Reduce number of synchronizations GODT-313

* [x] expononential cooldown of retries
* [x] do not trigger sync by counts
* [x] randomization of event poll interval

internal/store/cooldown.go

@@ -0,0 +1,65 @@
+// Copyright (c) 2020 Proton Technologies AG
+//
+// This file is part of ProtonMail Bridge.
+//
+// ProtonMail Bridge is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// ProtonMail Bridge is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with ProtonMail Bridge.  If not, see <https://www.gnu.org/licenses/>.
+
+package store
+
+import "time"
+
+type cooldown struct {
+	waitTimes []time.Duration
+	waitIndex int
+	lastTry   time.Time
+}
+
+func (c *cooldown) setExponentialWait(initial time.Duration, base int, maximum time.Duration) {
+	waitTimes := []time.Duration{}
+	t := initial
+	if base > 1 {
+		for t < maximum {
+			waitTimes = append(waitTimes, t)
+			t *= time.Duration(base)
+		}
+	}
+	waitTimes = append(waitTimes, maximum)
+	c.setWaitTimes(waitTimes...)
+}
+
+func (c *cooldown) setWaitTimes(newTimes ...time.Duration) {
+	c.waitTimes = newTimes
+	c.reset()
+}
+
+// isTooSoon™ returns whether the cooldown period is not yet over.
+func (c *cooldown) isTooSoon() bool {
+	if time.Since(c.lastTry) < c.waitTimes[c.waitIndex] {
+		return true
+	}
+	c.lastTry = time.Now()
+	return false
+}
+
+func (c *cooldown) increaseWaitTime() {
+	c.lastTry = time.Now()
+	if c.waitIndex+1 < len(c.waitTimes) {
+		c.waitIndex++
+	}
+}
+
+func (c *cooldown) reset() {
+	c.waitIndex = 0
+	c.lastTry = time.Time{}
+}