Harden field-caps request dispatcher (#108736)

ExceptionHelper#useAndSuppress can throw exceptions if both input 
exceptions having the same root cause. If this happens, the field-caps
request dispatcher might fail to notify the completion to the caller. I
found this while running ES|QL with disruptions.

Relates #107347

docs/changelog/108736.yaml

@@ -0,0 +1,5 @@
+pr: 108736
+summary: Harden field-caps request dispatcher
+area: Search
+type: bug
+issues: []

server/src/main/java/org/elasticsearch/action/fieldcaps/RequestDispatcher.java

@@ -92,8 +92,13 @@ final class RequestDispatcher {
         this.onComplete = new RunOnce(onComplete);
         this.indexSelectors = ConcurrentCollections.newConcurrentMap();
         for (String index : indices) {
-            final GroupShardsIterator<ShardIterator> shardIts = clusterService.operationRouting()
-                .searchShards(clusterState, new String[] { index }, null, null, null, null);
+            final GroupShardsIterator<ShardIterator> shardIts;
+            try {
+                shardIts = clusterService.operationRouting().searchShards(clusterState, new String[] { index }, null, null, null, null);
+            } catch (Exception e) {
+                onIndexFailure.accept(index, e);
+                continue;
+            }
             final IndexSelector indexResult = new IndexSelector(shardIts);
             if (indexResult.nodeToShards.isEmpty()) {
                 onIndexFailure.accept(index, new NoShardAvailableActionException(null, "index [" + index + "] has no active shard copy"));
@@ -168,7 +173,7 @@ private void sendRequestToNode(String nodeId, List<ShardId> shardIds) {
         assert node != null;
         LOGGER.debug("round {} sends field caps node request to node {} for shardIds {}", executionRound, node, shardIds);
         final ActionListener<FieldCapabilitiesNodeResponse> listener = ActionListener.wrap(
-            r -> onRequestResponse(shardIds, r),
+            this::onRequestResponse,
             failure -> onRequestFailure(shardIds, failure)
         );
         final FieldCapabilitiesNodeRequest nodeRequest = new FieldCapabilitiesNodeRequest(
@@ -188,7 +193,11 @@ private void sendRequestToNode(String nodeId, List<ShardId> shardIds) {
             nodeRequest,
             parentTask,
             TransportRequestOptions.EMPTY,
-            new ActionListenerResponseHandler<>(listener, FieldCapabilitiesNodeResponse::new, executor)
+            new ActionListenerResponseHandler<>(
+                ActionListener.runAfter(listener, () -> afterRequestsCompleted(shardIds.size())),
+                FieldCapabilitiesNodeResponse::new,
+                executor
+            )
         );
     }
 
@@ -201,7 +210,7 @@ private void afterRequestsCompleted(int numRequests) {
         }
     }
 
-    private void onRequestResponse(List<ShardId> shardIds, FieldCapabilitiesNodeResponse nodeResponse) {
+    private void onRequestResponse(FieldCapabilitiesNodeResponse nodeResponse) {
         for (FieldCapabilitiesIndexResponse indexResponse : nodeResponse.getIndexResponses()) {
             if (indexResponse.canMatch()) {
                 if (fieldCapsRequest.includeEmptyFields() == false) {
@@ -224,7 +233,6 @@ private void onRequestResponse(List<ShardId> shardIds, FieldCapabilitiesNodeResp
                 indexSelector.setFailure(e.getKey(), e.getValue());
             }
         }
-        afterRequestsCompleted(shardIds.size());
     }
 
     private void onRequestFailure(List<ShardId> shardIds, Exception e) {
@@ -234,7 +242,6 @@ private void onRequestFailure(List<ShardId> shardIds, Exception e) {
                 indexSelector.setFailure(shardId, e);
             }
         }
-        afterRequestsCompleted(shardIds.size());
     }
 
     private static class IndexSelector {
@@ -253,14 +260,23 @@ private static class IndexSelector {
         synchronized Exception getFailure() {
             Exception first = null;
             for (Exception e : failures.values()) {
-                first = ExceptionsHelper.useOrSuppress(first, e);
+                first = useOrSuppressIfDifferent(first, e);
+            }
+            return first;
+        }
+
+        static Exception useOrSuppressIfDifferent(Exception first, Exception second) {
+            if (first == null) {
+                return second;
+            } else if (ExceptionsHelper.unwrap(first) != ExceptionsHelper.unwrap(second)) {
+                first.addSuppressed(second);
             }
             return first;
         }
 
