.

wcfs/internal/xbtree/δbtail.go

@@ -138,9 +138,20 @@ const debugΔBtail = false
 // An example for tracked set is a set of visited BTree paths.
 // There is no requirement that tracked set belongs to only one single BTree.
 //
-// XXX concurrent use
-//
 // See also zodb.ΔTail and zdata.ΔFtail
+//
+//
+// Concurrency
+//
+// ΔBtail is safe to use in single-writer / multiple-readers mode. That is at
+// any time there should be either only sole writer, or, potentially several
+// simultaneous readers. The table below classifies operations:
+//
+//	Writers:  Update, ForgetPast
+//	Readers:  Track + all queries (SliceByRev, SliceByRootRev, GetAt)
+//
+// Note that, in particular, it is correct to run multiple Track and queries
+// requests simultaneously.
 type ΔBtail struct {
 	// raw ZODB changes; Kept to rebuild .byRoot after new Track.
 	// includes all changed objects, not only tracked ones.