Allow getting the latest stream of base module.

If base module name:stream is not found, consider `stream` as virtual stream
and return the latest (the one with highest stream_version) base module with
that virtual stream.

module_build_service/utils/mse.py

@@ -188,6 +188,39 @@ def _get_mmds_from_requires(requires, mmds, recursive=False,
     return mmds
 
 
+def _get_latest_base_module_mmd_with_virtual_stream(name, virtual_stream):
+    """
+    Returns the Modulemd instance of Module build with highest stream_version,
+    with name `name`, and containing the virtual stream `virtual_stream` in its
+    xmd["mbs"]["virtual_streams"] section
+
+    :param str name: Name of a module to return.
+    :param str virtual_stream: Name of virtual stream which must be included in
+        the virtual_streams xmd section.
+    :return Modulemd: Modulemd instance of such module with highest stream_version.
+    """
+    with models.make_session(conf) as session:
+        builds = models.ModuleBuild.get_last_build_in_all_streams(session, name)
+        if not builds:
+            return None
+
+        build_to_return = None
+        for build in builds:
+            mmd = build.mmd()
+            xmd = mmd.get_xmd()
+            if "mbs" not in xmd.keys() or "virtual_streams" not in xmd["mbs"].keys():
+                continue
+
+            virtual_streams = xmd["mbs"]["virtual_streams"]
+            if virtual_stream not in virtual_streams:
+                continue
+
+            if not build_to_return or build_to_return.stream_version < build.stream_version:
+                build_to_return = build
+
+        return build_to_return.mmd() if build_to_return else None
+
+
 def _get_base_module_mmds(mmd):
     """
     Returns list of MMDs of base modules buildrequired by `mmd` including the compatible
@@ -226,8 +259,18 @@ def _get_base_module_mmds(mmd):
                 # zero or one module build.
                 mmds = resolver.get_module_modulemds(name, stream)
                 if not mmds:
-                    continue
-                stream_mmd = mmds[0]
+                    # The given name:stream base module does not exist. It might be the case
+                    # when user requests name:virtual_stream (like platform:f29). In this case,
+                    # we need to find the latest (the one with highest stream_version) base
+                    # module.
+                    stream_mmd = _get_latest_base_module_mmd_with_virtual_stream(name, stream)
+                    if not stream_mmd:
+                        continue
+                    # The `stream` does not exist and we are going to use the stream name of
+                    # the latest base module with given virtual stream, so override it here.
+                    stream = stream_mmd.get_stream()
+                else:
+                    stream_mmd = mmds[0]
 
                 # In case there are no virtual_streams in the buildrequired name:stream,
                 # it is clear that there are no compatible streams, so return just this