.

go/zodb/db.go

@@ -124,7 +124,7 @@ func NewDB(stor IStorage) *DB {
 
 	watchq := make(chan CommitEvent)
 	at0 := stor.AddWatch(watchq)
-	db.δtail = NewΔTail(at0) // init δtail to (at0, at0]
+	db.δtail = NewΔTail(at0) // init to (at0, at0]
 
 	go db.watcher(watchq)
 	// XXX DelWatch? in db.Close() ?
@@ -243,36 +243,29 @@ func (db *DB) Open(ctx context.Context, opt *ConnOptions) (_ *Connection, err er
 	// find out db state we should open at
 	at := opt.At
 	if at == 0 {
-		head := Tid(0)
-
 		if opt.NoSync {
 			db.mu.Lock()
-			head = db.δtail.Head()	// = 0 if δtail was not yet initialized with first event
+			at = db.δtail.Head()
 			db.mu.Unlock()
-		}
-
-		// !NoSync or δtail !initialized
+		} else {
 			// sync storage for lastTid
-		if head == 0 {
 			var err error
 
 			// XXX stor.LastTid returns last_tid storage itself
 			// received on server, not last_tid on server.
 			// -> add stor.Sync() ?
-			head, err = db.stor.LastTid(ctx)
+			at, err = db.stor.LastTid(ctx)
 			if err != nil {
 				return nil, err
 			}
 		}
-
-		at = head
 	}
 
 	// proceed to open(at)
 	db.mu.Lock() // unlocked in *DBUnlock
 
 /*
-	err := db.needHeadOrDBUnlock(ctx, at) // XXX what if δtail !init yet?
+	err := db.needHeadOrDBUnlock(ctx, at)
 	if err != nil {
 		return nil, err
 	}
@@ -313,23 +306,15 @@ retry:
 
 		// no exact match - let's try to find nearest
 		δtail := db.δtail
-		δhead := db.δtail.Head()
 
 		// too far in the past, and we know there is no exact match
 		// -> new historic connection.
-		if at <= db.δtail.Tail() {
-			return newConnection(db, at), nil
-		}
-
-		// δtail !initialized yet
-		if db.δtail.Head() == 0 {
-			// XXX δtail could be not yet initialized, but e.g. last_tid changed
-			//     -> we have to wait for δtail not to loose just-released live cache
+		if at <= δtail.Tail() {
 			return newConnection(db, at), nil
 		}
 
 		// we have some δtail coverage, but at is ahead of that.
-		if at > δhead {
+		if at > δtail.Head() {
 			// wait till δtail.head is up to date covering ≥ at,
 			// and retry the loop (δtail.tail might go over at while we are waiting)
 			δready := make(chan struct{})
@@ -348,8 +333,6 @@ retry:
 			}
 		}
 
-		// XXX note: vvv at start δtail.Tail is not covering first committed txn
-
 		// at ∈ (δtail, δhead]	; try to get nearby idle connection or make a new one
 		conn = db.get(δtail.Tail(), at)
 		if conn == nil {

go/zodb/zodb.go

@@ -451,6 +451,7 @@ type Watcher interface {
 	//
 	// Multiple AddWatch calls with the same watchq register watchq only once.	XXX
 	//
+	// XXX ↑ guaranteed
 	// XXX watchq closed when stor.watchq closed?
 	AddWatch(watchq chan<- CommitEvent) (at0 Tid)