Build layered indexers.

The indexers will be extracted in a short while and wrapped accordingly with
these types.

Change-Id: I4d1abda4e46117210babad5aa0d42f9ca1f6594f

storage/metric/index.go

@@ -400,6 +400,203 @@ type MetricIndexer interface {
 	IndexMetrics(FingerprintMetricMapping) error
 }
 
+// IndexObserver listens and receives changes to a given
+// FingerprintMetricMapping.
+type IndexerObserver interface {
+	Observe(FingerprintMetricMapping) error
+}
+
+// IndexerProxy receives IndexMetric requests and proxies them to the underlying
+// MetricIndexer.  Upon success of the underlying receiver, the registered
+// IndexObservers are called serially.
+//
+// If an error occurs in the underlying MetricIndexer or any of the observers,
+// this proxy will not work any further and return the offending error in this
+// call or any subsequent ones.
+type IndexerProxy struct {
+	err error
+
+	i         MetricIndexer
+	observers []IndexerObserver
+}
+
+func (p *IndexerProxy) IndexMetrics(b FingerprintMetricMapping) error {
+	if p.err != nil {
+		return p.err
+	}
+	if p.err = p.i.IndexMetrics(b); p.err != nil {
+		return p.err
+	}
+
+	for _, o := range p.observers {
+		if p.err = o.Observe(b); p.err != nil {
+			return p.err
+		}
+	}
+
+	return nil
+}
+
+// Close closes the underlying indexer.
+func (p *IndexerProxy) Close() error {
+	if p.err != nil {
+		return p.err
+	}
+	if closer, ok := p.i.(io.Closer); ok {
+		p.err = closer.Close()
+		return p.err
+	}
+	return nil
+}
+
+// Close flushes the underlying index requests before closing.
+func (p *IndexerProxy) Flush() error {
+	if p.err != nil {
+		return p.err
+	}
+	if flusher, ok := p.i.(flusher); ok {
+		p.err = flusher.Flush()
+		return p.err
+	}
+	return nil
+}
+
+// NewIndexerProxy builds an IndexerProxy for the given configuration.
+func NewIndexerProxy(i MetricIndexer, o ...IndexerObserver) *IndexerProxy {
+	return &IndexerProxy{
+		i:         i,
+		observers: o,
+	}
+}
+
+// SynchronizedIndexer provides naive locking for any MetricIndexer.
+type SynchronizedIndexer struct {
+	mu sync.Mutex
+	i  MetricIndexer
+}
+
+func (i *SynchronizedIndexer) IndexMetrics(b FingerprintMetricMapping) error {
+	i.mu.Lock()
+	defer i.mu.Unlock()
+
+	return i.i.IndexMetrics(b)
+}
+
+type flusher interface {
+	Flush() error
+}
+
+func (i *SynchronizedIndexer) Flush() error {
+	if flusher, ok := i.i.(flusher); ok {
+		i.mu.Lock()
+		defer i.mu.Unlock()
+
+		return flusher.Flush()
+	}
+
+	return nil
+}
+
+func (i *SynchronizedIndexer) Close() error {
+	if closer, ok := i.i.(io.Closer); ok {
+		i.mu.Lock()
+		defer i.mu.Unlock()
+
+		return closer.Close()
+	}
+
+	return nil
+}
+
+// NewSynchronizedIndexer builds a new MetricIndexer.
+func NewSynchronizedIndexer(i MetricIndexer) *SynchronizedIndexer {
+	return &SynchronizedIndexer{
+		i: i,
+	}
+}
+
+// BufferedIndexer provides unsynchronized index buffering.
+//
+// If an error occurs in the underlying MetricIndexer or any of the observers,
+// this proxy will not work any further and return the offending error.
+type BufferedIndexer struct {
+	i MetricIndexer
+
+	limit int
+
+	buf []FingerprintMetricMapping
+
+	err error
+}
+
+func (i *BufferedIndexer) IndexMetrics(b FingerprintMetricMapping) error {
+	if i.err != nil {
+		return i.err
+	}
+
+	if len(i.buf) < i.limit {
+		i.buf = append(i.buf, b)
+
+		return nil
+	}
+
+	i.buf = append(i.buf)
+
+	i.err = i.Flush()
+
+	return i.err
+}
+
+// Flush writes all pending entries to the index.
+func (i *BufferedIndexer) Flush() error {
+	if i.err != nil {
+		return i.err
+	}
+
+	if len(i.buf) == 0 {
+		return nil
+	}
+
+	superset := FingerprintMetricMapping{}
+	for _, b := range i.buf {
+		for fp, m := range b {
+			if _, ok := superset[fp]; ok {
+				continue
+			}
+
+			superset[fp] = m
+		}
+	}
+
+	i.buf = make([]FingerprintMetricMapping, 0, i.limit)
+
+	i.err = i.i.IndexMetrics(superset)
+
+	return i.err
+}
+
+// Close flushes and closes the underlying buffer.
+func (i *BufferedIndexer) Close() error {
+	if err := i.Flush(); err != nil {
+		return err
+	}
+
+	if closer, ok := i.i.(io.Closer); ok {
+		return closer.Close()
+	}
+
+	return nil
+}
+
+func NewBufferedIndexer(i MetricIndexer, limit int) *BufferedIndexer {
+
+	return &BufferedIndexer{
+		i:     i,
+		limit: limit,
+		buf:   make([]FingerprintMetricMapping, 0, limit),
+	}
+}
+
 // TotalIndexer is a MetricIndexer that indexes all standard facets of a metric
 // that a user or the Prometheus subsystem would want to query against:
 //