Handle new explain output in tests (#1191)

The response document for the explain command has changed in 7.2 sharded clusters.
It introduces a top-level "shards" document that contains the explain output for
each shard. This patch supports this new field in tests of explain API in the
driver.

JAVA-5137

Author: jyemin

driver-core/src/test/functional/com/mongodb/internal/operation/AggregateOperationSpecification.groovy

@@ -320,7 +320,7 @@ class AggregateOperationSpecification extends OperationFunctionalSpecification {
         def result = execute(operation, async)
 
         then:
-        result.containsKey('stages') || result.containsKey('queryPlanner')
+        result.containsKey('stages') || result.containsKey('queryPlanner') || result.containsKey('shards')
 
         where:
         async << [true, false]

driver-sync/src/test/functional/com/mongodb/client/AbstractExplainTest.java

@@ -99,27 +99,50 @@ public void testExplainOfAggregateWithNewResponseStructure() {
         AggregateIterable<BsonDocument> iterable = collection
                 .aggregate(singletonList(Aggregates.match(Filters.eq("_id", 1))));
 
-        Document explainDocument = iterable.explain();
-        assertNotNull(explainDocument);
+        Document explainDocument = getAggregateExplainDocument(iterable.explain());
         assertTrue(explainDocument.containsKey("queryPlanner"));
         assertTrue(explainDocument.containsKey("executionStats"));
 
-        explainDocument = iterable.explain(ExplainVerbosity.QUERY_PLANNER);
+        explainDocument = getAggregateExplainDocument(iterable.explain(ExplainVerbosity.QUERY_PLANNER));
         assertNotNull(explainDocument);
         assertTrue(explainDocument.containsKey("queryPlanner"));
         assertFalse(explainDocument.containsKey("executionStats"));
 
-        BsonDocument explainBsonDocument = iterable.explain(BsonDocument.class);
+        BsonDocument explainBsonDocument = getAggregateExplainDocument(iterable.explain(BsonDocument.class));
         assertNotNull(explainBsonDocument);
         assertTrue(explainBsonDocument.containsKey("queryPlanner"));
         assertTrue(explainBsonDocument.containsKey("executionStats"));
 
-        explainBsonDocument = iterable.explain(BsonDocument.class, ExplainVerbosity.QUERY_PLANNER);
+        explainBsonDocument = getAggregateExplainDocument(iterable.explain(BsonDocument.class, ExplainVerbosity.QUERY_PLANNER));
         assertNotNull(explainBsonDocument);
         assertTrue(explainBsonDocument.containsKey("queryPlanner"));
         assertFalse(explainBsonDocument.containsKey("executionStats"));
     }
 
+    // Post-MongoDB 7.0, sharded cluster responses move the explain plan document into a "shards" document, which a plan for each shard.
+    // This method grabs the explain plan document from the first shard when this new structure is present.
+    private static Document getAggregateExplainDocument(final Document rootAggregateExplainDocument) {
+        assertNotNull(rootAggregateExplainDocument);
+        Document aggregateExplainDocument = rootAggregateExplainDocument;
+        if (rootAggregateExplainDocument.containsKey("shards")) {
+            Document shardDocument = rootAggregateExplainDocument.get("shards", Document.class);
+            String firstKey = shardDocument.keySet().iterator().next();
+            aggregateExplainDocument = shardDocument.get(firstKey, Document.class);
+        }
+        return aggregateExplainDocument;
+    }
+
+    private static BsonDocument getAggregateExplainDocument(final BsonDocument rootAggregateExplainDocument) {
+        assertNotNull(rootAggregateExplainDocument);
+        BsonDocument aggregateExplainDocument = rootAggregateExplainDocument;
+        if (rootAggregateExplainDocument.containsKey("shards")) {
+            BsonDocument shardDocument = rootAggregateExplainDocument.getDocument("shards");
+            String firstKey = shardDocument.getFirstKey();
+            aggregateExplainDocument = shardDocument.getDocument(firstKey);
+        }
+        return aggregateExplainDocument;
+    }
+
     @Test
     public void testExplainOfAggregateWithOldResponseStructure() {
         // Aggregate explain is supported on earlier versions, but the structure of the response on which we're asserting in this test