Add go template shell completion for --format

The --format flags accepts go template strings. I use this often but I
consistently forget the field names. This commit adds a way to provide
shell completion for the --format flag. It works by automatically
receiving the field names with the reflect package from the given
struct. This requires almost no maintenance since this ensures that we
always use the correct field names. This also works for nested structs.

```
$ podman ps --format "{{.P"
{{.Pid}}      {{.PIDNS}}    {{.Pod}}      {{.PodName}}  {{.Ports}}
```

NOTE: This only works when you use quotes otherwise the shell does not
provide completions. Also this does not work for fish at the moment.

Signed-off-by: Paul Holzinger <paul.holzinger@web.de>

1 files changed, 142 insertions, 0 deletions

diff --git a/cmd/podman/common/completion_test.go b/cmd/podman/common/completion_test.go
new file mode 100644
index 000000000..5bd627b85
--- /dev/null
+++ b/cmd/podman/common/completion_test.go
@@ -0,0 +1,142 @@
+package common_test
+
+import (
+	"testing"
+
+	"github.com/containers/podman/v3/cmd/podman/common"
+	"github.com/spf13/cobra"
+	"github.com/stretchr/testify/assert"
+)
+
+type Car struct {
+	Brand string
+	Stats struct {
+		HP           *int
+		Displacement int
+	}
+	Extras map[string]string
+}
+
+func (c Car) Type() string {
+	return ""
+}
+
+func (c Car) Color() string {
+	return ""
+}
+
+func TestAutocompleteFormat(t *testing.T) {
+	testStruct := struct {
+		Name string
+		Age  int
+		Car  *Car
+	}{}
+
+	testStruct.Car = &Car{}
+	testStruct.Car.Extras = map[string]string{"test": "1"}
+
+	tests := []struct {
+		name       string
+		toComplete string
+		expected   []string
+	}{
+		{
+			"empty completion",
+			"",
+			[]string{"json"},
+		},
+		{
+			"json completion",
+			"json",
+			[]string{"json"},
+		},
+		{
+			"invalid completion",
+			"blahblah",
+			nil,
+		},
+		{
+			"invalid completion",
+			"{{",
+			nil,
+		},
+		{
+			"invalid completion",
+			"{{  ",
+			nil,
+		},
+		{
+			"invalid completion",
+			"{{  ..",
+			nil,
+		},
+		{
+			"fist level struct field name",
+			"{{.",
+			[]string{"{{.Name}}", "{{.Age}}", "{{.Car."},
+		},
+		{
+			"fist level struct field name",
+			"{{ .",
+			[]string{"{{ .Name}}", "{{ .Age}}", "{{ .Car."},
+		},
+		{
+			"fist level struct field name",
+			"{{ .N",
+			[]string{"{{ .Name}}"},
+		},
+		{
+			"second level struct field name",
+			"{{ .Car.",
+			[]string{"{{ .Car.Brand}}", "{{ .Car.Stats.", "{{ .Car.Extras}}", "{{ .Car.Color}}", "{{ .Car.Type}}"},
+		},
+		{
+			"second level struct field name",
+			"{{ .Car.B",
+			[]string{"{{ .Car.Brand}}"},
+		},
+		{
+			"three level struct field name",
+			"{{ .Car.Stats.",
+			[]string{"{{ .Car.Stats.HP}}", "{{ .Car.Stats.Displacement}}"},
+		},
+		{
+			"three level struct field name",
+			"{{ .Car.Stats.D",
+			[]string{"{{ .Car.Stats.Displacement}}"},
+		},
+		{
+			"second level struct field name",
+			"{{ .Car.B",
+			[]string{"{{ .Car.Brand}}"},
+		},
+		{
+			"invalid field name",
+			"{{ .Ca.B",
+			nil,
+		},
+		{
+			"map key names don't work",
+			"{{ .Car.Extras.",
+			nil,
+		},
+		{
+			"two variables struct field name",
+			"{{ .Car.Brand }} {{ .Car.",
+			[]string{"{{ .Car.Brand }} {{ .Car.Brand}}", "{{ .Car.Brand }} {{ .Car.Stats.", "{{ .Car.Brand }} {{ .Car.Extras}}",
+				"{{ .Car.Brand }} {{ .Car.Color}}", "{{ .Car.Brand }} {{ .Car.Type}}"},
+		},
+		{
+			"only dot without variable",
+			".",
+			nil,
+		},
+	}
+
+	for _, test := range tests {
+		completion, directive := common.AutocompleteFormat(testStruct)(nil, nil, test.toComplete)
+		// directive should always be greater than ShellCompDirectiveNoFileComp
+		assert.GreaterOrEqual(t, directive, cobra.ShellCompDirectiveNoFileComp, "unexpected ShellCompDirective")
+		assert.Equal(t, test.expected, completion, test.name)
+	}
+}