+ @GuardedBy("unflushedSegments")
+ protected void ensureSegment(final StackedSegment first, final int offset) {
+ final int segmentOffset = offset / StackedSegment.SEGMENT_SIZE;
+ LOG.debug("Queue {} slow offset {} maps to {} segments {}", this, offset, segmentOffset, unflushedSegments.size());
+
+ for (int i = unflushedSegments.size(); i <= segmentOffset; ++i) {
+ final StackedSegment newSegment = StackedSegment.create(first.getBaseXid() + (StackedSegment.SEGMENT_SIZE * i));
+ LOG.debug("Adding segment {}", newSegment);
+ unflushedSegments.add(newSegment);
+ }
+
+ allocatedXid = uncompletedSegments.get(uncompletedSegments.size() - 1).getEndXid();
+ }
+
+ /*
+ * This method is expected to be called from multiple threads concurrently.
+ */
+ @Override
+ public Long reserveEntry() {
+ final long xid = LAST_XID_OFFSET_UPDATER.incrementAndGet(this);
+ final StackedSegment fastSegment = firstSegment;
+
+ if (xid >= fastSegment.getBaseXid() + StackedSegment.SEGMENT_SIZE) {
+ if (xid >= allocatedXid) {
+ // Multiple segments, this a slow path
+ LOG.debug("Queue {} falling back to slow reservation for XID {}", this, xid);
+
+ synchronized (unflushedSegments) {
+ LOG.debug("Queue {} executing slow reservation for XID {}", this, xid);
+
+ // Shutdown was scheduled, need to fail the reservation
+ if (shutdownOffset != null) {
+ LOG.debug("Queue {} is being shutdown, failing reservation", this);
+ return null;
+ }
+
+ // Ensure we have the appropriate segment for the specified XID
+ final StackedSegment slowSegment = firstSegment;
+ final int slowOffset = (int) (xid - slowSegment.getBaseXid());
+ Verify.verify(slowOffset >= 0);
+
+ // Now, we let's see if we need to allocate a new segment
+ ensureSegment(slowSegment, slowOffset);
+
+ LOG.debug("Queue {} slow reservation finished", this);
+ }
+ } else {
+ LOG.debug("Queue {} XID {} is already backed", this, xid);
+ }
+ }
+
+ LOG.trace("Queue {} allocated XID {}", this, xid);
+ return xid;
+ }
+