@@ -471,8 +471,8 @@ func (ctx *mossCtx) phase1(w http.ResponseWriter, r *http.Request, config *cmn.C
 	}
 
 	// to have a fresh full idle window for the eventual phase2 GET
-	xmoss.IncPending()
-	xmoss.DecPending()
+	xmoss.DemandBase.IncPending()
+	xmoss.DemandBase.DecPending()
 
 	// return xid (proxy will send it to phase 3 senders)
 	w.Header().Set(apc.HdrXactionID, xid)
@@ -519,7 +519,7 @@ func (ctx *mossCtx) phase3(w http.ResponseWriter, r *http.Request, config *cmn.C
 	}
 
 	ctx.smap = smap
-	xmoss.IncPending()
+	xmoss.DemandBase.IncPending()
 	go ctx._startSend(xmoss, tsi)
 }
 
@@ -529,7 +529,7 @@ func (ctx *mossCtx) _startSend(xmoss *xs.XactMoss, tsi *meta.Snode) {
 			xmoss.AddErr(fmt.Errorf("send wid=%s: %v", ctx.wid, err), 4, cos.ModXs)
 		}
 	}
-	xmoss.DecPending()
+	xmoss.DemandBase.DecPending()
 }
 
 func _checkMossEmpty(req *apc.MossReq) (err error) {
 
@@ -208,7 +208,7 @@ type (
 		pending     sync.Map // [wid => *basewi]
 		abortedWIDs sync.Map // wid => error from DT
 		xact.DemandBase
-		activeWG     sync.WaitGroup // when pending
+		activeWG     sync.WaitGroup // active asm()
 		lastLog      atomic.Int64   // last log timestamp (sparse)
 		advNextCheck atomic.Int64
 		pendingCnt   atomic.Int64
@@ -350,12 +350,12 @@ func (r *XactMoss) housekeep(now int64) time.Duration {
 
 func (*XactMoss) Run(*sync.WaitGroup) { debug.Assert(false) }
 
-func (r *XactMoss) IncPending() {
+func (r *XactMoss) incPending() {
 	r.DemandBase.IncPending()
 	r.activeWG.Add(1)
 }
 
-func (r *XactMoss) DecPending() {
+func (r *XactMoss) decPending() {
 	r.DemandBase.DecPending()
 	r.activeWG.Done()
 }
@@ -371,23 +371,8 @@ func (r *XactMoss) Abort(err error) bool {
 
 	nlog.Infoln(r.Name(), "aborting:", err)
 
-	// best-effort wait for pending WIs to drain; never block indefinitely
-	const (
-		pollIval = 100 * time.Millisecond
-	)
-	{
-		var (
-			total   time.Duration
-			maxWait = max(cmn.Rom.MaxKeepalive(), 8*time.Second)
-		)
-		for r.pendingCnt.Load() > 0 && total < maxWait {
-			time.Sleep(pollIval)
-			total += pollIval
-		}
-		if r.pendingCnt.Load() > 0 {
-			nlog.Warningln(r.Name(), "abort: still", r.pendingCnt.Load(), "pending after", maxWait, "- proceeding anyway")
-		}
-	}
+	r.activeWG.Wait()
+	r._abortQuiesce()
 
 	r.pending.Range(r.cleanup)
 	r.pending.Clear()
@@ -401,6 +386,22 @@ func (r *XactMoss) Abort(err error) bool {
 	return true
 }
 
+// best-effort (and extremely unlikely): quiesce prior to cleanups
+func (r *XactMoss) _abortQuiesce() {
+	const pollIval = cos.PollSleepShort // 100ms
+	var (
+		total   time.Duration
+		maxWait = max(cmn.Rom.MaxKeepalive(), 8*time.Second)
+	)
+	for r.Pending() > 0 && total < maxWait {
+		time.Sleep(pollIval)
+		total += pollIval
+	}
+	if p := r.Pending(); p > 0 {
+		nlog.Warningln(r.Name(), "abort: still", p, "demand-pending after", maxWait, "- proceeding anyway")
+	}
+}
+
 // (Abort, fini) => pending (sync.Map's) callback to cleanup all pending work items
 func (*XactMoss) cleanup(_, value any) bool {
 	wi := value.(*basewi)
@@ -674,11 +675,20 @@ func (r *XactMoss) Assemble(req *apc.MossReq, w http.ResponseWriter, wid string)
 		return err
 	}
 
+	if r.IsAborted() || r.IsDone() {
+		wi.owned.CAS(wiownASM, wiownNone) // Abort/fini owns final cleanup
+		return nil
+	}
+
 	// asm() path owns wi; DecPending below
-	debug.Assert(wid == wi.wid)
-	r.IncPending()
-	defer r.DecPending()
+	r.incPending()
+	defer r.decPending()
+	if r.IsAborted() || r.IsDone() {
+		wi.owned.CAS(wiownASM, wiownNone) // ditto
+		return nil
+	}
 
+	debug.Assert(wid == wi.wid)
 	err := r.asm(req, w, wi)
 	if err != nil {
 		if r.shouldAbortWI(wi) {
Original file line number	Diff line number	Diff line change
`@@ -471,8 +471,8 @@ func (ctx mossCtx) phase1(w http.ResponseWriter, r http.Request, config *cmn.C`
`471`	`471`	`}`
`472`	`472`
`473`	`473`	`// to have a fresh full idle window for the eventual phase2 GET`
`474`		`- xmoss.IncPending()`
`475`		`- xmoss.DecPending()`
	`474`	`+ xmoss.DemandBase.IncPending()`
	`475`	`+ xmoss.DemandBase.DecPending()`
`476`	`476`
`477`	`477`	`// return xid (proxy will send it to phase 3 senders)`
`478`	`478`	`w.Header().Set(apc.HdrXactionID, xid)`
`@@ -519,7 +519,7 @@ func (ctx mossCtx) phase3(w http.ResponseWriter, r http.Request, config *cmn.C`
`519`	`519`	`}`
`520`	`520`
`521`	`521`	`ctx.smap = smap`
`522`		`- xmoss.IncPending()`
	`522`	`+ xmoss.DemandBase.IncPending()`
`523`	`523`	`go ctx._startSend(xmoss, tsi)`
`524`	`524`	`}`
`525`	`525`
`@@ -529,7 +529,7 @@ func (ctx mossCtx) _startSend(xmoss xs.XactMoss, tsi *meta.Snode) {`
`529`	`529`	`xmoss.AddErr(fmt.Errorf("send wid=%s: %v", ctx.wid, err), 4, cos.ModXs)`
`530`	`530`	`}`
`531`	`531`	`}`
`532`		`- xmoss.DecPending()`
	`532`	`+ xmoss.DemandBase.DecPending()`
`533`	`533`	`}`
`534`	`534`
`535`	`535`	`func _checkMossEmpty(req *apc.MossReq) (err error) {`