When decrypting a part that is gpg signed and encrypted and the key used to sign the part is not in the local keyring, then "gpg --decrypt" has an exit code of 2. gpg_decrypt in gmime_gpg_context.c interprets this as a failure and returns NULL.

As you can see in the example gpg output below, the status fd contains the correct information (DECRYPTION_OK and ERRSIG).

Instead of returning NULL gpg_decrypt should return a GMimeDecryptResult with a signature list with a GMimeSignature with a status of GMIME_SIGNATURE_STATUS_ERROR or something similar.

Example gpg output:

$ LANG=C gpg --verbose --no-secmem-warning --no-greeting --no-tty --charset=UTF-8 --status-fd=2 --decrypt --output /dev/null 
gpg: armor header: Version: GnuPG v1.4.10 (GNU/Linux)
gpg: public key is 936691BB
[GNUPG:] ENC_TO ED34A39B936691BB 1 0
gpg: no secret subkey for public subkey 12E71AE1 - ignoring
gpg: no secret subkey for public subkey 11EBA649 - ignoring
gpg: using subkey 936691BB instead of primary key 8C7740AF
[GNUPG:] USERID_HINT ED34A39B936691BB Gaudenz Steinlin <gaudenz.steinlin@soziologie.ch>
[GNUPG:] NEED_PASSPHRASE ED34A39B936691BB DC5A28E48C7740AF 1 0
gpg: using subkey 936691BB instead of primary key 8C7740AF
[GNUPG:] GOOD_PASSPHRASE
gpg: encrypted with 2048-bit RSA key, ID 936691BB, created 2006-04-20
      "Gaudenz Steinlin <gaudenz.steinlin@soziologie.ch>"
[GNUPG:] BEGIN_DECRYPTION
gpg: AES256 encrypted data
[GNUPG:] DECRYPTION_INFO 2 9
gpg: original file name=''
[GNUPG:] PLAINTEXT 62 1338368705 
gpg: Signature made Wed May 30 11:05:05 2012 CEST using RSA key ID E43EED4F
[GNUPG:] ERRSIG 01301861E43EED4F 1 2 00 1338368705 9
[GNUPG:] NO_PUBKEY 01301861E43EED4F
gpg: Can't check signature: public key not found
[GNUPG:] DECRYPTION_OKAY
[GNUPG:] GOODMDC
[GNUPG:] END_DECRYPTION
$ echo $?
2

Thanks,
Gaudenz