repos.recursive_buid_report: Try breaking cycles

Instead of falling back to alphabetical order when
cycle is detected, we try to break it and form
the topological order again. Even if cycles are
broken at the wrong edge, this order might still
be more useful.

All the cycles found are included into the report.

port_stats/repos.py

@@ -393,11 +393,18 @@ def recursive_build_report(from_repo, to_repo, *source_names,
             dep_sources.add(b.source_name)
         full_req[source] = dep_sources
 
+    cycles = []
+    while True:
         try:
             order = list(graphlib.TopologicalSorter(full_req).static_order())
         except graphlib.CycleError as ex:
-        LOG.error("Cycle detected: %s", ex)
-        order = sorted(build_source_deps)
+            LOG.warning("Cycle detected: %s", ex)
+            cycle = ex.args[1]
+            cycles.append(cycle)
+            # break the cycle and retry
+            full_req[cycle[1]].remove(cycle[0])
+        else:
+            break
 
     reports.append('\n== SUMMARY ==\n')
     for source in order:
@@ -412,6 +419,11 @@ def recursive_build_report(from_repo, to_repo, *source_names,
         reports.extend('\t%s' % source.decode()
                        for source in sorted(self_dependent))
 
+    if cycles:
+        reports.append('\nCycles:')
+        reports.extend('\t%s' % ' '.join(source.decode() for source in c)
+                       for c in cycles)
+
     special = set()
     for source_name in order:
         source = to_repo.sources.get(source_name)