lianetm commented on code in PR #16885:
URL: https://github.com/apache/kafka/pull/16885#discussion_r1727373275


##########
clients/src/main/java/org/apache/kafka/clients/consumer/internals/events/ApplicationEventProcessor.java:
##########
@@ -256,14 +271,176 @@ private void process(final UnsubscribeEvent event) {
         }
     }
 
-    private void process(final ResetPositionsEvent event) {
-        CompletableFuture<Void> future = 
requestManagers.offsetsRequestManager.resetPositionsIfNeeded();
-        future.whenComplete(complete(event.future()));
+    /**
+     *
+     * Fetch committed offsets and use them to update positions in the 
subscription state. If no
+     * committed offsets available, fetch offsets from the leader.
+     */
+    private void process(final UpdateFetchPositionsEvent 
updateFetchPositionsEvent) {
+        try {
+            // The event could be completed in the app thread before it got to 
be
+            // processed in the background (ex. interrupted)
+            if (updateFetchPositionsEvent.future().isCompletedExceptionally()) 
{
+                log.debug("UpdateFetchPositions event {} was completed 
exceptionally before it " +
+                    "got time to be processed.", updateFetchPositionsEvent);
+                return;
+            }
+
+            // Validate positions using the partition leader end offsets, to 
detect if any partition
+            // has been truncated due to a leader change. This will trigger an 
OffsetForLeaderEpoch
+            // request, retrieve the partition end offsets, and validate the 
current position
+            // against it. It will throw an exception if log truncation is 
detected.
+            requestManagers.offsetsRequestManager.validatePositionsIfNeeded();
+
+            boolean hasAllFetchPositions = 
subscriptions.hasAllFetchPositions();
+            if (hasAllFetchPositions) {
+                updateFetchPositionsEvent.future().complete(true);
+                return;
+            }
+
+            // Reset positions using committed offsets retrieved from the 
group coordinator, for any
+            // partitions which do not have a valid position and are not 
awaiting reset. This will
+            // trigger an OffsetFetch request and update positions with the 
offsets retrieved. This
+            // will only do a coordinator lookup if there are partitions which 
have missing
+            // positions, so a consumer with manually assigned partitions can 
avoid a coordinator
+            // dependence by always ensuring that assigned partitions have an 
initial position.
+            if (requestManagers.commitRequestManager.isPresent()) {
+                CompletableFuture<Void> initWithCommittedOffsetsResult = 
initWithCommittedOffsetsIfNeeded(updateFetchPositionsEvent);
+                initWithCommittedOffsetsResult.whenComplete((__, error) -> {
+                    if (error == null) {
+                        // Retrieve partition offsets to init positions for 
partitions that still
+                        // don't have a valid position
+                        
initWithPartitionOffsetsIfNeeded(updateFetchPositionsEvent);
+                    } else {
+                        
updateFetchPositionsEvent.future().completeExceptionally(error);
+                    }
+                });
+            } else {
+                initWithPartitionOffsetsIfNeeded(updateFetchPositionsEvent);
+            }
+
+        } catch (Exception e) {
+            
updateFetchPositionsEvent.future().completeExceptionally(maybeWrapAsKafkaException(e));
+        }
     }
 
-    private void process(final ValidatePositionsEvent event) {
-        CompletableFuture<Void> future = 
requestManagers.offsetsRequestManager.validatePositionsIfNeeded();
-        future.whenComplete(complete(event.future()));
+    private void initWithPartitionOffsetsIfNeeded(final 
UpdateFetchPositionsEvent updateFetchPositionsEvent) {
+        try {
+            // If there are partitions still needing a position and a reset 
policy is defined,
+            // request reset using the default policy. If no reset strategy is 
defined and there
+            // are partitions with a missing position, then we will raise a 
NoOffsetForPartitionException exception.
+            subscriptions.resetInitializingPositions();
+
+            // Reset positions using partition offsets retrieved from the 
leader, for any partitions
+            // which are awaiting reset. This will trigger a ListOffset 
request, retrieve the
+            // partition offsets according to the strategy (ex. earliest, 
latest), and update the
+            // positions.
+            CompletableFuture<Void> resetPositionsFuture = 
requestManagers.offsetsRequestManager.resetPositionsIfNeeded();
+
+            resetPositionsFuture.whenComplete((result, error) -> {
+                if (updateFetchPositionsEvent.future().isDone()) {
+                    log.debug("UpdateFetchPositions event {} had already 
expired when reset " +
+                        "positions completed.", updateFetchPositionsEvent);
+                    return;
+                }
+                if (error == null) {
+                    updateFetchPositionsEvent.future().complete(false);
+                } else {
+                    
updateFetchPositionsEvent.future().completeExceptionally(error);
+                }
+            });
+        } catch (Exception e) {
+            updateFetchPositionsEvent.future().completeExceptionally(e);

Review Comment:
   uhm I wasn't completely sure about this, but after checking in detail seems 
to make sense to wrap here.  I was concerned that maybe we would mistakenly 
wrap something that was expected from the API, given that we know perform in 
the background lots of operations that were in the app thread before. But we do 
handle the expected errors in the updatePositions flow and requests, and use 
them to complete the future). So this would really be for unexpected errors in 
the flow I would say, and ok to wrap. Done



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