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/config.go

@@ -29,7 +29,7 @@ import (
 
 const (
 	OldestHandledVersion = 10
-	CurrentVersion       = 19
+	CurrentVersion       = 20
 	MaxRescanIntervalS   = 365 * 24 * 60 * 60
 )
 
@@ -292,6 +292,9 @@ func (cfg *Configuration) clean() error {
 	if cfg.Version == 18 {
 		convertV18V19(cfg)
 	}
+	if cfg.Version == 19 {
+		convertV19V20(cfg)
+	}
 
 	// Build a list of available devices
 	existingDevices := make(map[protocol.DeviceID]bool)
@@ -341,6 +344,18 @@ func (cfg *Configuration) clean() error {
 	return nil
 }
 
+func convertV19V20(cfg *Configuration) {
+	cfg.Options.MinHomeDiskFree = Size{Value: cfg.Options.DeprecatedMinHomeDiskFreePct, Unit: "%"}
+	cfg.Options.DeprecatedMinHomeDiskFreePct = 0
+
+	for i := range cfg.Folders {
+		cfg.Folders[i].MinDiskFree = Size{Value: cfg.Folders[i].DeprecatedMinDiskFreePct, Unit: "%"}
+		cfg.Folders[i].DeprecatedMinDiskFreePct = 0
+	}
+
+	cfg.Version = 20
+}
+
 func convertV18V19(cfg *Configuration) {
 	// Triggers a database tweak
 	cfg.Version = 19
@@ -537,7 +552,7 @@ func convertV11V12(cfg *Configuration) {
 func convertV10V11(cfg *Configuration) {
 	// Set minimum disk free of existing folders to 1%
 	for i := range cfg.Folders {
-		cfg.Folders[i].MinDiskFreePct = 1
+		cfg.Folders[i].DeprecatedMinDiskFreePct = 1
 	}
 	cfg.Version = 11
 }