LogDog: Add collector service implementation.

server/internal/logdog/collector/coordinator/cache.go

Index: server/internal/logdog/collector/coordinator/cache.go
diff --git a/server/internal/logdog/collector/coordinator/cache.go b/server/internal/logdog/collector/coordinator/cache.go
new file mode 100644
index 0000000000000000000000000000000000000000..24b3cef9fca2bdcbc067f0f42005ab33b9047bf2
--- /dev/null
+++ b/server/internal/logdog/collector/coordinator/cache.go
@@ -0,0 +1,195 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package coordinator
+
+import (
+	"sync/atomic"
+	"time"
+
+	"github.com/luci/luci-go/common/clock"
+	"github.com/luci/luci-go/common/logdog/types"
+	log "github.com/luci/luci-go/common/logging"
+	"github.com/luci/luci-go/common/lru"
+	"github.com/luci/luci-go/common/promise"
+	"github.com/luci/luci-go/common/proto/logdog/logpb"
+	"golang.org/x/net/context"
+)
+
+const (
+	// DefaultSize is the default (maximum) size of the LRU cache.
+	DefaultSize = 1024 * 1024
+
+	// DefaultExpiration is the default expiration value.
+	DefaultExpiration = 10 * time.Minute
+)
+
+// cache is a Coordinator interface implementation for the Collector service
+// that caches remote results locally.
+type cache struct {
+	Coordinator
+
+	// Size is the number of stream states to hold in the cache. If zero,
+	// DefaultCacheSize will be used.
+	size int
+
+	// expiration is the maximum lifespan of a cache entry. If an entry is older
+	// than this, it will be discarded. If zero, DefaultExpiration will be used.
+	expiration time.Duration
+
+	// cache is the LRU state cache.
+	lru *lru.Cache
+}
+
+// NewCache creates a new Coordinator instance that wraps another Coordinator
+// instance with a cache that retains the latest remote Coordiantor state in a
+// client-side LRU cache.
+func NewCache(c Coordinator, size int, expiration time.Duration) Coordinator {
+	if size <= 0 {
+		size = DefaultSize
+	}
+	if expiration <= 0 {
+		expiration = DefaultExpiration
+	}
+
+	return &cache{
+		Coordinator: c,
+		expiration:  expiration,
+		lru:         lru.New(size),
+	}
+}
+
+// RegisterStream invokes the wrapped Coordinator's RegisterStream method and
+// caches the result. It uses a Promise to cause all simultaneous identical
+// RegisterStream requests to block on a single RPC.
+func (c *cache) RegisterStream(ctx context.Context, st *LogStreamState, d *logpb.LogStreamDescriptor) (
+	*LogStreamState, error) {
+	now := clock.Now(ctx)
+
+	// Get the cacheEntry from our cache. If it is expired, doesn't exist, or
+	// we're forcing, ignore any existing entry and replace with a Promise pending
+	// Coordinator sync.
+	entry := c.lru.Mutate(st.Path, func(current interface{}) interface{} {
+		// Don't replace an existing entry, unless it has an error or has expired.
+		if current != nil {
+			curEntry := current.(*cacheEntry)
+			if !curEntry.hasError() && now.Before(curEntry.expiresAt) {
+				return current
+			}
+		}
+
+		p := promise.New(func() (interface{}, error) {
+			st, err := c.Coordinator.RegisterStream(ctx, st, d)
+			if err != nil {
+				return nil, err
+			}
+
+			return &LogStreamState{
+				Path:          st.Path,
+				ProtoVersion:  st.ProtoVersion,
+				Secret:        st.Secret,
+				TerminalIndex: types.MessageIndex(st.TerminalIndex),
+				Archived:      st.Archived,
+				Purged:        st.Purged,
+			}, nil
+		})
+
+		return &cacheEntry{
+			terminalIndex: -1,
+			p:             p,
+			path:          st.Path,
+			expiresAt:     now.Add(c.expiration),
+		}
+	}).(*cacheEntry)
+
+	// If there was an error, purge the erroneous entry from the cache so that
+	// the next "update" will re-fetch it.
+	st, err := entry.get(ctx)
+	if err != nil {
+		log.Fields{
+			log.ErrorKey: err,
+		}.Errorf(ctx, "Error retrieving stream state.")
+		return nil, err
+	}
+	return st, nil
+}
+
+func (c *cache) TerminateStream(ctx context.Context, st *LogStreamState) error {
+	// Immediately update our state cache to record the terminal index, if
+	// we have a state cache.
+	c.lru.Mutate(st.Path, func(current interface{}) (r interface{}) {
+		// Always return the current entry. We're just atomically examining it to
+		// load it with a terminal index.
+		r = current
+		if r != nil {
+			r.(*cacheEntry).loadTerminalIndex(st.TerminalIndex)
+		}
+		return
+	})
+
+	return c.Coordinator.TerminateStream(ctx, st)
+}
+
+// cacheEntry is the value stored in the cache. It contains a Promise
+// representing the value and an optional invalidation singleton to ensure that
+// if the state failed to fetch, it will be invalidated at most once.
+//
+// In addition to remote caching via Promise, the state can be updated locally
+// by calling the cache's "put" method. In this case, the Promise will be nil,
+// and the state value will be populated.
+type cacheEntry struct {
+	// terminalIndex is the loaded terminal index set via loadTerminalIndex. It
+	// will be applied to returned LogStreamState objects so that once a terminal
+	// index has been set, we become aware of it in the Collector.
+	//
+	// This MUST be the first field in the struct in order to comply with atomic's
+	// 64-bit alignment requirements.
+	terminalIndex int64
+
+	// p is a Promise that is blocking pending a Coordiantor stream state
+	// response. Upon successful resolution, it will contain a *LogStreamState.
+	p         promise.Promise
+	path      types.StreamPath
+	expiresAt time.Time
+}
+
+// get returns the cached state that this entry owns, blocking until resolution
+// if necessary.
+//
+// The returned state is a shallow copy of the cached state, and may be
+// modified by the caller.
+func (e *cacheEntry) get(ctx context.Context) (*LogStreamState, error) {
+	promisedSt, err := e.p.Get(ctx)
+	if err != nil {
+		return nil, err
+	}
+
+	// Create a clone of our cached value (not deep, so secret is not cloned, but
+	// the Collector will not modify that). If we have a local terminal index
+	// cached, apply that to the response.
+	//
+	// We need to lock around our terminalIndex.
+	rp := *(promisedSt.(*LogStreamState))
+	if rp.TerminalIndex < 0 {
+		rp.TerminalIndex = types.MessageIndex(atomic.LoadInt64(&e.terminalIndex))
+	}
+
+	return &rp, nil
+}
+
+// hasError tests if this entry has completed evaluation with an error state.
+// This is non-blocking, so if the evaluation hasn't completed, it will return
+// false.
+func (e *cacheEntry) hasError() bool {
+	if _, err := e.p.Peek(); err != nil && err != promise.ErrNoData {
+		return true
+	}
+	return false
+}
+
+// loadTerminalIndex loads a local cache of the stream's terminal index. This
+// will be applied to all future get requests.
+func (e *cacheEntry) loadTerminalIndex(tidx types.MessageIndex) {
+	atomic.StoreInt64(&e.terminalIndex, int64(tidx))
+}