Improve builtin generation rules.

Makefile

@@ -48,10 +48,12 @@ $(MYGOBIN)/goimports:
 	cd api; \
 	go install golang.org/x/tools/cmd/goimports
 
-# Version pinned by api/go.mod
+# To pin pluginator, use this recipe instead:
+#   cd api;
+#   go install sigs.k8s.io/kustomize/pluginator/v2
 $(MYGOBIN)/pluginator:
-	cd api; \
-	go install sigs.k8s.io/kustomize/pluginator/v2
+	cd pluginator; \
+	go install .
 
 # Install kustomize from whatever is checked out.
 $(MYGOBIN)/kustomize:
@@ -66,23 +68,91 @@ install-tools: \
 	$(MYGOBIN)/pluginator \
 	$(MYGOBIN)/stringer
 
-# Builtin plugins are generated code.
-# Add new items here to create new builtins.
-builtinplugins = \
-	api/builtins/annotationstransformer.go \
-	api/builtins/configmapgenerator.go \
-	api/builtins/hashtransformer.go \
-	api/builtins/imagetagtransformer.go \
-	api/builtins/inventorytransformer.go \
-	api/builtins/labeltransformer.go \
-	api/builtins/legacyordertransformer.go \
-	api/builtins/namespacetransformer.go \
-	api/builtins/patchjson6902transformer.go \
-	api/builtins/patchstrategicmergetransformer.go \
-	api/builtins/patchtransformer.go \
-	api/builtins/prefixsuffixtransformer.go \
-	api/builtins/replicacounttransformer.go \
-	api/builtins/secretgenerator.go
+### Begin kustomize plugin rules.
+#
+# The rules to deal with builtin plugins are a bit
+# complicated because
+#
+# - Every builtin plugin is a Go plugin -
+#   meaning it gets its own module directory
+#   (outside of the api module) with Go
+#   code in a 'main' package per Go plugin rules.
+# - kustomize locates plugins using the
+#   'apiVersion' and 'kind' fields from the
+#   plugin config file.
+# - k8s wants CamelCase in 'kind' fields.
+# - The module name (the last name in the path)
+#   must be the lowercased 'kind' of the
+#   plugin because Go and related tools
+#   demand lowercase in import paths, but
+#   allow CamelCase in file names.
+# - the generated code must live in the api
+#   module (it's linked into the api).
+
+# Where all generated builtin plugin code should go.
+pGen=api/builtins
+# Where the builtin Go plugin modules live.
+pSrc=plugin/builtin
+
+_builtinplugins = \
+	AnnotationsTransformer.go \
+	ConfigMapGenerator.go \
+	HashTransformer.go \
+	ImageTagTransformer.go \
+	InventoryTransformer.go \
+	LabelTransformer.go \
+	LegacyOrderTransformer.go \
+	NamespaceTransformer.go \
+	PatchJson6902Transformer.go \
+	PatchStrategicMergeTransformer.go \
+	PatchTransformer.go \
+	PrefixSuffixTransformer.go \
+	ReplicaCountTransformer.go \
+	SecretGenerator.go
+
+# Maintaining this explicit list of generated files, and
+# adding it as a dependency to a few targets, to assure
+# they get recreated if deleted.  The rules below on how
+# to make them don't, by themselves, assure they will be
+# recreated if deleted.
+builtinplugins = $(patsubst %,$(pGen)/%,$(_builtinplugins))
+
+# These rules are verbose, but assure that if a source file
+# is modified, the corresponding generated file, and only
+# that file, will be recreated.
+$(pGen)/AnnotationsTransformer.go: $(pSrc)/annotationstransformer/AnnotationsTransformer.go
+$(pGen)/ConfigMapGenerator.go: $(pSrc)/configmapgenerator/ConfigMapGenerator.go
+$(pGen)/HashTransformer.go: $(pSrc)/hashtransformer/HashTransformer.go
+$(pGen)/ImageTagTransformer.go: $(pSrc)/imagetagtransformer/ImageTagTransformer.go
+$(pGen)/InventoryTransformer.go: $(pSrc)/inventorytransformer/InventoryTransformer.go
+$(pGen)/LabelTransformer.go: $(pSrc)/labeltransformer/LabelTransformer.go
+$(pGen)/LegacyOrderTransformer.go: $(pSrc)/legacyordertransformer/LegacyOrderTransformer.go
+$(pGen)/NamespaceTransformer.go: $(pSrc)/namespacetransformer/NamespaceTransformer.go
+$(pGen)/PatchJson6902Transformer.go: $(pSrc)/patchjson6902transformer/PatchJson6902Transformer.go
+$(pGen)/PatchStrategicMergeTransformer.go: $(pSrc)/patchstrategicmergetransformer/PatchStrategicMergeTransformer.go
+$(pGen)/PatchTransformer.go: $(pSrc)/patchtransformer/PatchTransformer.go
+$(pGen)/PrefixSuffixTransformer.go: $(pSrc)/prefixsuffixtransformer/PrefixSuffixTransformer.go
+$(pGen)/ReplicaCountTransformer.go: $(pSrc)/replicacounttransformer/ReplicaCountTransformer.go
+$(pGen)/SecretGenerator.go: $(pSrc)/secretgenerator/SecretGenerator.go
+
+# The (verbose but portable) Makefile way to convert to lowercase.
+toLowerCase = $(subst A,a,$(subst B,b,$(subst C,c,$(subst D,d,$(subst E,e,$(subst F,f,$(subst G,g,$(subst H,h,$(subst I,i,$(subst J,j,$(subst K,k,$(subst L,l,$(subst M,m,$(subst N,n,$(subst O,o,$(subst P,p,$(subst Q,q,$(subst R,r,$(subst S,s,$(subst T,t,$(subst U,u,$(subst V,v,$(subst W,w,$(subst X,x,$(subst Y,y,$(subst Z,z,$1))))))))))))))))))))))))))
+
+$(pGen)/%.go: $(MYGOBIN)/pluginator
+	@echo "generating $*"
+	( \
+		set -e; \
+		cd $(pSrc)/$(call toLowerCase,$*); \
+		go generate .; \
+		cd ../../../$(pGen); \
+		$(MYGOBIN)/goimports -w $*.go \
+	)
+
+# Target is for debugging.
+.PHONY: generate-kustomize-builtin-plugins
+generate-kustomize-builtin-plugins: $(builtinplugins)
+
+### End kustomize plugin rules.
 
 .PHONY: lint-kustomize
 lint-kustomize: install-tools $(builtinplugins)
@@ -93,22 +163,6 @@ lint-kustomize: install-tools $(builtinplugins)
 	cd pluginator; \
 	$(MYGOBIN)/golangci-lint-kustomize -c ../.golangci-kustomize.yml run ./...
 
-# TODO: modify rule to trigger on source material.  E.g. editting
-#   plugin/builtin/namespacetransformer/NamespaceTransformer.go
-# should trigger regeneration of
-#   api/builtins/namespacetransformer.go
-# To faciliate a simple rule, rename the name of the generated
-# file to CamelCase format like the source material.
-api/builtins/%.go: $(MYGOBIN)/pluginator
-	@echo "generating $*"
-	( \
-		set -e; \
-		cd plugin/builtin/$*; \
-		go generate .; \
-		cd ../../../api/builtins; \
-		$(MYGOBIN)/goimports -w $*.go \
-	)
-
 .PHONY: test-unit-kustomize-api
 test-unit-kustomize-api: $(builtinplugins)
 	cd api; go test ./...