kvdb: add ForAll

A new method to allow efficient range queries for backends that support
it.

kvdb/interface.go

@@ -109,6 +109,12 @@ type ExtendedRBucket interface {
 
 	// Prefetch will attempt to prefetch all values under a path.
 	Prefetch(paths ...[]string)
+
+	// ForAll is an optimized version of ForEach.
+	//
+	// NOTE: ForAll differs from ForEach in that no additional queries can
+	// be executed within the callback.
+	ForAll(func(k, v []byte) error) error
 }
 
 // Prefetch will attempt to prefetch all values under a path from the passed
@@ -119,6 +125,16 @@ func Prefetch(b RBucket, paths ...[]string) {
 	}
 }
 
+// ForAll is an optimized version of ForEach with the limitation that no
+// additional queries can be executed within the callback.
+func ForAll(b RBucket, cb func(k, v []byte) error) error {
+	if bucket, ok := b.(ExtendedRBucket); ok {
+		return bucket.ForAll(cb)
+	}
+
+	return b.ForEach(cb)
+}
+
 // RootBucket is a wrapper to ExtendedRTx.RootBucket which does nothing if
 // the implementation doesn't have ExtendedRTx.
 func RootBucket(t RTx) RBucket {