[gnome-keysign: 10/15] gpgmeh: create exceptions for issued during signature import

[Tobias Mueller <tobiasmue src gnome org>]

commit c362bcabfc458144d178e97d8fb30d4f1e63eeae
Author: Tobias Mueller <muelli cryptobitch de>
Date:   Fri Apr 8 14:36:50 2022 +0200

    gpgmeh: create exceptions for issued during signature import
    
    This allows for the UI to provide rich feedback about the case at hand.
    For example, the UI can choose to ignore when a signature has already
    been imported, i.e. NoNewSignatures. Or fail very loudly when a
    revocation has been imported.

 keysign/gpgmeh.py | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)
---
diff --git a/keysign/gpgmeh.py b/keysign/gpgmeh.py
index 61279aa..25f2b00 100755
--- a/keysign/gpgmeh.py
+++ b/keysign/gpgmeh.py
@@ -550,6 +550,13 @@ def sign_keydata_and_encrypt(keydata, error_cb=None, homedir=None):
                 yield (UID.from_gpgme(uid), ciphertext, uid_data)
 
 
+class NoNewSignatures(GPGMEError):
+    pass
+class NewRevocations(GPGMEError):
+    pass
+class NewSubkey(GPGMEError):
+    pass
+
 def decrypt_signature(encrypted_sig, homedir=None):
     """
     Takes an encrypted signture, tries to decrypt it, and returns the
@@ -569,9 +576,12 @@ def decrypt_signature(encrypted_sig, homedir=None):
         log.warning("Trying to import a new key instead of a signature!")
         raise GPGMEError
 
-    if result.new_signatures == 0 or result.new_revocations != 0 or result.new_sub_keys != 0:
-        log.warning("The signature that we were importing is not as we expected!")
-        raise GPGMEError
+    if result.new_signatures == 0:
+        raise NoNewSignatures()
+    if result.new_revocations != 0:
+        raise NewRevocations()
+    if result.new_sub_keys != 0:
+        raise NewSubkey()
 
     return decrypted_sig