From 5d8515403007b5e10fbd162c9cfe5de664a73330 Mon Sep 17 00:00:00 2001
From: Gregory Eremin <g@erem.in>
Date: Tue, 9 Sep 2014 17:40:41 +0400
Subject: [PATCH] Storage subpackage

---
 storage/counter.go |  68 +++++++++++++++++++++
 storage/storage.go | 144 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 212 insertions(+)
 create mode 100644 storage/counter.go
 create mode 100644 storage/storage.go

diff --git a/storage/counter.go b/storage/counter.go
new file mode 100644
index 0000000..2aab039
--- /dev/null
+++ b/storage/counter.go
@@ -0,0 +1,68 @@
+package storage
+
+import (
+	"sync"
+)
+
+const (
+	maxIndex = ^uint(0) // Max unit value
+)
+
+type (
+	// Counter is responsible for operating queue read and write indexes
+	counter struct {
+		write uint // Number of the record last written to the queue
+		read  uint // Number of the record last read from the queue
+		// If write index is greater than read index then there are unread messages
+		// If write index is less tham read index then max index was reached
+
+		mutex     sync.Mutex
+		stream    chan uint
+		streaming *sync.Cond
+	}
+)
+
+func newCounter(wi, ri uint) (c *counter) {
+	m := &sync.Mutex{}
+	m.Lock()
+
+	c = &counter{
+		write:     wi,
+		read:      ri,
+		stream:    make(chan uint),
+		streaming: sync.NewCond(m),
+	}
+
+	go c.increment()
+
+	return
+}
+
+func (c *counter) tryWrite(fn func(i uint) bool) {
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
+
+	if ok := fn(c.write + 1); ok {
+		c.write++
+		c.streaming.Signal()
+	}
+}
+
+func (c *counter) distance() uint {
+	d := c.write - c.read
+	if d < 0 {
+		d += maxIndex
+	}
+	return d
+}
+
+func (c *counter) increment() {
+	for {
+		if c.distance() == 0 {
+			c.streaming.Wait()
+		}
+
+		c.stream <- c.read + 1
+		c.read++
+	}
+}
diff --git a/storage/storage.go b/storage/storage.go
new file mode 100644
index 0000000..5e85f2f
--- /dev/null
+++ b/storage/storage.go
@@ -0,0 +1,144 @@
+package storage
+
+import (
+	"encoding/json"
+	"strconv"
+	"strings"
+	"time"
+
+	"bitbucket.org/ww/cabinet"
+)
+
+const (
+	stateMetaKey      = "state"
+	stateSaveInterval = 1 // seconds
+)
+
+type (
+	Storage struct {
+		kyoto    *cabinet.KCDB
+		counters map[string]*counter
+	}
+)
+
+func New(path string) (s *Storage, err error) {
+	kyoto := cabinet.New()
+	if err = kyoto.Open(path, cabinet.KCOWRITER|cabinet.KCOCREATE); err != nil {
+		return
+	}
+
+	s = &Storage{
+		kyoto:    kyoto,
+		counters: make(map[string]*counter),
+	}
+
+	return
+}
+
+func (s *Storage) Get(queue string, abort <-chan struct{}) (message []byte, ok bool) {
+	if _, ok := s.counters[queue]; !ok {
+		s.counters[queue] = newCounter(0, 0)
+	}
+
+	var index uint
+
+	select {
+	case index = <-s.counters[queue].stream:
+	case <-abort:
+		return
+	}
+
+	key := makeKey(queue, index)
+	message, err := s.kyoto.Get(key)
+	if err != nil {
+		return
+	}
+
+	if err = s.kyoto.Remove(key); err != nil {
+		return
+	}
+	ok = true
+
+	return
+}
+
+func (s *Storage) Put(queue string, message []byte) (err error) {
+	if _, ok := s.counters[queue]; !ok {
+		s.counters[queue] = newCounter(0, 0)
+	}
+
+	s.counters[queue].tryWrite(func(index uint) bool {
+		key := makeKey(queue, index)
+		err = s.kyoto.Set(key, message)
+
+		return (err == nil)
+	})
+
+	return
+}
+
+func (s *Storage) Close() (err error) {
+	if err = s.kyoto.Sync(true); err != nil {
+		return
+	}
+	err = s.kyoto.Close()
+
+	return
+}
+
+// State
+
+func (s *Storage) saveState() (err error) {
+	state := make(map[string]map[string]uint)
+	for queue, ctr := range s.counters {
+		state[queue] = map[string]uint{
+			"wi": ctr.write,
+			"ri": ctr.read,
+		}
+	}
+
+	jsn, _ := json.Marshal(state)
+	err = s.kyoto.Set([]byte(stateMetaKey), jsn)
+
+	return
+}
+
+func (s *Storage) loadState() (err error) {
+	var (
+		jsn   []byte
+		state = make(map[string]map[string]uint)
+	)
+
+	if jsn, err = s.kyoto.Get([]byte(stateMetaKey)); err != nil {
+		return
+	}
+	if err = json.Unmarshal(jsn, &state); err != nil {
+		return
+	}
+
+	for queue, meta := range state {
+		s.counters[queue] = newCounter(meta["wi"], meta["ri"])
+	}
+
+	return
+}
+
+func (s *Storage) keepStatePersisted() {
+	t := time.NewTicker(stateSaveInterval * time.Second)
+
+	for {
+		select {
+		case <-t.C:
+			if err := s.saveState(); err != nil {
+				panic("Failed to persist state")
+			}
+			if err := s.kyoto.Sync(false); err != nil {
+				panic("Failed to sync storage")
+			}
+		}
+	}
+}
+
+func makeKey(queue string, index uint) []byte {
+	return []byte(strings.Join([]string{queue, strconv.FormatUint(uint64(index), 10)}, "_"))
+}