feat: enforce monotonicity in terraform provider

Previous value must come from env var. To read tfstate requires
changing param from a `data` block to a `resource` block

Author: Emyrk

provider/parameter.go

@@ -144,7 +144,13 @@ func parameterDataSource() *schema.Resource {
 				input = &envValue
 			}
 
-			value, diags := parameter.ValidateInput(input)
+			var previous *string
+			envPreviousValue, ok := os.LookupEnv(ParameterEnvironmentVariablePrevious(parameter.Name))
+			if ok {
+				previous = &envPreviousValue
+			}
+
+			value, diags := parameter.ValidateInput(input, previous)
 			if diags.HasError() {
 				return diags
 			}
@@ -393,7 +399,7 @@ func valueIsType(typ OptionType, value string) error {
 	return nil
 }
 
-func (v *Parameter) ValidateInput(input *string) (string, diag.Diagnostics) {
+func (v *Parameter) ValidateInput(input *string, previous *string) (string, diag.Diagnostics) {
 	var err error
 	var optionType OptionType
 
@@ -436,7 +442,7 @@ func (v *Parameter) ValidateInput(input *string) (string, diag.Diagnostics) {
 		forcedValue = *value
 	}
 
-	d := v.validValue(forcedValue, optionType, optionValues, cty.Path{})
+	d := v.validValue(forcedValue, previous, optionType, optionValues, cty.Path{})
 	if d.HasError() {
 		return "", d
 	}
@@ -500,7 +506,7 @@ func (v *Parameter) ValidOptions(optionType OptionType) (map[string]struct{}, di
 	return optionValues, nil
 }
 
-func (v *Parameter) validValue(value string, optionType OptionType, optionValues map[string]struct{}, path cty.Path) diag.Diagnostics {
+func (v *Parameter) validValue(value string, previous *string, optionType OptionType, optionValues map[string]struct{}, path cty.Path) diag.Diagnostics {
 	// name is used for constructing more precise error messages.
 	name := "Value"
 	if path.Equals(defaultValuePath) {
@@ -567,7 +573,7 @@ func (v *Parameter) validValue(value string, optionType OptionType, optionValues
 
 	if len(v.Validation) == 1 {
 		validCheck := &v.Validation[0]
-		err := validCheck.Valid(v.Type, value)
+		err := validCheck.Valid(v.Type, value, previous)
 		if err != nil {
 			return diag.Diagnostics{
 				{
@@ -583,7 +589,7 @@ func (v *Parameter) validValue(value string, optionType OptionType, optionValues
 	return nil
 }
 
-func (v *Validation) Valid(typ OptionType, value string) error {
+func (v *Validation) Valid(typ OptionType, value string, previous *string) error {
 	if typ != OptionTypeNumber {
 		if !v.MinDisabled {
 			return fmt.Errorf("a min cannot be specified for a %s type", typ)
@@ -633,6 +639,28 @@ func (v *Validation) Valid(typ OptionType, value string) error {
 		if v.Monotonic != "" && v.Monotonic != ValidationMonotonicIncreasing && v.Monotonic != ValidationMonotonicDecreasing {
 			return fmt.Errorf("number monotonicity can be either %q or %q", ValidationMonotonicIncreasing, ValidationMonotonicDecreasing)
 		}
+
+		switch v.Monotonic {
+		case "":
+			// No monotonicity check
+		case ValidationMonotonicIncreasing, ValidationMonotonicDecreasing:
+			if previous != nil { // Only check if previous value exists
+				previousNum, err := strconv.Atoi(*previous)
+				if err != nil {
+					return fmt.Errorf("previous value %q is not a number", *previous)
+				}
+
+				if v.Monotonic == ValidationMonotonicIncreasing && !(num >= previousNum) {
+					return fmt.Errorf("parameter value '%d' must be equal or greater than previous value: %d", num, previousNum)
+				}
+
+				if v.Monotonic == ValidationMonotonicDecreasing && !(num <= previousNum) {
+					return fmt.Errorf("parameter value '%d' must be equal or lower than previous value: %d", num, previousNum)
+				}
+			}
+		default:
+			return fmt.Errorf("number monotonicity can be either %q or %q", ValidationMonotonicIncreasing, ValidationMonotonicDecreasing)
+		}
 	case OptionTypeListString:
 		var listOfStrings []string
 		err := json.Unmarshal([]byte(value), &listOfStrings)
@@ -660,6 +688,15 @@ func ParameterEnvironmentVariable(name string) string {
 	return "CODER_PARAMETER_" + hex.EncodeToString(sum[:])
 }
 
