backend: worker to not call keygen for source builds

In the commit d87321b we skipped the
"signing" job itself, but the problem also happens for downloading
pubkey from copr-keygen.

Complements: d87321b
Fixes: fedora-copr#2942

backend/copr_backend/background_worker_build.py

@@ -2,6 +2,7 @@
 BuildBackgroundWorker class + internals.
 """
 
+import functools
 import glob
 import logging
 import os
@@ -110,6 +111,18 @@ def filter(self, record):
 
         return 1
 
+
+def skipped_for_source_build(f):
+    """ Mark method no-op for the source build (srpm-builds dir) """
+    @functools.wraps(f)
+    def wrapper(self, *args, **kwargs):
+        if self.job.chroot == 'srpm-builds':
+            self.log.debug("Skipping method %s for source build", f.__name__)
+            return None
+        return f(self, *args, **kwargs)
+    return wrapper
+
+
 class BuildBackgroundWorker(BackendBackgroundWorker):
     """
     The (S)RPM build logic.
@@ -593,6 +606,7 @@ def _check_build_success(self):
         if not os.path.exists(successfile):
             raise BackendError("No success file => build failure")
 
+    @skipped_for_source_build
     def _sign_built_packages(self):
         """
             Sign built rpms
@@ -703,6 +717,7 @@ def _get_build_details(self, job):
 
         return build_details
 
+    @skipped_for_source_build
     def _add_pubkey(self):
         """
         Adds pubkey.gpg with public key to ``chroot_dir`` using

backend/copr_backend/sign.py

@@ -154,10 +154,6 @@ def sign_rpms_in_dir(username, projectname, path, chroot, opts, log):
     :raises: :py:class:`backend.exceptions.CoprSignError` failed to sign at least one package
     """
 
-    if chroot == 'srpm-builds':
-        log.info("Source builds are not signed")
-        return
-
     rpm_list = [
         os.path.join(path, filename)
         for filename in os.listdir(path)