Add treapstore, a treap-based in-memory data store

common/data/treapstore/store.go

+// Copyright 2016 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+// Package treapstore is a lightweight append-only in-memory key-value store
+// built on top a treap (tree + heap) implementation.
+//
+// treapstore is specifically focused on supporting the in-memory datastore
+// implementation at "github.com/luci/gae/impl/memory".
+package treapstore
+
+import (
+        "math/rand"
+        "sort"
+        "sync"
+        "sync/atomic"
+
+        "github.com/luci/gtreap"

[Vadim Sh., 2016/12/28 01:33:40]

do we need to fork it? IIRC, we forked gkvlite to fix some bugs that upstream
ignored. luci/gtreap looks to be identical to steveyen/gtreap.

[Vadim Sh., 2016/12/28 01:34:27]

nvm, saw another CL :)

[dnj, 2016/12/28 03:01:06]

Acknowledged. Yeah will actually put that forward as a pull request post-review
and if steveyen merges it back in will un-fork.

+)
+
+// Store is a generaly-purpose concurrently-accessible copy-on-write store built
+// on top of the github.com/steveyen/gtreap treap (tree + heap) implementation.
+//
+// A Store is composed of a series of named Collection instances, each of which
+// can hold data elements.
+//
+// A Store is created by calling New, and is initially in a read/write mode.
+// Derivative Stores can be created by calling a Store's Snapshot capability.
+// These Stores will be read-only, meaning their collections and those
+// collections' data data may only be accessed, not modified.
+type Store struct {
+        compare  gtreap.Compare
+        colls    map[string]*Collection
+        collLock *sync.RWMutex
+}
+
+// New creates a new read/write Store.
+func New(c gtreap.Compare) *Store {
+        return &Store{
+                compare:  c,
+                collLock: &sync.RWMutex{},
+        }
+}
+
+// IsReadOnly returns true if this Store is read-only.
+func (s *Store) IsReadOnly() bool { return s.collLock == nil }
+
+func (s *Store) assertNotReadOnly() {
+        if s.IsReadOnly() {
+                panic("store is read-only")
+        }
+}
+
+// Snapshot creates a snapshot of the Store and all of its Collection instances.

[Vadim Sh., 2016/12/28 01:33:40]

nit: "Snapshot creates a read-only copy of the Store ..."

This is more informative.

[dnj, 2016/12/28 03:01:06]

Done.

+// Because a Store is copy-on-write, this is a cheap operation.
+func (s *Store) Snapshot() *Store {
+        if s.IsReadOnly() {
+                return s
+        }
+
+        s.collLock.RLock()
+        defer s.collLock.RUnlock()
+
+        // Return a read-only Store with the new Collection set.
+        snap := &Store{
+                compare:  s.compare,
+                colls:    make(map[string]*Collection, len(s.colls)),
+                collLock: nil,
+        }
+
+        // Create a read-only Collection for each coll.
+        for k, coll := range s.colls {
+                newColl := &Collection{
+                        s:    snap,
+                        name: coll.name,
+                }
+                newColl.setRoot(coll.currentRoot())
+                snap.colls[k] = newColl
+        }
+        return snap
+}
+
+// GetCollectionNames returns the names of the Collections in this Store, sorted
+// alphabetically.
+func (s *Store) GetCollectionNames() []string {
+        if !s.IsReadOnly() {
+                s.collLock.RLock()
+                defer s.collLock.RUnlock()
+        }
+
+        names := make([]string, 0, len(s.colls))
+        for k := range s.colls {
+                names = append(names, k)
+        }
+        sort.Strings(names)
+        return names
+}
+
+// GetCollection returns the Collection with the specified name. If no such
+// Collection exists, GetCollection will return nil.
+func (s *Store) GetCollection(name string) *Collection {
+        if !s.IsReadOnly() {
+                s.collLock.RLock()
+                defer s.collLock.RUnlock()
+        }
+        return s.colls[name]
+}
+
+// GetOrCreateCollection returns a Collection with the specified name, creating
+// it if it does not exist. If s is read-only, this will panic regardless of
+// whether the named Collection exists.
+func (s *Store) GetOrCreateCollection(name string) *Collection {
+        s.assertNotReadOnly()
+
+        // Fast path: get the Collection using read lock.
+        coll := s.GetCollection(name)
+        if coll != nil {
+                return coll
+        }
+
+        // Might have to create the Collection. Grab the write lock.
+        s.collLock.Lock()
+        defer s.collLock.Unlock()
+
+        // Try to get the existing Collection again now that we hold the write lock.
+        coll = s.colls[name]
+        if coll != nil {
+                return coll
+        }
+
+        // Create a new read/write Collection.
+        coll = &Collection{
+                s:       s,
+                name:    name,
+                putLock: &sync.Mutex{},
+        }
+        coll.setRoot(gtreap.NewTreap(s.compare))
+
+        if s.colls == nil {
+                s.colls = make(map[string]*Collection)
+        }
+        s.colls[name] = coll
+        return coll
+}
+
+// Collection is a collection of Items.
+//
+// Collections belonging to read/write Store instances are, themselves,
+// read/write. Collections belonging to Snapshot instances are read-only.
+type Collection struct {
+        s    *Store

