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Compare cursor serials by identity so returning to an earlier cursor updates

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This commit is contained in:
Viktor Liu
2026-08-29 10:25:09 +02:00
parent 1ecd1dab8b
commit 827098c3d2
2 changed files with 91 additions and 10 deletions
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client/vnc/server/session_cursor.go
+11 -10
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@@ -40,21 +40,22 @@ func (s *session) pendingCursorRect() []byte {
if buf == nil {
return nil
}
// Re-check the serial under the write lock so a concurrent update
// from another goroutine can't be silently overwritten with a stale
// value: if someone advanced it past `serial` while we were encoding,
// keep their value and drop this rect.
// Re-check under the write lock so a cursor another goroutine already
// sent is not sent again.
//
// Compared for equality only, never for order. A serial identifies the
// cursor, it does not count upwards: X11 passes through the XFixes
// cursor-serial, which is a property of the cursor itself, so switching
// back to a cursor shown earlier produces a *lower* value than the one on
// screen. Treating that as stale left the client stuck on whichever cursor
// happened to have the highest serial, typically the I-beam after a text
// field. macOS and Windows use their own counters, which only ever move
// forwards, so equality is the right test for all three.
s.encMu.Lock()
if serial == s.lastCursorSerial {
s.encMu.Unlock()
return nil
}
if uint64(serial-s.lastCursorSerial) > 1<<63 {
// `serial` is older than the current value (wraparound-aware
// comparison). Drop it.
s.encMu.Unlock()
return nil
}
s.lastCursorSerial = serial
s.encMu.Unlock()
return buf