find_apksigner():
     if set_command_in_config('apksigner'):
         return config['apksigner']
     build_tools_path = os.path.join(config['sdk_path'], 'build-tools')
+    if not os.path.isdir(build_tools_path):
+        return None
     for f in sorted(os.listdir(build_tools_path), reverse=True):
+        if not os.path.isdir(os.path.join(build_tools_path, f)):
+            continue
         if LooseVersion(f) < LooseVersion(MINIMUM_AAPT_BUILD_TOOLS_VERSION):
             return None
         if os.path.exists(os.path.join(build_tools_path, f, 'apksigner')):

tests/common.TestCase

@@ -675,6 +675,8 @@ class CommonTest(unittest.TestCase):
     def test_sign_apk_targetsdk_30(self):
         fdroidserver.common.config = None
         config = fdroidserver.common.read_config(fdroidserver.common.options)
+        if not fdroidserver.common.find_apksigner():
+            self.skipTest('SKIPPING as apksigner is not installed!')
         config['jarsigner'] = fdroidserver.common.find_sdk_tools_cmd('jarsigner')
         config['keyalias'] = 'sova'
         config['keystorepass'] = 'r9aquRHYoI8+dYz6jKrLntQ5/NJNASFBacJh7Jv2BlI='