X treediff: Fix BUG while computing AB coverage

Access to node entries is valid only after the node was activated.
And it fails otherwise.

--- FAIL: TestΔBTail (0.63s)
    --- FAIL: TestΔBTail/rebuild/T/T1/B0:a-B1:b→T/T1/T-T/B0:c-B1:d (0.00s)
        --- FAIL: TestΔBTail/rebuild/T/T1/B0:a-B1:b→T/T1/T-T/B0:c-B1:d/_T{};R (0.00s)
            --- FAIL: TestΔBTail/rebuild/T/T1/B0:a-B1:b→T/T1/T-T/B0:c-B1:d/_T{};R/_→T/T/B0:a (0.00s)
panic: BUG: T000000000000003a points to T000000000000000c as parent in trackSet, but not found in T000000000000000c children [recovered]
        panic: BUG: T000000000000003a points to T000000000000000c as parent in trackSet, but not found in T000000000000000c children

wcfs/internal/xbtree/treediff.go

@@ -487,6 +487,9 @@ func diffT(ctx context.Context, A, B *Tree, δZTC SetOid, trackSet PPTreeSubSet)
 ABcov:
 	for i := len(ABpath)-2; i >= 0; i-- {
 		xparent, err := node.PJar().Get(ctx, ABpath[i]);  /*X*/if err != nil { return nil,nil,nil, err }
+		err = xparent.PActivate(ctx);  /*X*/if err != nil { return nil,nil,nil, err}
+		defer xparent.PDeactivate()
+
 		parent := xparent.(*Tree) // must succeed
 		// find node in parent children and constrain ABlo/ABhi accordingly
 		entryv := parent.Entryv()
@@ -512,7 +515,14 @@ ABcov:
 			}
 		}
 
-		panicf("BUG: T%s points to T%s as parent in trackSet, but not found in T%s children", node.POid(), parent.POid(), parent.POid())
+		emsg := fmt.Sprintf("BUG: T%s points to T%s as parent in trackSet, but not found in T%s children\n", node.POid(), parent.POid(), parent.POid())
+		children := []string{}
+		for _, entry := range entryv {
+			children = append(children, vnode(entry.Child()))
+		}
+		emsg += fmt.Sprintf("T%s children: %v\n", parent.POid(), children)
+		emsg += fmt.Sprintf("trackSet: %s\n", trackSet)
+		panic(emsg)
 	}
 
 

wcfs/internal/xbtree/δbtail_test.go

@@ -1084,6 +1084,7 @@ func xverifyΔBTail_rebuild(t *testing.T, db *zodb.DB, treeRoot zodb.Oid, t0, t1
 								/* trackSet=*/ t2.xkv.trackSet(keys12R2),
 								/*vδT=*/ δkv1_k12R2, δkv2_k12R2)
 
+							// XXX move vvv to separate test (out of this inner loop)
 							// ΔBtail.Clone had bug that aliased klon data to orig
 							assertΔTtail(t, "BUG: after clone check", δbtail, db, t2, treeRoot, xat,
 								/*vδT=*/ δkv1_k1R2, δkv2_k1R2)