Add storage.Queryable and storage.QueryableFunc

In order to compose different querier implementations more easily, this
change introduces a separate storage.Queryable interface grouping the
query (Querier) function of the storage.

Furthermore, it adds a QueryableFunc type to ease writing very simple
queryable implementations.

storage/interface.go

@@ -31,12 +31,11 @@ var (
 // Storage ingests and manages samples, along with various indexes. All methods
 // are goroutine-safe. Storage implements storage.SampleAppender.
 type Storage interface {
+	Queryable
+
 	// StartTime returns the oldest timestamp stored in the storage.
 	StartTime() (int64, error)
 
-	// Querier returns a new Querier on the storage.
-	Querier(ctx context.Context, mint, maxt int64) (Querier, error)
-
 	// Appender returns a new appender against the storage.
 	Appender() (Appender, error)
 
@@ -44,6 +43,12 @@ type Storage interface {
 	Close() error
 }
 
+// A Queryable handles queries against a storage.
+type Queryable interface {
+	// Querier returns a new Querier on the storage.
+	Querier(ctx context.Context, mint, maxt int64) (Querier, error)
+}
+
 // Querier provides reading access to time series data.
 type Querier interface {
 	// Select returns a set of series that matches the given label matchers.
@@ -56,6 +61,15 @@ type Querier interface {
 	Close() error
 }
 
+// QueryableFunc is an adapter to allow the use of ordinary functions as
+// Queryables. It follows the idea of http.HandlerFunc.
+type QueryableFunc func(ctx context.Context, mint, maxt int64) (Querier, error)
+
+// Querier calls f() with the given parameters.
+func (f QueryableFunc) Querier(ctx context.Context, mint, maxt int64) (Querier, error) {
+	return f(ctx, mint, maxt)
+}
+
 // Appender provides batched appends against a storage.
 type Appender interface {
 	Add(l labels.Labels, t int64, v float64) (uint64, error)