caucase.utils: Tolerate broken symlinks

Otherwise, this causes an IOError to be raised in _getPEMTypeDict.
So move file existence check inside the loop. Also rely on isdir returning
False on non-existent inputs.
This may for example happen if openssl-rehash is used on these directories:
caucase-updater may delete an expired CA, breaking its symlink, triggering
this bug and crashing caucase-updater.

CHANGES.txt

@@ -3,6 +3,7 @@
 * Janitorial: make updated code checkers happier.
 * Explicitly depend on ipaddress module on python 2.7 .
 * Be less sensitive to small client/server time disagreements.
+* Fix crashes when encountering broken symlinks in directories of PEM files.
 
 0.9.13 (2021-12-22)
 ===================

caucase/test.py

@@ -3384,6 +3384,9 @@ class CaucaseTest(TestCase):
       utils.getCRLList(self._client_ca_dir),
       [crl0_pem, crl1_pem],
     )
+    # Broken symlink: does not get deleted, and does not cause errors
+    broken_symlink = os.path.join(self._client_ca_dir, 'is_a_trap')
+    os.symlink('does_not_exist', broken_symlink)
     # On with the test for the CA side
     utils.saveCertList(self._client_ca_dir, [crt0_pem])
     self.assertTrue(os.path.exists(self._client_ca_dir))
@@ -3404,6 +3407,7 @@ class CaucaseTest(TestCase):
     utils.saveCertList(self._client_ca_dir, [crt0_pem])
     self.assertTrue(os.path.exists(kept_file_path))
     self.assertFalse(os.path.exists(deleted_file_path))
+    self.assertTrue(os.path.lexists(broken_symlink))
     os.unlink(kept_file_path)
     # Storing and loading multiple certificates
     utils.saveCertList(self._client_ca_dir, [crt0_pem, crt1_pem])
@@ -3430,6 +3434,8 @@ class CaucaseTest(TestCase):
       utils.getCRLList(self._client_ca_dir),
       [crl0_pem, crl1_pem],
     )
+    # The borken symlink is still present
+    self.assertTrue(os.path.lexists(broken_symlink))
 
   def testHttpSSLRenewal(self):
     """

caucase/utils.py

@@ -152,17 +152,17 @@ def getCRLList(crl_path):
   return _getPEMListFromPath(crl_path, pem.CertificateRevocationList)
 
 def _getPEMListFromPath(path, pem_type):
-  if not os.path.exists(path):
-    return []
-  return [
-    pem_object.as_bytes()
+  result = []
   for file_name in (
     [os.path.join(path, x) for x in os.listdir(path)]
     if os.path.isdir(path) else
     [path]
-    )
-    for pem_object in _getPEMTypeDict(file_name).get(pem_type, ())
-  ]
+  ):
+    # path is missing, or a symlink in path (if path is a directory) is broken
+    if os.path.exists(file_name):
+      for pem_object in _getPEMTypeDict(file_name).get(pem_type, ()):
+        result.append(pem_object.as_bytes())
+  return result
 
 def saveCertList(crt_path, cert_pem_list):
   """