Create a generator for defaulters

Given an object that wishes to have a top level defaulter, traverse the
object looking for nested fields that have defaulters and generate a
single function for that type that invokes all defaulters. The function
will have the name `SetObjectDefaults_NAME`.

Types use `// +k8s:defaulter-gen=true` to indicate they wish a defaulter
generated. If a function already exists with the desired name
`SetObjectDefaults_NAME` then no generation will occur. At a package
level, authors can bulk select the types to generate by setting the
value of the comment to the name of a field - all objects with that
field name without `// +k8s:defaulter-gen=false` defined on the type
will get a defaulter.

Because the defaulting behavior from conversions happens recursively,
all defaulters are expected to be invoked. We call these defaulters
"non-covering" (other defaulters may be invoked beneath them). The
defaulters we generate, by comparison, are "covering" - no nested
defaulters should be invoked. To distinguish between these two types, we
introduce the `// +k8s:defaulter-gen=covers` comment on a defaulter
function which will instruct the generator that the function should
terminate recursion.

This sets the stage for future defaulter generation from comments by
subsuming our existing generators

cmd/libs/go2idl/defaulter-gen/main.go

@@ -0,0 +1,78 @@
+/*
+Copyright 2016 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// defaulter-gen is a tool for auto-generating Defaulter functions.
+//
+// Given a list of input directories, it will scan for top level types
+// and generate efficient defaulters for an entire object from the sum
+// of the SetDefault_* methods contained in the object tree.
+//
+// Generation is governed by comment tags in the source.  Any package may
+// request defaulter generation by including one or more comment tags at
+// the package comment level:
+//
+//   // +k8s:defaulter-gen=<field-name-to-flag>
+//
+// which will create defaulters for any type that contains the provided
+// field name (if the type has defaulters). Any type may request explicit
+// defaulting by providing the comment tag:
+//
+//   // +k8s:defaulter-gen=true|false
+//
+// An existing defaulter method (`SetDefaults_TYPE`) can provide the
+// comment tag:
+//
+//   // +k8s:defaulter-gen=covers
+//
+// to indicate that the defaulter does not or should not call any nested
+// defaulters.
+package main
+
+import (
+	"path/filepath"
+
+	"k8s.io/gengo/args"
+	"k8s.io/gengo/examples/defaulter-gen/generators"
+
+	"github.com/golang/glog"
+	"github.com/spf13/pflag"
+)
+
+func main() {
+	arguments := args.Default()
+
+	// Override defaults.
+	arguments.OutputFileBaseName = "zz_generated.defaults"
+	arguments.GoHeaderFilePath = filepath.Join(args.DefaultSourceTree(), "k8s.io/kubernetes/hack/boilerplate/boilerplate.go.txt")
+
+	// Custom args.
+	customArgs := &generators.CustomArgs{
+		ExtraPeerDirs: []string{},
+	}
+	pflag.CommandLine.StringSliceVar(&customArgs.ExtraPeerDirs, "extra-peer-dirs", customArgs.ExtraPeerDirs,
+		"Comma-separated list of import paths which are considered, after tag-specified peers, for conversions.")
+	arguments.CustomArgs = customArgs
+
+	// Run it.
+	if err := arguments.Execute(
+		generators.NameSystems(),
+		generators.DefaultNameSystem(),
+		generators.Packages,
+	); err != nil {
+		glog.Fatalf("Error: %v", err)
+	}
+	glog.V(2).Info("Completed successfully.")
+}