shell completion --format: work with nil structs

AutocompleteFormat() takes the format struct as argument. Often the structs
are deeply nested and contain other structs. Up until now if there was a
pointer to a struct the logic was not able to get the field names from
that, simply because the pointer was nil. However it is possible to
create a new initialized type with reflect.New(). This allows us to
complete all struct fields/functions even when there nil pointers.
Therefore we can drop the extra initialization which was done by some
callers.

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

2 files changed, 38 insertions, 15 deletions

diff --git a/cmd/podman/common/completion.go b/cmd/podman/common/completion.go
index c7d5d6d60..6f13eb5a2 100644
--- a/cmd/podman/common/completion.go
+++ b/cmd/podman/common/completion.go
@@ -987,14 +987,12 @@ func AutocompleteFormat(o interface{}) func(cmd *cobra.Command, args []string, t
 		fields := strings.Split(field[len(field)-1], ".")
 		f := reflect.ValueOf(o)
 		for i := 1; i < len(fields); i++ {
-			if f.Kind() == reflect.Ptr {
-				f = f.Elem()
-			}
-
-			// the only supported type is struct
-			if f.Kind() != reflect.Struct {
+			val := getActualStructType(f)
+			if val == nil {
+				// no struct return nothing to complete
 				return nil, cobra.ShellCompDirectiveNoFileComp
 			}
+			f = *val
 
 			// last field get all names to suggest
 			if i == len(fields)-1 {
@@ -1012,17 +1010,38 @@ func AutocompleteFormat(o interface{}) func(cmd *cobra.Command, args []string, t
 	}
 }
 
-// getStructFields reads all struct field names and method names and returns them.
-func getStructFields(f reflect.Value, prefix string) []string {
-	suggestions := []string{}
+// getActualStructType take the value and check if it is a struct,
+// if it is pointer it will dereference it and when it is nil,
+// it will create a new value from it to get the actual struct
+// returns nil when type is not a struct
+func getActualStructType(f reflect.Value) *reflect.Value {
 	// follow the pointer first
 	if f.Kind() == reflect.Ptr {
+		// if the pointer is nil we create a new value from the elements type
+		// this allows us to follow nil pointers and get the actual struct fields
+		if f.IsNil() {
+			f = reflect.New(f.Type().Elem())
+		}
 		f = f.Elem()
 	}
 	// we only support structs
 	if f.Kind() != reflect.Struct {
 		return nil
 	}
+	return &f
+}
+
+// getStructFields reads all struct field names and method names and returns them.
+func getStructFields(f reflect.Value, prefix string) []string {
+	suggestions := []string{}
+
+	val := getActualStructType(f)
+	if val == nil {
+		// no struct return nothing to complete
+		return nil
+	}
+	f = *val
+
 	// loop over all field names
 	for j := 0; j < f.NumField(); j++ {
 		field := f.Type().Field(j)
@@ -1043,7 +1062,7 @@ func getStructFields(f reflect.Value, prefix string) []string {
 		}
 		// if field is anonymous add the child fields as well
 		if field.Anonymous {
-			suggestions = append(suggestions, getStructFields(f.FieldByIndex([]int{j}), prefix)...)
+			suggestions = append(suggestions, getStructFields(f.Field(j), prefix)...)
 		}
 	}
 
diff --git a/cmd/podman/common/completion_test.go b/cmd/podman/common/completion_test.go
index d28ac3928..bfff9a96e 100644
--- a/cmd/podman/common/completion_test.go
+++ b/cmd/podman/common/completion_test.go
@@ -34,10 +34,9 @@ func TestAutocompleteFormat(t *testing.T) {
 		Name string
 		Age  int
 		Car  *Car
+		Car2 *Car
 		*Anonymous
-	}{
-		Anonymous: &Anonymous{},
-	}
+	}{}
 
 	testStruct.Car = &Car{}
 	testStruct.Car.Extras = map[string]string{"test": "1"}
@@ -80,12 +79,12 @@ func TestAutocompleteFormat(t *testing.T) {
 		{
 			"fist level struct field name",
 			"{{.",
-			[]string{"{{.Name}}", "{{.Age}}", "{{.Car.", "{{.Anonymous.", "{{.Hello}}"},
+			[]string{"{{.Name}}", "{{.Age}}", "{{.Car.", "{{.Car2.", "{{.Anonymous.", "{{.Hello}}"},
 		},
 		{
 			"fist level struct field name",
 			"{{ .",
-			[]string{"{{ .Name}}", "{{ .Age}}", "{{ .Car.", "{{ .Anonymous.", "{{ .Hello}}"},
+			[]string{"{{ .Name}}", "{{ .Age}}", "{{ .Car.", "{{ .Car2.", "{{ .Anonymous.", "{{ .Hello}}"},
 		},
 		{
 			"fist level struct field name",
@@ -103,6 +102,11 @@ func TestAutocompleteFormat(t *testing.T) {
 			[]string{"{{ .Car.Brand}}"},
 		},
 		{
+			"second level nil struct field name",
+			"{{ .Car2.",
+			[]string{"{{ .Car2.Brand}}", "{{ .Car2.Stats.", "{{ .Car2.Extras}}", "{{ .Car2.Color}}", "{{ .Car2.Type}}"},
+		},
+		{
 			"three level struct field name",
 			"{{ .Car.Stats.",
 			[]string{"{{ .Car.Stats.HP}}", "{{ .Car.Stats.Displacement}}"},