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_test.go

@@ -0,0 +1,72 @@
+// 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 "testing"
+
+func TestParseSize(t *testing.T) {
+	cases := []struct {
+		in  string
+		ok  bool
+		val float64
+		pct bool
+	}{
+		// We accept upper case SI units
+		{"5K", true, 5e3, false}, // even when they should be lower case
+		{"4 M", true, 4e6, false},
+		{"3G", true, 3e9, false},
+		{"2 T", true, 2e12, false},
+		// We accept lower case SI units out of user friendliness
+		{"1 k", true, 1e3, false},
+		{"2m", true, 2e6, false},
+		{"3 g", true, 3e9, false},
+		{"4t", true, 4e12, false},
+		// Fractions are OK
+		{"123.456 k", true, 123.456e3, false},
+		{"0.1234 m", true, 0.1234e6, false},
+		{"3.45 g", true, 3.45e9, false},
+		// We don't parse negative numbers
+		{"-1", false, 0, false},
+		{"-1k", false, 0, false},
+		{"-0.45g", false, 0, false},
+		// We accept various unit suffixes on the unit prefix
+		{"100 KBytes", true, 100e3, false},
+		{"100 Kbps", true, 100e3, false},
+		{"100 MAU", true, 100e6, false},
+		// Percentages are OK
+		{"1%", true, 1, true},
+		{"200%", true, 200, true},    // even large ones
+		{"200K%", true, 200e3, true}, // even with prefixes, although this makes no sense
+		{"2.34%", true, 2.34, true},  // fractions are A-ok
+		// The empty string is a valid zero
+		{"", true, 0, false},
+		{"  ", true, 0, false},
+	}
+
+	for _, tc := range cases {
+		size, err := ParseSize(tc.in)
+
+		if !tc.ok {
+			if err == nil {
+				t.Errorf("Unexpected nil error in UnmarshalText(%q)", tc.in)
+			}
+			continue
+		}
+
+		if err != nil {
+			t.Errorf("Unexpected error in UnmarshalText(%q): %v", tc.in, err)
+			continue
+		}
+		if size.BaseValue() > tc.val*1.001 || size.BaseValue() < tc.val*0.999 {
+			// Allow 0.1% slop due to floating point multiplication
+			t.Errorf("Incorrect value in UnmarshalText(%q): %v, wanted %v", tc.in, size.BaseValue(), tc.val)
+		}
+		if size.Percentage() != tc.pct {
+			t.Errorf("Incorrect percentage bool in UnmarshalText(%q): %v, wanted %v", tc.in, size.Percentage(), tc.pct)
+		}
+	}
+}