Extract conflict detection to it's own interface.

This PR
 - defines a patch conflict detector interface,
 - extracts implementations of the interface from the
   merginator code, making the merginator code
   independent of --enable_kyaml.
 - injects those implementations into kustomize
   as a function of --enable_kyaml.

So, instead of using different merginators to combine
resmaps, this pr allows the use of a single patch merge
code path that uses different conflict detectors.

So instead of debating how to merge, we're now only
considering whether to warn on conflict detection
in one transformer.

This PR is in service of #3304, eliminating seven
instances where --enable_kyaml was consulted.  These
were cases where conflict detection wasn't an issue
(but merging patches was).

api/resmap/factory.go

@@ -12,24 +12,18 @@ import (
 	"sigs.k8s.io/kustomize/kyaml/yaml"
 )
 
-// Merginator merges resources.
-type Merginator interface {
-	// Merge creates a new ResMap by merging incoming resources.
-	// Error if conflict found.
-	Merge([]*resource.Resource) (ResMap, error)
-}
-
 // Factory makes instances of ResMap.
 type Factory struct {
 	// Makes resources.
 	resF *resource.Factory
-	// Makes ResMaps via merging.
-	pm Merginator
+	// Makes ConflictDetectors.
+	cdf resource.ConflictDetectorFactory
 }
 
 // NewFactory returns a new resmap.Factory.
-func NewFactory(rf *resource.Factory, pm Merginator) *Factory {
-	return &Factory{resF: rf, pm: pm}
+func NewFactory(
+	rf *resource.Factory, cdf resource.ConflictDetectorFactory) *Factory {
+	return &Factory{resF: rf, cdf: cdf}
 }
 
 // RF returns a resource.Factory.
@@ -134,8 +128,8 @@ func (rmF *Factory) FromSecretArgs(
 
 // Merge creates a new ResMap by merging incoming resources.
 // Error if conflict found.
-func (rmF *Factory) Merge(patches []*resource.Resource) (ResMap, error) {
-	return rmF.pm.Merge(patches)
+func (rmF *Factory) Merge(incoming []*resource.Resource) (ResMap, error) {
+	return (&merginator{cdf: rmF.cdf}).Merge(incoming)
 }
 
 func newResMapFromResourceSlice(

api/resmap/merginator.go

@@ -0,0 +1,118 @@
+// Copyright 2020 The Kubernetes Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+package resmap
+
+import (
+	"fmt"
+
+	"sigs.k8s.io/kustomize/api/resource"
+)
+
+// merginator coordinates merging the resources in incoming to the result.
+type merginator struct {
+	incoming []*resource.Resource
+	cdf      resource.ConflictDetectorFactory
+	result   ResMap
+}
+
+func (m *merginator) Merge(in []*resource.Resource) (ResMap, error) {
+	m.result = New()
+	m.incoming = in
+	for index := range m.incoming {
+		alreadyInResult, err := m.appendIfNoMatch(index)
+		if err != nil {
+			return nil, err
+		}
+		if alreadyInResult != nil {
+			// The resource at index has the same resId as a previously
+			// considered resource.
+			//
+			// If they conflict with each other (e.g. they both want to change
+			// the image name in a Deployment, but to different values),
+			// return an error.
+			//
+			// If they don't conflict, then merge them into a single resource,
+			// since they both target the same item, and we want cumulative
+			// behavior. E.g. say both patches modify a map.  Without a merge,
+			// the last patch wins, replacing the entire map.
+			err = m.mergeWithExisting(index, alreadyInResult)
+			if err != nil {
+				return nil, err
+			}
+		}
+	}
+	return m.result, nil
+}
+
+func (m *merginator) appendIfNoMatch(index int) (*resource.Resource, error) {
+	candidate := m.incoming[index]
+	matchedResources := m.result.GetMatchingResourcesByOriginalId(
+		candidate.OrgId().Equals)
+	if len(matchedResources) == 0 {
+		m.result.Append(candidate)
+		return nil, nil
+	}
+	if len(matchedResources) > 1 {
+		return nil, fmt.Errorf("multiple resources targeted by patch")
+	}
+	return matchedResources[0], nil
+}
+
+func (m *merginator) mergeWithExisting(
+	index int, alreadyInResult *resource.Resource) error {
+	candidate := m.incoming[index]
+	cd, err := m.cdf.New(candidate.OrgId().Gvk)
+	if err != nil {
+		return err
+	}
+	hasConflict, err := cd.HasConflict(candidate, alreadyInResult)
+	if err != nil {
+		return err
+	}
+	if hasConflict {
+		return m.makeError(cd, index)
+	}
+	merged, err := cd.MergePatches(alreadyInResult, candidate)
+	if err != nil {
+		return err
+	}
+	_, err = m.result.Replace(merged)
+	return err
+}
+
+// Make an error message describing the conflict.
+func (m *merginator) makeError(cd resource.ConflictDetector, index int) error {
+	conflict, err := m.findConflict(cd, index)
+	if err != nil {
+		return err
+	}
+	if conflict == nil {
+		return fmt.Errorf("expected conflict for %s", m.incoming[index].OrgId())
+	}
+	return fmt.Errorf(
+		"conflict between %#v at index %d and %#v",
+		m.incoming[index].Map(), index, conflict.Map())
+}
+
+// findConflict looks for a conflict in a resource slice.
+// It returns the first conflict between the resource at index
+// and some other resource.  Two resources can only conflict if
+// they have the same original ResId.
+func (m *merginator) findConflict(
+	cd resource.ConflictDetector, index int) (*resource.Resource, error) {
+	targetId := m.incoming[index].OrgId()
+	for i, p := range m.incoming {
+		if i == index || !targetId.Equals(p.OrgId()) {
+			continue
+		}
+		conflict, err := cd.HasConflict(p, m.incoming[index])
+		if err != nil {
+			return nil, err
+		}
+		if conflict {
+			return p, nil
+		}
+	}
+	return nil, nil
+}