pull: Cleanly error out on unknown schemes

Previous to this we'd trip an assertion `abort()` deep in the curl code if e.g.
a user did `ostree remote add foo htttp://...` etc.

Motivated by considering supporting "external remotes" where code outside
ostree does a pull, but we want to reuse the signing verification infrastructure.

src/libostree/ostree-fetcher-uri.c

@@ -117,3 +117,21 @@ _ostree_fetcher_uri_to_string (OstreeFetcherURI *uri)
 {
   return soup_uri_to_string ((SoupURI*)uri, FALSE);
 }
+
+
+/* Only accept http, https, and file; particularly curl has a ton of other
+ * backends like sftp that we don't want, and this also gracefully filters
+ * out invalid input.
+ */
+gboolean
+_ostree_fetcher_uri_validate (OstreeFetcherURI *uri, GError **error) 
+{
+  const char *scheme = soup_uri_get_scheme ((SoupURI*)uri);
+  // TODO only allow file if explicitly requested by a higher level
+  if (!(g_str_equal (scheme, "http") || g_str_equal (scheme, "https") || g_str_equal (scheme, "file")))
+    {
+      g_autofree char *s = _ostree_fetcher_uri_to_string (uri);
+      return glnx_throw (error, "Invalid URI scheme in %s", s);
+    }
+  return TRUE;
+}

src/libostree/ostree-fetcher.h

@@ -90,6 +90,9 @@ _ostree_fetcher_uri_get_path (OstreeFetcherURI *uri);
 char *
 _ostree_fetcher_uri_to_string (OstreeFetcherURI *uri);
 
+gboolean
+_ostree_fetcher_uri_validate (OstreeFetcherURI *uri, GError **error);
+
 GType   _ostree_fetcher_get_type (void) G_GNUC_CONST;
 
 OstreeFetcher *_ostree_fetcher_new (int                      tmpdir_dfd,

src/libostree/ostree-repo-pull.c

@@ -3446,6 +3446,9 @@ compute_effective_mirrorlist (OstreeRepo    *self,
       if (!baseuri)
         return FALSE;
 
+      if (!_ostree_fetcher_uri_validate (baseuri, error))
+        return FALSE;
+
       *out_mirrorlist =
         g_ptr_array_new_with_free_func ((GDestroyNotify) _ostree_fetcher_uri_free);
       g_ptr_array_add (*out_mirrorlist, g_steal_pointer (&baseuri));