[PATCH b4] Delay dns.resolver.get_default_resolver call

["Kyle Meyer" <kyle@kyleam.com>]

When b4.attestation-policy is 'off', calling `b4 am` with
--use-local-mbox is conceptually an offline operation, but it still
requires being online because get_default_resolver() is called at the
top-level of b4/__init__.py.

Support offline operation by delaying the call to
get_default_resolver() until it's needed.  Note that
get_default_resolver() already caches the result, so there's no need
for b4 to store the result as a global variable on its end.

Signed-off-by: Kyle Meyer <kyle@kyleam.com>
---
 b4/__init__.py | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/b4/__init__.py b/b4/__init__.py
index 17fae0e..49304fd 100644
--- a/b4/__init__.py
+++ b/b4/__init__.py
@@ -33,10 +33,8 @@
     import dkim
 
     can_dkim_verify = True
-    _resolver = dns.resolver.get_default_resolver()
 except ModuleNotFoundError:
     can_dkim_verify = False
-    _resolver = None
 
 __VERSION__ = '0.6.3-dev'
 
@@ -1703,7 +1701,8 @@ def __init__(self, msg):
             ddata = get_parts_from_header(dks)
             self.attestor = LoreAttestorDKIM(ddata['d'])
             # Do we have a resolve method?
-            if _resolver and hasattr(_resolver, 'resolve'):
+            resolver = get_resolver()
+            if resolver and hasattr(resolver, 'resolve'):
                 res = dkim.verify(self.msg.as_bytes(), dnsfunc=dkim_get_txt)
             else:
                 res = dkim.verify(self.msg.as_bytes())
@@ -2448,13 +2447,19 @@ def validate_gpg_signature(output, trustmodel):
     return good, valid, trusted, attestor, sigdate, errors
 
 
+def get_resolver():
+    if can_dkim_verify:
+        return dns.resolver.get_default_resolver()
+
+
 def dkim_get_txt(name: bytes, timeout: int = 5):
     global _DKIM_DNS_CACHE
     if name not in _DKIM_DNS_CACHE:
         lookup = name.decode()
         logger.debug('DNS-lookup: %s', lookup)
+        resolver = get_resolver()
         try:
-            a = _resolver.resolve(lookup, dns.rdatatype.TXT, raise_on_no_answer=False, lifetime=timeout, search=True)
+            a = resolver.resolve(lookup, dns.rdatatype.TXT, raise_on_no_answer=False, lifetime=timeout, search=True)
             for r in a.response.answer:
                 if r.rdtype == dns.rdatatype.TXT:
                     for item in r.items:

base-commit: f93bbd3e50b1fb4507aa537f4004da545af9d890
-- 
2.30.0