compose: Make --print-only happen earlier and be quiet

Don't print the version or other warnings; ensure we output clean JSON.

Prep for fixing https://github.com/coreos/fedora-coreos-config/pull/24
(This helps, but we still need to expand `${basearch}`)

Closes: #1648
Approved by: jlebon

src/app/rpmostree-compose-builtin-tree.c

@@ -591,6 +591,12 @@ impl_install_tree (RpmOstreeTreeComposeContext *self,
                    GCancellable    *cancellable,
                    GError         **error)
 {
+  if (opt_print_only)
+    {
+      g_print ("%s\n", ror_treefile_get_json_string (self->treefile_rs));
+      return TRUE; /* Note early return */
+    }
+
   /* Print version number */
   g_printerr ("RPM-OSTree Version: %s\n", PACKAGE_VERSION);
 
@@ -618,19 +624,6 @@ impl_install_tree (RpmOstreeTreeComposeContext *self,
    */
   *out_changed = FALSE;
 
-  if (opt_print_only)
-    {
-      glnx_unref_object JsonGenerator *generator = json_generator_new ();
-      g_autoptr(GOutputStream) stdout = g_unix_output_stream_new (1, FALSE);
-
-      json_generator_set_pretty (generator, TRUE);
-      json_generator_set_root (generator, self->treefile_rootval);
-      (void) json_generator_to_stream (generator, stdout, NULL, NULL);
-
-      /* Note early return */
-      return TRUE;
-    }
-
   /* Read the previous commit */
   if (self->ref)
     {

tests/compose-tests/test-basic-unified.sh

@@ -6,6 +6,11 @@ dn=$(cd $(dirname $0) && pwd)
 . ${dn}/libcomposetest.sh
 
 prepare_compose_test "basic-unified"
+# Test --print-only, currently requires --repo
+rpm-ostree compose tree --repo=${repobuild} --print-only ${treefile} > treefile.json
+# Verify it's valid JSON
+jq -r .ref < treefile.json > ref.txt
+assert_file_has_content_literal ref.txt "${treeref}"
 # Test metadata json with objects, arrays, numbers
 cat > metadata.json <<EOF
 {