lib/ur: Implement crash (panic) reporting (fixes #959) (#5702)

* lib/ur: Implement crash (panic) reporting (fixes #959)

This implements a simple crash reporting method. It piggybacks on the
panic log files created by the monitor process, picking these up and
uploading them from the usage reporting routine.

A new config value points to the crash receiver base URL, which defaults
to "https://crash.syncthing.net/newcrash" (following the pattern of
"https://data.syncthing.net/newdata" for usage reports, but allowing us
to separate the service as required).

lib/config/migrations_test.go

@@ -0,0 +1,34 @@
+// Copyright (C) 2019 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 TestMigrateCrashReporting(t *testing.T) {
+	// When migrating from pre-crash-reporting configs, crash reporting is
+	// enabled if global discovery is enabled or if usage reporting is
+	// enabled (not just undecided).
+	cases := []struct {
+		opts    OptionsConfiguration
+		enabled bool
+	}{
+		{opts: OptionsConfiguration{URAccepted: 0, GlobalAnnEnabled: true}, enabled: true},
+		{opts: OptionsConfiguration{URAccepted: -1, GlobalAnnEnabled: true}, enabled: true},
+		{opts: OptionsConfiguration{URAccepted: 1, GlobalAnnEnabled: true}, enabled: true},
+		{opts: OptionsConfiguration{URAccepted: 0, GlobalAnnEnabled: false}, enabled: false},
+		{opts: OptionsConfiguration{URAccepted: -1, GlobalAnnEnabled: false}, enabled: false},
+		{opts: OptionsConfiguration{URAccepted: 1, GlobalAnnEnabled: false}, enabled: true},
+	}
+
+	for i, tc := range cases {
+		cfg := Configuration{Version: 28, Options: tc.opts}
+		migrations.apply(&cfg)
+		if cfg.Options.CREnabled != tc.enabled {
+			t.Errorf("%d: unexpected result, CREnabled: %v != %v", i, cfg.Options.CREnabled, tc.enabled)
+		}
+	}
+}