Fix --watch-only of a single item with table output

pkg/kubectl/cmd/get/BUILD

@@ -22,6 +22,7 @@ go_library(
         "get.go",
         "get_flags.go",
         "humanreadable_flags.go",
+        "skip_printer.go",
         "sorter.go",
         "table_printer.go",
     ],
@@ -60,6 +61,7 @@ go_library(
         "//vendor/github.com/spf13/cobra:go_default_library",
         "//vendor/k8s.io/klog:go_default_library",
         "//vendor/k8s.io/utils/integer:go_default_library",
+        "//vendor/k8s.io/utils/pointer:go_default_library",
         "//vendor/vbom.ml/util/sortorder:go_default_library",
     ],
 )
@@ -97,7 +99,6 @@ go_test(
         "//staging/src/k8s.io/apimachinery/pkg/apis/meta/v1beta1:go_default_library",
         "//staging/src/k8s.io/apimachinery/pkg/runtime:go_default_library",
         "//staging/src/k8s.io/apimachinery/pkg/runtime/schema:go_default_library",
-        "//staging/src/k8s.io/apimachinery/pkg/runtime/serializer/json:go_default_library",
         "//staging/src/k8s.io/apimachinery/pkg/runtime/serializer/streaming:go_default_library",
         "//staging/src/k8s.io/apimachinery/pkg/util/diff:go_default_library",
         "//staging/src/k8s.io/apimachinery/pkg/watch:go_default_library",

pkg/kubectl/cmd/get/get.go

@@ -49,12 +49,13 @@ import (
 	"k8s.io/kubernetes/pkg/api/legacyscheme"
 	cmdutil "k8s.io/kubernetes/pkg/kubectl/cmd/util"
 	"k8s.io/kubernetes/pkg/kubectl/util/i18n"
+	utilpointer "k8s.io/utils/pointer"
 )
 
 // GetOptions contains the input to the get command.
 type GetOptions struct {
 	PrintFlags             *PrintFlags
-	ToPrinter              func(*meta.RESTMapping, bool, bool) (printers.ResourcePrinterFunc, error)
+	ToPrinter              func(*meta.RESTMapping, *bool, bool, bool) (printers.ResourcePrinterFunc, error)
 	IsHumanReadablePrinter bool
 	PrintWithOpenAPICols   bool
 
@@ -230,7 +231,7 @@ func (o *GetOptions) Complete(f cmdutil.Factory, cmd *cobra.Command, args []stri
 		o.IsHumanReadablePrinter = true
 	}
 
-	o.ToPrinter = func(mapping *meta.RESTMapping, withNamespace bool, withKind bool) (printers.ResourcePrinterFunc, error) {
+	o.ToPrinter = func(mapping *meta.RESTMapping, outputObjects *bool, withNamespace bool, withKind bool) (printers.ResourcePrinterFunc, error) {
 		// make a new copy of current flags / opts before mutating
 		printFlags := o.PrintFlags.Copy()
 
@@ -257,6 +258,9 @@ func (o *GetOptions) Complete(f cmdutil.Factory, cmd *cobra.Command, args []stri
 		if o.Sort {
 			printer = &SortingPrinter{Delegate: printer, SortField: sortBy}
 		}
+		if outputObjects != nil {
+			printer = &skipPrinter{delegate: printer, output: outputObjects}
+		}
 		if o.ServerPrint {
 			printer = &TablePrinter{Delegate: printer}
 		}
@@ -539,7 +543,7 @@ func (o *GetOptions) Run(f cmdutil.Factory, cmd *cobra.Command, args []string) e
 				fmt.Fprintln(o.ErrOut)
 			}
 
-			printer, err = o.ToPrinter(mapping, printWithNamespace, printWithKind)
+			printer, err = o.ToPrinter(mapping, nil, printWithNamespace, printWithKind)
 			if err != nil {
 				if !errs.Has(err.Error()) {
 					errs.Insert(err.Error())
@@ -635,7 +639,8 @@ func (o *GetOptions) watch(f cmdutil.Factory, cmd *cobra.Command, args []string)
 
 	info := infos[0]
 	mapping := info.ResourceMapping()
-	printer, err := o.ToPrinter(mapping, o.AllNamespaces, false)
+	outputObjects := utilpointer.BoolPtr(!o.WatchOnly)
+	printer, err := o.ToPrinter(mapping, outputObjects, o.AllNamespaces, false)
 	if err != nil {
 		return err
 	}
@@ -664,25 +669,29 @@ func (o *GetOptions) watch(f cmdutil.Factory, cmd *cobra.Command, args []string)
 	tableGK := metainternal.SchemeGroupVersion.WithKind("Table").GroupKind()
 
 	// print the current object
-	if !o.WatchOnly {
-		var objsToPrint []runtime.Object
-
-		if isList {
-			objsToPrint, _ = meta.ExtractList(obj)
-		} else {
-			objsToPrint = append(objsToPrint, obj)
+	var objsToPrint []runtime.Object
+	if isList {
+		objsToPrint, _ = meta.ExtractList(obj)
+	} else {
+		objsToPrint = append(objsToPrint, obj)
+	}
+	for _, objToPrint := range objsToPrint {
+		if o.IsHumanReadablePrinter && objToPrint.GetObjectKind().GroupVersionKind().GroupKind() != tableGK {
+			// printing anything other than tables always takes the internal version, but the watch event uses externals
+			internalGV := mapping.GroupVersionKind.GroupKind().WithVersion(runtime.APIVersionInternal).GroupVersion()
+			objToPrint = attemptToConvertToInternal(objToPrint, legacyscheme.Scheme, internalGV)
 		}
-		for _, objToPrint := range objsToPrint {
-			if o.IsHumanReadablePrinter && objToPrint.GetObjectKind().GroupVersionKind().GroupKind() != tableGK {
-				// printing anything other than tables always takes the internal version, but the watch event uses externals
-				internalGV := mapping.GroupVersionKind.GroupKind().WithVersion(runtime.APIVersionInternal).GroupVersion()
-				objToPrint = attemptToConvertToInternal(objToPrint, legacyscheme.Scheme, internalGV)
-			}
-			if err := printer.PrintObj(objToPrint, writer); err != nil {
-				return fmt.Errorf("unable to output the provided object: %v", err)
-			}
+		if err := printer.PrintObj(objToPrint, writer); err != nil {
+			return fmt.Errorf("unable to output the provided object: %v", err)
 		}
-		writer.Flush()
+	}
+	writer.Flush()
+	if isList {
+		// we can start outputting objects now, watches started from lists don't emit synthetic added events
+		*outputObjects = true
+	} else {
+		// suppress output, since watches started for individual items emit a synthetic ADDED event first
+		*outputObjects = false
 	}
 
