gpg: Avoid error diagnostics with --override-session-key when verifying

* src/engine-gpg.c (gpg_decrypt): only send --no-keyring when we are
not verifying.

--

Without this change, the signature verification would fail.  This
problem was introduced in bded8ebc59 in
an attempt to avoid an error when *not* verifying.  Clearly more test
suite coverage is needed to avoid introducing this sort of problem in
the future.

GnuPG-bug-id: 3464
Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>

src/engine-gpg.c

@@ -1695,12 +1695,15 @@ gpg_decrypt (void *engine,
                                          strlen (override_session_key), 1);
           if (!err)
             {
-              /* We add --no-keyring because a keyring is not required
-               * when we are overriding the session key.  It would
+              /* When we are not trying to verify signatures as well,
+               * we add --no-keyring because a keyring is not required
+               * for decryption when overriding the session key.  It would
                * work without that option but --no-keyring avoids that
                * gpg return a failure due to a missing key log_error()
                * diagnostic.  --no-keyring is supported since 2.1.14. */
-              err = add_arg (gpg, "--no-keyring");
+
+              if (!(flags & GPGME_DECRYPT_VERIFY))
+                  err = add_arg (gpg, "--no-keyring");
               if (!err)
                 err = add_arg (gpg, "--override-session-key-fd");
               if (!err)