[RFC] Add method to get reference number for TSDB Appender (#8600)

* Add method to get reference number for TSDB Appender

In situations where we need to copy labels before calling Add(),
GetRef() allows to check first, then call AddFast() in the case that the series
is already known.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

* Add explicit interface for GetRef() method

Suggested in code review by @bwplotka

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

* Rename OptionalGetRef to GetRef

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

* Simplify return value of GetRef()

0 can be relied on to mean 'no reference'

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

storage/interface.go

@@ -180,6 +180,14 @@ type Appender interface {
 	ExemplarAppender
 }
 
+// GetRef is an extra interface on Appenders used by downstream projects
+// (e.g. Cortex) to avoid maintaining a parallel set of references.
+type GetRef interface {
+	// Returns reference number that can be used to pass to Appender.Append().
+	// 0 means the appender does not have a reference to this series.
+	GetRef(lset labels.Labels) uint64
+}
+
 // ExemplarAppender provides an interface for adding samples to exemplar storage, which
 // within Prometheus is in-memory only.
 type ExemplarAppender interface {

tsdb/db.go

@@ -795,6 +795,15 @@ type dbAppender struct {
 	db *DB
 }
 
+var _ storage.GetRef = dbAppender{}
+
+func (a dbAppender) GetRef(lset labels.Labels) uint64 {
+	if g, ok := a.Appender.(storage.GetRef); ok {
+		return g.GetRef(lset)
+	}
+	return 0
+}
+
 func (a dbAppender) Commit() error {
 	err := a.Appender.Commit()
 

tsdb/head.go

@@ -1107,6 +1107,15 @@ func (a *initAppender) AppendExemplar(ref uint64, l labels.Labels, e exemplar.Ex
 	return a.app.AppendExemplar(ref, l, e)
 }
 
+var _ storage.GetRef = &initAppender{}
+
+func (a *initAppender) GetRef(lset labels.Labels) uint64 {
+	if g, ok := a.app.(storage.GetRef); ok {
+		return g.GetRef(lset)
+	}
+	return 0
+}
+
 func (a *initAppender) Commit() error {
 	if a.app == nil {
 		return nil
@@ -1331,6 +1340,16 @@ func (a *headAppender) AppendExemplar(ref uint64, _ labels.Labels, e exemplar.Ex
 	return s.ref, nil
 }
 
+var _ storage.GetRef = &headAppender{}
+
+func (a *headAppender) GetRef(lset labels.Labels) uint64 {
+	s := a.head.series.getByHash(lset.Hash(), lset)
+	if s == nil {
+		return 0
+	}
+	return s.ref
+}
+
 func (a *headAppender) log() error {
 	if a.head.wal == nil {
 		return nil