SERVER-19758 add support for "executionStats" and "allPlansExecution" to agg explain

Like other explainable commands, aggregate can now be
explained using the explain command, e.g.
db.runCommand({explain: {aggregate: ...}, verbosity:
"executionStats"}). The existing explain:true flag
corresponds to "queryPlanner" mode and is still supported.
However, explain:true cannot be specified when explaining
aggregate via the explain command.

Additional execution information is provided only in the
$cursor section of the aggregation explain output. Having
aggregation stages themselves track and report execution
info is further work.

1 files changed, 35 insertions, 0 deletions

diff --git a/src/mongo/dbtests/documentsourcetests.cpp b/src/mongo/dbtests/documentsourcetests.cpp
index a5add096d27..58df14b77b4 100644
--- a/src/mongo/dbtests/documentsourcetests.cpp
+++ b/src/mongo/dbtests/documentsourcetests.cpp
@@ -310,6 +310,40 @@ public:
     }
 };
 
+class SerializationRespectsExplainModes : public Base {
+public:
+    void run() {
+        createSource();
+
+        {
+            // Nothing serialized when no explain mode specified.
+            auto explainResult = source()->serialize();
+            ASSERT_TRUE(explainResult.missing());
+        }
+
+        {
+            auto explainResult = source()->serialize(ExplainOptions::Verbosity::kQueryPlanner);
+            ASSERT_FALSE(explainResult["$cursor"]["queryPlanner"].missing());
+            ASSERT_TRUE(explainResult["$cursor"]["executionStats"].missing());
+        }
+
+        {
+            auto explainResult = source()->serialize(ExplainOptions::Verbosity::kExecStats);
+            ASSERT_FALSE(explainResult["$cursor"]["queryPlanner"].missing());
+            ASSERT_FALSE(explainResult["$cursor"]["executionStats"].missing());
+            ASSERT_TRUE(explainResult["$cursor"]["executionStats"]["allPlansExecution"].missing());
+        }
+
+        {
+            auto explainResult =
+                source()->serialize(ExplainOptions::Verbosity::kExecAllPlans).getDocument();
+            ASSERT_FALSE(explainResult["$cursor"]["queryPlanner"].missing());
+            ASSERT_FALSE(explainResult["$cursor"]["executionStats"].missing());
+            ASSERT_FALSE(explainResult["$cursor"]["executionStats"]["allPlansExecution"].missing());
+        }
+    }
+};
+
 }  // namespace DocumentSourceCursor
 
 class All : public Suite {
@@ -325,6 +359,7 @@ public:
         add<DocumentSourceCursor::IndexScanProvidesSortOnKeys>();
         add<DocumentSourceCursor::ReverseIndexScanProvidesSort>();
         add<DocumentSourceCursor::CompoundIndexScanProvidesMultipleSorts>();
+        add<DocumentSourceCursor::SerializationRespectsExplainModes>();
     }
 };