signapi: Change API to also return a success message

This is the dual of https://github.com/ostreedev/ostree/pull/2129/commits/1f3c8c5b3de978f6e069c24938967f823cce7ee8
where we output more detail when signapi fails to validate.

Extend the API to return a string for success, which we output
to stdout.

This will help the test suite *and* end users validate that the expected
thing is happening.

In order to make this cleaner, split the "verified commit" set
in the pull code into GPG and signapi verified sets, and have
the signapi verified set contain the verification string.

We're not doing anything with the verification string in the
pull code *yet* but I plan to add something like
`ostree pull --verbose` which would finally print this.

src/libostree/ostree-repo-pull-verify.c

@@ -261,12 +261,15 @@ gboolean
 _sign_verify_for_remote (GPtrArray *verifiers,
                          GBytes *signed_data,
                          GVariant *metadata,
+                         char    **out_success_message,
                          GError **error)
 {
   guint n_invalid_signatures = 0;
   g_autoptr (GError) last_sig_error = NULL;
   gboolean found_sig = FALSE;
 
+  g_assert (out_success_message == NULL || *out_success_message == NULL);
+
   g_assert_cmpuint (verifiers->len, >=, 1);
   for (guint i = 0; i < verifiers->len; i++)
     {
@@ -282,17 +285,21 @@ _sign_verify_for_remote (GPtrArray *verifiers,
 
       found_sig = TRUE;
 
+      g_autofree char *success_message = NULL;
         /* Return true if any signature fit to pre-loaded public keys.
           * If no keys configured -- then system configuration will be used */
       if (!ostree_sign_data_verify (sign,
                                     signed_data,
                                     signatures,
+                                    &success_message,
                                     last_sig_error ? NULL : &last_sig_error))
         {
           n_invalid_signatures++;
           continue;
         }
       /* Accept the first valid signature */
+      if (out_success_message)
+        *out_success_message = g_steal_pointer (&success_message);
       return TRUE;
     }
 
@@ -348,11 +355,10 @@ _verify_unwritten_commit (OtPullData                 *pull_data,
                           GCancellable               *cancellable,
                           GError                    **error)
 {
-
-  if (pull_data->gpg_verify || pull_data->signapi_commit_verifiers)
-    /* Shouldn't happen, but see comment in process_gpg_verify_result() */
-    if (g_hash_table_contains (pull_data->verified_commits, checksum))
-      return TRUE;
+  /* Shouldn't happen, but see comment in process_gpg_verify_result() */
+  if ((!pull_data->gpg_verify || g_hash_table_contains (pull_data->verified_commits, checksum))
+      && (!pull_data->signapi_commit_verifiers || g_hash_table_contains (pull_data->signapi_verified_commits, checksum)))
+    return TRUE;
 
   g_autoptr(GBytes) signed_data = g_variant_get_data_as_bytes (commit);
 
@@ -382,12 +388,13 @@ _verify_unwritten_commit (OtPullData                 *pull_data,
       if (detached_metadata == NULL)
         return glnx_throw (error, "Can't verify commit without detached metadata");
 
-      if (!_sign_verify_for_remote (pull_data->signapi_commit_verifiers, signed_data, detached_metadata, error))
+      g_autofree char *success_message = NULL;
+      if (!_sign_verify_for_remote (pull_data->signapi_commit_verifiers, signed_data, detached_metadata, &success_message, error))
         return glnx_prefix_error (error, "Can't verify commit");
 
       /* Mark the commit as verified to avoid double verification
        * see process_verify_result () for rationale */
-      g_hash_table_add (pull_data->verified_commits, g_strdup (checksum));
+      g_hash_table_insert (pull_data->signapi_verified_commits, g_strdup (checksum), g_steal_pointer (&success_message));
     }
 
   return TRUE;