configstruct: add SetAny to parse config from the rc

Now that we have unified the config, we can make a much more
convenient rc interface which mirrors the command line exactly, rather
than using the structure of the internal Go structs.

fs/config/configstruct/configstruct.go

@@ -86,6 +86,62 @@ func StringToInterface(def any, in string) (newValue any, err error) {
 	return newValue, nil
 }
 
+// InterfaceToString turns in into a string
+//
+// This supports a subset of builtin types, string, integer types,
+// bool, time.Duration and []string.
+//
+// Builtin types are expected to be encoding as their natural
+// stringificatons as produced by fmt.Sprint except for []string which
+// is expected to be encoded a a CSV with empty array encoded as "".
+//
+// Any other types are expected to be encoded by their String()
+// methods and decoded by their `Set(s string) error` methods.
+func InterfaceToString(in any) (strValue string, err error) {
+	switch x := in.(type) {
+	case string:
+		// return strings unmodified
+		strValue = x
+	case int, int8, int16, int32, int64,
+		uint, uint8, uint16, uint32, uint64, uintptr,
+		float32, float64:
+		strValue = fmt.Sprint(in)
+	case bool:
+		strValue = fmt.Sprint(in)
+	case time.Duration:
+		strValue = fmt.Sprint(in)
+	case []string:
+		// CSV encode arrays of strings - ideally we would use
+		// fs.CommaSepList here but we can't as it would cause
+		// a circular import.
+		if len(x) == 0 {
+			strValue = ""
+		} else if len(x) == 1 && len(x[0]) == 0 {
+			strValue = `""`
+		} else {
+			var buf strings.Builder
+			w := csv.NewWriter(&buf)
+			err := w.Write(x)
+			if err != nil {
+				return "", err
+			}
+			w.Flush()
+			strValue = strings.TrimSpace(buf.String())
+		}
+	default:
+		// Try using a String method
+		if do, ok := in.(fmt.Stringer); ok {
+			strValue = do.String()
+		} else {
+			err = errors.New("don't know how to convert this")
+		}
+	}
+	if err != nil {
+		return "", fmt.Errorf("interpreting %T as string failed: %w", in, err)
+	}
+	return strValue, nil
+}
+
 // Item describes a single entry in the options structure
 type Item struct {
 	Name  string    // snake_case
@@ -157,6 +213,22 @@ func Items(opt any) (items []Item, err error) {
 	return items, nil
 }
 
+// setValue sets newValue to configValue returning an updated newValue
+func setValue(newValue any, configValue string) (any, error) {
+	newNewValue, err := StringToInterface(newValue, configValue)
+	if err != nil {
+		// Mask errors if setting an empty string as
+		// it isn't valid for all types.  This makes
+		// empty string be the equivalent of unset.
+		if configValue != "" {
+			return nil, err
+		}
+	} else {
+		newValue = newNewValue
+	}
+	return newValue, nil
+}
+
 // Set interprets the field names in defaults and looks up config
 // values in the config passed in.  Any values found in config will be
 // set in the opt structure.
@@ -178,18 +250,61 @@ func Set(config configmap.Getter, opt any) (err error) {
 	for _, defaultItem := range defaultItems {
 		newValue := defaultItem.Value
 		if configValue, ok := config.Get(defaultItem.Name); ok {
-			var newNewValue any
+			newValue, err = setValue(newValue, configValue)
-			newNewValue, err = StringToInterface(newValue, configValue)
 			if err != nil {
-				// Mask errors if setting an empty string as
-				// it isn't valid for all types.  This makes
-				// empty string be the equivalent of unset.
-				if configValue != "" {
 				return fmt.Errorf("couldn't parse config item %q = %q as %T: %w", defaultItem.Name, configValue, defaultItem.Value, err)
 			}
-			} else {
+		}
-				newValue = newNewValue
+		defaultItem.Set(newValue)
-			}
+	}
+	return nil
+}
+
+// setIfSameType set aPtr with b if they are the same type or returns false.
+func setIfSameType(aPtr interface{}, b interface{}) bool {
+	aVal := reflect.ValueOf(aPtr).Elem()
+	bVal := reflect.ValueOf(b)
+
+	if aVal.Type() != bVal.Type() {
+		return false
+	}
+	aVal.Set(bVal)
+	return true
+}
+
+// SetAny interprets the field names in defaults and looks up config
+// values in the config passed in.  Any values found in config will be
+// set in the opt structure.
+//
+// opt must be a pointer to a struct.  The struct should have entirely
+// public fields.  The field names are converted from CamelCase to
+// snake_case and looked up in the config supplied or a
+// `config:"field_name"` is looked up.
+//
+// If items are found then they are set directly if the correct type,
+// otherwise they are converted to string and then converted from
+// string to native types and set in opt.
+//
+// All the field types in the struct must implement fmt.Scanner.
+func SetAny(config map[string]any, opt any) (err error) {
+	defaultItems, err := Items(opt)
+	if err != nil {
+		return err
+	}
+	for _, defaultItem := range defaultItems {
+		newValue := defaultItem.Value
+		if configValue, ok := config[defaultItem.Name]; ok {
+			if !setIfSameType(&newValue, configValue) {
+				// Convert the config value to be a string
+				stringConfigValue, err := InterfaceToString(configValue)
+				if err != nil {
+					return err
+				}
+				newValue, err = setValue(newValue, stringConfigValue)
+				if err != nil {
+					return fmt.Errorf("couldn't parse config item %q = %q as %T: %w", defaultItem.Name, stringConfigValue, defaultItem.Value, err)
+				}
+			}
 		}
 		defaultItem.Set(newValue)
 	}

