gather: Link files in order for dependant variants

If there are variants that depend on another, they should be processed
in order to make sure packages from the base variant are linked first.
That way the srpm cache is populated and any package in layered variant
but with source in base will have access to correct epoch information.

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

pungi/compose.py

@@ -32,6 +32,7 @@ from productmd.images import Images
 from dogpile.cache import make_region
 
 
+from pungi.graph import SimpleAcyclicOrientedGraph
 from pungi.wrappers.variants import VariantsXmlParser
 from pungi.paths import Paths
 from pungi.wrappers.scm import get_file_from_scm
@@ -428,3 +429,40 @@ class Compose(kobo.log.LoggingBase):
         """
         path = os.path.join(self.paths.work.tmp_dir(arch=arch, variant=variant))
         return tempfile.mkdtemp(suffix=suffix, prefix=prefix, dir=path)
+
+
+def get_ordered_variant_uids(compose):
+    if not hasattr(compose, "_ordered_variant_uids"):
+        ordered_variant_uids = _prepare_variant_as_lookaside(compose)
+        # Some variants were not mentioned in configuration value
+        # 'variant_as_lookaside' and its run order is not crucial (that
+        # means there are no dependencies inside this group). They will be
+        # processed first. A-Z sorting is for reproducibility.
+        unordered_variant_uids = sorted(
+            set(compose.all_variants.keys()) - set(ordered_variant_uids)
+        )
+        setattr(
+            compose,
+            "_ordered_variant_uids",
+            unordered_variant_uids + ordered_variant_uids
+        )
+    return getattr(compose, "_ordered_variant_uids")
+
+
+def _prepare_variant_as_lookaside(compose):
+    """
+    Configuration value 'variant_as_lookaside' contains variant pairs <variant,
+    its lookaside>. In that pair lookaside variant have to be processed first.
+    Structure can be represented as a oriented graph. Its spanning line shows
+    order how to process this set of variants.
+    """
+    variant_as_lookaside = compose.conf.get("variant_as_lookaside", [])
+    graph = SimpleAcyclicOrientedGraph()
+    for variant, lookaside_variant in variant_as_lookaside:
+        try:
+            graph.add_edge(variant, lookaside_variant)
+        except ValueError as e:
+            raise ValueError("There is a bad configuration in 'variant_as_lookaside': %s" % e)
+
+    variant_processing_order = reversed(graph.prune_graph())
+    return list(variant_processing_order)