etcdserver: check invalid range in txn

storage/kv.go

@@ -24,6 +24,11 @@ type KV interface {
 	// Rev returns the current revision of the KV.
 	Rev() int64
 
+	// FirstRev returns the first revision of the KV.
+	// After a compaction, the first revision increases to the compaction
+	// revision.
+	FirstRev() int64
+
 	// Range gets the keys in the range at rangeRev.
 	// If rangeRev <=0, range gets the keys at currentRev.
 	// If `end` is nil, the request returns the key.