[Vadim Sh., 2016/12/28 01:33:39]

nit: this is unnecessary, just use 'putLock == nil' as a check for the read-only
bit, as in Store.

[dnj, 2016/12/28 03:01:06]

Done.

+        name string
+
+        root    *atomic.Value

[Vadim Sh., 2016/12/28 01:33:39]

I'm not sure atomic.Value is necessary since you use putLock too. Is there a
reason to use atomic.Value in addition to the lock?

[dnj, 2016/12/28 03:01:06]

Readers will grab this value w/out taking out a lock. I suppose I could use
RWLock. Probably cleaner, better performance. I'll do that.

+        putLock *sync.Mutex
+}
+
+// Name returns this Collection's name.
+func (c *Collection) Name() string { return c.name }
+
+// IsReadOnly returns true if this Collection is read-only.
+func (c *Collection) IsReadOnly() bool { return c.s.IsReadOnly() }
+
+// Min returns the smallest item in the collection.
+func (c *Collection) Min() gtreap.Item { return c.currentRoot().Min() }
+
+// Max returns the largest item in the collection.
+func (c *Collection) Max() gtreap.Item { return c.currentRoot().Max() }
+
+// Get returns the item in the Store that matches i, or nil if no such item
+// exists.
+func (c *Collection) Get(i gtreap.Item) gtreap.Item { return c.currentRoot().Get(i) }
+
+// Put adds an item to the Store.
+//
+// If the Store is read-only, Put will panic.
+func (c *Collection) Put(i gtreap.Item) {
+        c.s.assertNotReadOnly()
+
+        // Lock around the entire Upsert operation to serialize Puts.
+        c.putLock.Lock()
+        c.setRoot(c.currentRoot().Upsert(i, rand.Int()))

[Vadim Sh., 2016/12/28 01:33:40]

nit: do rand.Int() outside the lock

[dnj, 2016/12/28 03:01:06]

um ... yes done. /ashamed

+        c.putLock.Unlock()
+}
+
+// Delete deletes an item from the Collection, if such an item exists.
+func (c *Collection) Delete(i gtreap.Item) {
+        c.s.assertNotReadOnly()
+
+        c.putLock.Lock()
+        c.setRoot(c.currentRoot().Delete(i))
+        c.putLock.Unlock()
+}
+
+func (c *Collection) currentRoot() *gtreap.Treap {
+        if c.root == nil {
+                panic("collection has no root")
+        }
+        return c.root.Load().(*gtreap.Treap)
+}
+
+func (c *Collection) setRoot(root *gtreap.Treap) {
+        if c.root == nil {
+                c.root = &atomic.Value{}
+        }
+        c.root.Store(root)
+}
+
+// Iterator returns an iterator over the Store, starting at the supplied pivot
+// item.
+func (c *Collection) Iterator(pivot gtreap.Item) *gtreap.Iterator {
+        return c.currentRoot().Iterator(pivot)
+}