gui, lib/config, lib/model: Allow absolute values for minimum disk free space (fixes #3307)

This deprecates the current minDiskFreePct setting and introduces
minDiskFree. The latter is, in it's serialized form, a string with a
unit. We accept percentages ("2.35%") and absolute values ("250 k", "12.5
Gi"). Common suffixes are understood. The config editor lets the user
enter the string, and validates it.

We still default to "1 %", but the user can change that to an absolute
value at will.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4087
LGTM: AudriusButkevicius, imsodin

lib/config/size.go

@@ -0,0 +1,75 @@
+// Copyright (C) 2017 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package config
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+type Size struct {
+	Value float64 `json:"value" xml:",chardata"`
+	Unit  string  `json:"unit" xml:"unit,attr"`
+}
+
+func ParseSize(s string) (Size, error) {
+	s = strings.TrimSpace(s)
+	if len(s) == 0 {
+		return Size{}, nil
+	}
+
+	var num, unit string
+	for i := 0; i < len(s) && (s[i] >= '0' && s[i] <= '9' || s[i] == '.' || s[i] == ','); i++ {
+		num = s[:i+1]
+	}
+	var i = len(num)
+	for i < len(s) && s[i] == ' ' {
+		i++
+	}
+	unit = s[i:]
+
+	val, err := strconv.ParseFloat(num, 64)
+	if err != nil {
+		return Size{}, err
+	}
+
+	return Size{val, unit}, nil
+}
+
+func (s Size) BaseValue() float64 {
+	unitPrefix := s.Unit
+	if len(unitPrefix) > 1 {
+		unitPrefix = unitPrefix[:1]
+	}
+
+	mult := 1.0
+	switch unitPrefix {
+	case "k", "K":
+		mult = 1000
+	case "m", "M":
+		mult = 1000 * 1000
+	case "g", "G":
+		mult = 1000 * 1000 * 1000
+	case "t", "T":
+		mult = 1000 * 1000 * 1000 * 1000
+	}
+
+	return s.Value * mult
+}
+
+func (s Size) Percentage() bool {
+	return strings.Contains(s.Unit, "%")
+}
+
+func (s Size) String() string {
+	return fmt.Sprintf("%v %s", s.Value, s.Unit)
+}
+
+func (Size) ParseDefault(s string) (interface{}, error) {
+	return ParseSize(s)
+}