[b4][PATCH 2/2] Include dkim log output when -d/--debug argument is passed

[Paul Barker <paul@pbarker.dev>]

We can pass a logger object to dkim.verify() which will be used to
report internal errors and debugging info. This can be helpful when
investigating DKIM verification issues but is probably not wanted during
normal operation so the log level of each message is reset to DEBUG.
Each message is also prefixed with 'DKIM: ' to identify its origin when
debug output is enabled.

Signed-off-by: Paul Barker <paul@pbarker.dev>
---
 b4/__init__.py | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/b4/__init__.py b/b4/__init__.py
index 168b722..9721e22 100644
--- a/b4/__init__.py
+++ b/b4/__init__.py
@@ -47,6 +47,17 @@ __VERSION__ = '0.8-dev'
 
 logger = logging.getLogger('b4')
 
+def _dkim_log_filter(record):
+    # Hide all dkim logging output in normal operation by setting the level to
+    # DEBUG. If debugging output has been enabled then prefix dkim logging
+    # output to make its origin clear.
+    record.levelno = logging.DEBUG
+    record.levelname = 'DEBUG'
+    record.msg = 'DKIM: ' + record.msg
+    return True
+dkimlogger = logger.getChild('dkim')
+dkimlogger.addFilter(_dkim_log_filter)
+
 HUNK_RE = re.compile(r'^@@ -\d+(?:,(\d+))? \+\d+(?:,(\d+))? @@')
 FILENAME_RE = re.compile(r'^(---|\+\+\+) (\S+)')
 
@@ -1028,7 +1039,7 @@ class LoreMessage:
                     signtime = self.date
 
             self.msg._headers.append((hn, hval))  # noqa
-            res = dkim.verify(self.msg.as_bytes())
+            res = dkim.verify(self.msg.as_bytes(), logger=dkimlogger)
 
             attestor = LoreAttestorDKIM(res, identity, signtime, errors)
             logger.debug('DKIM verify results: %s=%s', identity, res)
-- 
2.31.1