+// ParameterEnvironmentVariablePrevious returns the environment variable to
+// specify for a parameter's previous value. This is used for workspace
+// subsequent builds after the first. Primarily to validate monotonicity in the
+// `validation` block.
+func ParameterEnvironmentVariablePrevious(name string) string {
+	sum := sha256.Sum256([]byte(name))
+	return "CODER_PARAMETER_PREVIOUS_" + hex.EncodeToString(sum[:])
+}
+
 func takeFirstError(errs ...error) error {
 	for _, err := range errs {
 		if err != nil {

provider/parameter_test.go

@@ -839,7 +839,7 @@ func TestParameterValidation(t *testing.T) {
 		t.Run(tc.Name, func(t *testing.T) {
 			t.Parallel()
 			value := &tc.Value
-			_, diags := tc.Parameter.ValidateInput(value)
+			_, diags := tc.Parameter.ValidateInput(value, nil)
 			if tc.ExpectError != nil {
 				require.True(t, diags.HasError())
 				errMsg := fmt.Sprintf("%+v", diags[0]) // close enough
@@ -919,11 +919,19 @@ func TestParameterValidationEnforcement(t *testing.T) {
 
 		var validation *provider.Validation
 		if columns[5] != "" {
-			// Min-Max validation should look like:
-			//	1-10    :: min=1, max=10
-			//	-10     :: max=10
-			//	1-      :: min=1
-			if validMinMax.MatchString(columns[5]) {
+			switch {
+			case columns[5] == provider.ValidationMonotonicIncreasing || columns[5] == provider.ValidationMonotonicDecreasing:
+				validation = &provider.Validation{
+					MinDisabled: true,
+					MaxDisabled: true,
+					Monotonic:   columns[5],
+					Error:       "monotonicity",
+				}
+			case validMinMax.MatchString(columns[5]):
+				// Min-Max validation should look like:
+				//	1-10    :: min=1, max=10
+				//	-10     :: max=10
+				//	1-      :: min=1
 				parts := strings.Split(columns[5], "-")
 				min, _ := strconv.ParseInt(parts[0], 10, 64)
 				max, _ := strconv.ParseInt(parts[1], 10, 64)
@@ -936,7 +944,7 @@ func TestParameterValidationEnforcement(t *testing.T) {
 					Regex:       "",
 					Error:       "{min} < {value} < {max}",
 				}
-			} else {
+			default:
 				validation = &provider.Validation{
 					Min:         0,
 					MinDisabled: true,
@@ -1067,6 +1075,7 @@ func TestValueValidatesType(t *testing.T) {
 		Name                     string
 		Type                     provider.OptionType
 		Value                    string
+		Previous                 *string
 		Regex                    string
 		RegexError               string
 		Min                      int
@@ -1154,6 +1163,56 @@ func TestValueValidatesType(t *testing.T) {
 		Min:       0,
 		Max:       2,
 		Monotonic: "decreasing",
+	}, {
+		Name:        "IncreasingMonotonicityEqual",
+		Type:        "number",
+		Previous:    ptr("1"),
+		Value:       "1",
+		Monotonic:   "increasing",
+		MinDisabled: true,
+		MaxDisabled: true,
+	}, {
+		Name:        "DecreasingMonotonicityEqual",
+		Type:        "number",
+		Value:       "1",
+		Previous:    ptr("1"),
+		Monotonic:   "decreasing",
+		MinDisabled: true,
+		MaxDisabled: true,
+	}, {
+		Name:        "IncreasingMonotonicityGreater",
+		Type:        "number",
+		Previous:    ptr("0"),
+		Value:       "1",
+		Monotonic:   "increasing",
+		MinDisabled: true,
+		MaxDisabled: true,
+	}, {
+		Name:        "DecreasingMonotonicityGreater",
+		Type:        "number",
+		Value:       "1",
+		Previous:    ptr("0"),
+		Monotonic:   "decreasing",
+		MinDisabled: true,
+		MaxDisabled: true,
+		Error:       regexp.MustCompile("must be equal or"),
+	}, {
+		Name:        "IncreasingMonotonicityLesser",
+		Type:        "number",
+		Previous:    ptr("2"),
+		Value:       "1",
+		Monotonic:   "increasing",
+		MinDisabled: true,
+		MaxDisabled: true,
+		Error:       regexp.MustCompile("must be equal or"),
+	}, {
+		Name:        "DecreasingMonotonicityLesser",
+		Type:        "number",
+		Value:       "1",
+		Previous:    ptr("2"),
+		Monotonic:   "decreasing",
+		MinDisabled: true,
+		MaxDisabled: true,
 	}, {
 		Name:        "ValidListOfStrings",
 		Type:        "list(string)",
@@ -1205,7 +1264,7 @@ func TestValueValidatesType(t *testing.T) {
 				Regex:       tc.Regex,
 				Error:       tc.RegexError,
 			}
-			err := v.Valid(tc.Type, tc.Value)
+			err := v.Valid(tc.Type, tc.Value, tc.Previous)
 			if tc.Error != nil {
 				require.Error(t, err)
 				require.True(t, tc.Error.MatchString(err.Error()), "got: %s", err.Error())