Ensure that "null" can only be specified in "set-" commands (#2363)

This allows a check function to be set to determine whether it is
allowed to be set. This also ensures that if null is allowed it is
either the only value or not set.

internal/cmd/common/flags_test.go

@@ -3,6 +3,7 @@ package common
 import (
 	"encoding/json"
 	"fmt"
+	"strings"
 	"testing"
 
 	"github.com/hashicorp/boundary/internal/cmd/base"
@@ -422,3 +423,103 @@ func TestHandleAttributeFlags(t *testing.T) {
 		}
 	}
 }
+
+func TestNullableStringSlice(t *testing.T) {
+	makeStringSlicePointer := func(in ...string) *[]string {
+		return &in
+	}
+	tests := []struct {
+		name        string
+		cmd         string
+		args        []string
+		expected    base.StringSliceVar
+		expectedErr string
+	}{
+		{
+			name: "not-set-no-null",
+			cmd:  "add-values",
+			args: []string{"-val", "foobar", "-val", "barfoo", "-val", "boobaz", "-val", "bazboo"},
+			expected: base.StringSliceVar{
+				Target: makeStringSlicePointer("foobar", "barfoo", "boobaz", "bazboo"),
+			},
+		},
+		{
+			name:        "not-set-null",
+			cmd:         "add-values",
+			args:        []string{"-val", "foobar", "-val", "null", "-val", "boobaz", "-val", "bazboo"},
+			expectedErr: `"null" is not an allowed value`,
+		},
+		{
+			name: "set-no-null",
+			cmd:  "set-values",
+			args: []string{"-val", "foobar", "-val", "barfoo", "-val", "boobaz", "-val", "bazboo"},
+			expected: base.StringSliceVar{
+				Target: makeStringSlicePointer("foobar", "barfoo", "boobaz", "bazboo"),
+			},
+		},
+		{
+			name: "set-only-null",
+			cmd:  "set-values",
+			args: []string{"-val", "null"},
+			expected: base.StringSliceVar{
+				Target: makeStringSlicePointer("null"),
+			},
+		},
+		{
+			name:        "set-null-and-others-beginning",
+			cmd:         "set-values",
+			args:        []string{"-val", "null", "-val", "barfoo", "-val", "boobaz", "-val", "bazboo"},
+			expectedErr: `"null" cannot be combined with other values`,
+		},
+		{
+			name:        "set-null-and-others-middle",
+			cmd:         "set-values",
+			args:        []string{"-val", "foobar", "-val", "null", "-val", "boobaz", "-val", "bazboo"},
+			expectedErr: `"null" cannot be combined with other values`,
+		},
+		{
+			name:        "set-null-and-others-end",
+			cmd:         "set-values",
+			args:        []string{"-val", "foobar", "-val", "barfoo", "-val", "boobaz", "-val", "null"},
+			expectedErr: `"null" cannot be combined with other values`,
+		},
+		{
+			name: "set-null-multiple",
+			cmd:  "set-values",
+			args: []string{"-val", "null", "-val", "null"},
+			expected: base.StringSliceVar{
+				Target: makeStringSlicePointer("null"),
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			assert, require := assert.New(t), require.New(t)
+
+			// Note: we do the setup on each run to make sure we aren't carrying
+			// state over; just like in the real CLI where each run would have
+			// pristine state.
+			c := new(base.Command)
+			flagSet := c.FlagSet(base.FlagSetNone)
+			f := flagSet.NewFlagSet("Stringsssss")
+			var target []string
+			ssVar := &base.StringSliceVar{
+				Name:   "val",
+				Target: &target,
+				NullCheck: func() bool {
+					return strings.HasPrefix(tt.cmd, "set-")
+				},
+			}
+			f.StringSliceVar(ssVar)
+
+			err := flagSet.Parse(tt.args)
+			if tt.expectedErr != "" {
+				require.Error(err)
+				assert.Contains(err.Error(), tt.expectedErr)
+				return
+			}
+			require.NoError(err)
+			assert.Equal(*tt.expected.Target, *ssVar.Target)
+		})
+	}
+}