fs/config/configstruct/configstruct_test.go

@@ -176,6 +176,39 @@ func TestSetFull(t *testing.T) {
 	assert.Equal(t, want, in)
 }
 
+func TestSetAnyFull(t *testing.T) {
+	in := &Conf2{
+		PotatoPie:      "yum",
+		BeanStew:       true,
+		RaisinRoll:     42,
+		SausageOnStick: 101,
+		ForbiddenFruit: 6,
+		CookingTime:    fs.Duration(42 * time.Second),
+		TotalWeight:    fs.SizeSuffix(17 << 20),
+	}
+	m := map[string]any{
+		"spud_pie":         "YUM",
+		"bean_stew":        false,
+		"raisin_roll":      "43 ",
+		"sausage_on_stick": " 102 ",
+		"forbidden_fruit":  "0x7",
+		"cooking_time":     43 * time.Second,
+		"total_weight":     "18M",
+	}
+	want := &Conf2{
+		PotatoPie:      "YUM",
+		BeanStew:       false,
+		RaisinRoll:     43,
+		SausageOnStick: 102,
+		ForbiddenFruit: 7,
+		CookingTime:    fs.Duration(43 * time.Second),
+		TotalWeight:    fs.SizeSuffix(18 << 20),
+	}
+	err := configstruct.SetAny(m, in)
+	require.NoError(t, err)
+	assert.Equal(t, want, in)
+}
+
 func TestStringToInterface(t *testing.T) {
 	item := struct{ A int }{2}
 	for _, test := range []struct {
@@ -227,3 +260,47 @@ func TestStringToInterface(t *testing.T) {
 		}
 	}
 }
+
+func TestInterfaceToString(t *testing.T) {
+	item := struct{ A int }{2}
+	for _, test := range []struct {
+		in   any
+		want string
+		err  string
+	}{
+		{nil, "", "interpreting <nil> as string failed: don't know how to convert this"},
+		{"", "", ""},
+		{"   string   ", "   string   ", ""},
+		{int(123), "123", ""},
+		{int(0x123), "291", ""},
+		{int(-123), "-123", ""},
+		{false, "false", ""},
+		{true, "true", ""},
+		{uint(123), "123", ""},
+		{int64(123), "123", ""},
+		{item, "", "interpreting struct { A int } as string failed: don't know how to convert this"},
+		{fs.Duration(time.Second), "1s", ""},
+		{fs.Duration(61 * time.Second), "1m1s", ""},
+		{[]string{}, ``, ""},
+		{[]string{""}, `""`, ""},
+		{[]string{"", ""}, `,`, ""},
+		{[]string{"hello"}, `hello`, ""},
+		{[]string{"hello", "world"}, `hello,world`, ""},
+		{[]string{"hello", "", "world"}, `hello,,world`, ""},
+		{[]string{`hello, world`, `goodbye, world!`}, `"hello, world","goodbye, world!"`, ""},
+		{time.Second, "1s", ""},
+		{61 * time.Second, "1m1s", ""},
+		{fs.Mebi, "1Mi", ""},
+		{fs.Gibi, "1Gi", ""},
+	} {
+		what := fmt.Sprintf("interpret %#v as string", test.in)
+		got, err := configstruct.InterfaceToString(test.in)
+		if test.err == "" {
+			require.NoError(t, err, what)
+			assert.Equal(t, test.want, got, what)
+		} else {
+			assert.Equal(t, "", got)
+			assert.EqualError(t, err, test.err, what)
+		}
+	}
+}