 	// print watched changes
@@ -691,18 +700,11 @@ func (o *GetOptions) watch(f cmdutil.Factory, cmd *cobra.Command, args []string)
 		return err
 	}
 
-	first := true
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 	intr := interrupt.New(nil, cancel)
 	intr.Run(func() error {
 		_, err := watchtools.UntilWithoutRetry(ctx, w, func(e watch.Event) (bool, error) {
-			if !isList && first {
-				// drop the initial watch event in the single resource case
-				first = false
-				return false, nil
-			}
-
 			// printing always takes the internal version, but the watch event uses externals
 			// TODO fix printing to use server-side or be version agnostic
 			objToPrint := e.Object
@@ -714,6 +716,8 @@ func (o *GetOptions) watch(f cmdutil.Factory, cmd *cobra.Command, args []string)
 				return false, err
 			}
 			writer.Flush()
+			// after processing at least one event, start outputting objects
+			*outputObjects = true
 			return false, nil
 		})
 		return err
@@ -750,7 +754,7 @@ func (o *GetOptions) printGeneric(r *resource.Result) error {
 		return utilerrors.Reduce(utilerrors.Flatten(utilerrors.NewAggregate(errs)))
 	}
 
-	printer, err := o.ToPrinter(nil, false, false)
+	printer, err := o.ToPrinter(nil, nil, false, false)
 	if err != nil {
 		return err
 	}

pkg/kubectl/cmd/get/skip_printer.go

@@ -0,0 +1,48 @@
+/*
+Copyright 2019 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.
+*/
+
+package get
+
+import (
+	"io"
+
+	metav1beta1 "k8s.io/apimachinery/pkg/apis/meta/v1beta1"
+	"k8s.io/apimachinery/pkg/runtime"
+	"k8s.io/cli-runtime/pkg/printers"
+)
+
+// skipPrinter allows conditionally suppressing object output via the output field.
+// table objects are suppressed by setting their Rows to nil (allowing column definitions to propagate to the delegate).
+// non-table objects are suppressed by not calling the delegate at all.
+type skipPrinter struct {
+	delegate printers.ResourcePrinter
+	output   *bool
+}
+
+func (p *skipPrinter) PrintObj(obj runtime.Object, writer io.Writer) error {
+	if *p.output {
+		return p.delegate.PrintObj(obj, writer)
+	}
+
+	table, isTable := obj.(*metav1beta1.Table)
+	if !isTable {
+		return nil
+	}
+
+	table = table.DeepCopy()
+	table.Rows = nil
+	return p.delegate.PrintObj(table, writer)
+}

pkg/printers/tablegenerator.go

@@ -48,10 +48,11 @@ type handlerEntry struct {
 // will only be printed if the object type changes. This makes it useful for printing items
 // received from watches.
 type HumanReadablePrinter struct {
-	handlerMap  map[reflect.Type]*handlerEntry
-	options     PrintOptions
-	lastType    interface{}
-	lastColumns []metav1beta1.TableColumnDefinition
+	handlerMap     map[reflect.Type]*handlerEntry
+	options        PrintOptions
+	lastType       interface{}
+	lastColumns    []metav1beta1.TableColumnDefinition
+	printedHeaders bool
 }
 
 var _ TableGenerator = &HumanReadablePrinter{}

pkg/printers/tableprinter.go

@@ -80,18 +80,22 @@ func (h *HumanReadablePrinter) PrintObj(obj runtime.Object, output io.Writer) er
 	if table, ok := obj.(*metav1beta1.Table); ok {
 		// Do not print headers if this table has no column definitions, or they are the same as the last ones we printed
 		localOptions := h.options
-		if len(table.ColumnDefinitions) == 0 || reflect.DeepEqual(table.ColumnDefinitions, h.lastColumns) {
+		if h.printedHeaders && (len(table.ColumnDefinitions) == 0 || reflect.DeepEqual(table.ColumnDefinitions, h.lastColumns)) {
 			localOptions.NoHeaders = true
 		}
 
 		if len(table.ColumnDefinitions) == 0 {
 			// If this table has no column definitions, use the columns from the last table we printed for decoration and layout.
 			// This is done when receiving tables in watch events to save bandwidth.
-			localOptions.NoHeaders = true
 			table.ColumnDefinitions = h.lastColumns
-		} else {
+		} else if !reflect.DeepEqual(table.ColumnDefinitions, h.lastColumns) {
 			// If this table has column definitions, remember them for future use.
 			h.lastColumns = table.ColumnDefinitions
+			h.printedHeaders = false
+		}
+
+		if len(table.Rows) > 0 {
+			h.printedHeaders = true
 		}
 
 		if err := decorateTable(table, localOptions); err != nil {