         synchronized void setFailure(ShardId shardId, Exception failure) {
             assert unmatchedShardIds.contains(shardId) == false : "Shard " + shardId + " was unmatched already";
-            failures.compute(shardId, (k, curr) -> ExceptionsHelper.useOrSuppress(curr, failure));
+            failures.compute(shardId, (k, curr) -> useOrSuppressIfDifferent(curr, failure));
         }
 
         synchronized void addUnmatchedShardId(ShardId shardId) {

server/src/test/java/org/elasticsearch/action/fieldcaps/RequestDispatcherTests.java

@@ -56,6 +56,7 @@
 import org.elasticsearch.threadpool.ThreadPool;
 import org.elasticsearch.transport.TcpTransport;
 import org.elasticsearch.transport.Transport;
+import org.elasticsearch.transport.TransportException;
 import org.elasticsearch.transport.TransportInterceptor;
 import org.elasticsearch.transport.TransportRequest;
 import org.elasticsearch.transport.TransportRequestOptions;
@@ -599,6 +600,63 @@ public <T extends TransportResponse> void sendRequest(
         }
     }
 
+    public void testFailWithSameException() throws Exception {
+        final List<String> allIndices = IntStream.rangeClosed(1, 5).mapToObj(n -> "index_" + n).toList();
+        final ClusterState clusterState;
+        {
+            DiscoveryNodes.Builder discoNodes = DiscoveryNodes.builder();
+            int numNodes = randomIntBetween(1, 10);
+            for (int i = 0; i < numNodes; i++) {
+                discoNodes.add(newNode("node_" + i, VersionUtils.randomVersion(random()), IndexVersionUtils.randomVersion()));
+            }
+            Metadata.Builder metadata = Metadata.builder();
+            for (String index : allIndices) {
+                metadata.put(
+                    IndexMetadata.builder(index).settings(indexSettings(IndexVersions.MINIMUM_COMPATIBLE, between(1, 10), between(0, 3)))
+                );
+            }
+            clusterState = newClusterState(metadata.build(), discoNodes.build());
+        }
+        try (TestTransportService transportService = TestTransportService.newTestTransportService()) {
+            final List<String> targetIndices = randomSubsetOf(between(1, allIndices.size()), allIndices);
+            final ResponseCollector responseCollector = new ResponseCollector();
+            boolean withFilter = randomBoolean();
+            final RequestDispatcher dispatcher = new RequestDispatcher(
+                mockClusterService(clusterState),
+                transportService,
+                newRandomParentTask(),
+                randomFieldCapRequest(withFilter),
+                OriginalIndices.NONE,
+                randomNonNegativeLong(),
+                targetIndices.toArray(new String[0]),
+                transportService.threadPool.executor(ThreadPool.Names.SEARCH_COORDINATION),
+                responseCollector::addIndexResponse,
+                responseCollector::addIndexFailure,
+                responseCollector::onComplete
+            );
+            final RequestTracker requestTracker = new RequestTracker(dispatcher, clusterState.routingTable(), withFilter);
+            transportService.requestTracker.set(requestTracker);
+
+            RuntimeException ex = new RuntimeException("shared");
+            transportService.setTransportInterceptor(new TransportInterceptor.AsyncSender() {
+                @Override
+                public <T extends TransportResponse> void sendRequest(
+                    Transport.Connection connection,
+                    String action,
+                    TransportRequest request,
+                    TransportRequestOptions options,
+                    TransportResponseHandler<T> handler
+                ) {
+                    Exception failure = randomFrom(ex, new RuntimeException("second"), new IllegalStateException("third"));
+                    handler.executor().execute(() -> handler.handleException(new TransportException(failure)));
+                }
+            });
+            dispatcher.execute();
+            responseCollector.awaitCompletion();
+            assertThat(responseCollector.failures.keySet(), equalTo(Sets.newHashSet(targetIndices)));
+        }
+    }
+
     private static class NodeRequest {
         final int round;
         final DiscoveryNode node;