.

wcfs/internal/xbtree/δbtail.go

@@ -69,9 +69,12 @@ package xbtree
 //
 // trackSet, ktrackNew, krebuildJobs + explain rebuild algo (query lock, check ktrackNew, krebuildJob, go/merge/...)
 //
+// XXX queries work on vδT snapshot. When updated, vδT is modified via RCU ...
+//
 // This locking organization allows non-overlapping queries/track-requests to
-// run simultaneously. Without this property WCFS would not be able to serve
-// several non-overlapping READ requests to one file in parallel.
+// run simultaneously. This property is essential to WCFS because otherwise WCFS
+// would not be able to serve several non-overlapping READ requests to one file
+// in parallel.
 //
 // --------
 //
@@ -188,6 +191,12 @@ type _ΔTtail struct {
 	krebuildJobs _RangedMap_RebuildJob // {} keycov -> job
 }
 
+// _RebuildJob represents currently in-progress vδT rebuilding job.
+type _RebuildJob struct {
+	ready chan struct{} // closed when job completes
+	err   error
+}
+
 // _ΔBroots represents roots-only part of ΔB.
 //
 // It describes which trees were changed, but does not provide δkv details for changed trees.
@@ -209,14 +218,6 @@ type ΔTree struct {
 }
 
 
-// _RebuildJob represents currently in-progress vδT rebuilding job.
-// XXX place
-type _RebuildJob struct {
-	ready chan struct{} // closed when job completes
-	err   error
-}
-
-
 // NewΔBtail creates new empty ΔBtail object.
 //
 // Initial tracked set is empty.
@@ -470,14 +471,12 @@ func (δBtail *ΔBtail) vδTSnapForTrackedKey(root zodb.Oid, key Key) (vδT []Δ
 	// key ∈ ktrackNew  ->  this goroutine becomes responsible to start rebuilding vδT for it
 	// run rebuild job for all keys queued in ktrackNew so far
 	err = δTtail._runRebuildJob(root, δBtail)
-	if err != nil {
-		return nil, err
+	if err == nil {
+		vδT = δTtail.vδT
 	}
-
-	vδT = δTtail.vδT
 	δBtail.mu.Unlock()
 
-	return vδT, nil
+	return vδT, err
 }
 
 // vδTSnapForTracked returns vδT snapshot for root that takes into account all
@@ -579,7 +578,7 @@ func (δTtail *_ΔTtail) _runRebuildJob(root zodb.Oid, δBtail *ΔBtail) (err er
 	// we are done
 	job.err = err
 	close(job.ready)
-	return job.err
+	return err
 }
 
 // rebuild1 rebuilds ΔBtail for single root.