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, 44 insertions, 1 deletions

diff --git a/src/mongo/s/commands/cluster_pipeline_cmd.cpp b/src/mongo/s/commands/cluster_pipeline_cmd.cpp
index e533ae05392..21571bd6e4f 100644
--- a/src/mongo/s/commands/cluster_pipeline_cmd.cpp
+++ b/src/mongo/s/commands/cluster_pipeline_cmd.cpp
@@ -35,6 +35,7 @@
 #include "mongo/base/status.h"
 #include "mongo/db/auth/authorization_session.h"
 #include "mongo/db/commands.h"
+#include "mongo/rpc/metadata/server_selection_metadata.h"
 #include "mongo/s/commands/cluster_aggregate.h"
 
 namespace mongo {
@@ -80,13 +81,55 @@ public:
                      BSONObjBuilder& result) {
         const NamespaceString nss(parseNsCollectionRequired(dbname, cmdObj));
 
+        auto request = AggregationRequest::parseFromBSON(nss, cmdObj);
+        if (!request.isOK()) {
+            return appendCommandStatus(result, request.getStatus());
+        }
+
         ClusterAggregate::Namespaces nsStruct;
         nsStruct.requestedNss = nss;
         nsStruct.executionNss = std::move(nss);
-        auto status = ClusterAggregate::runAggregate(opCtx, nsStruct, cmdObj, options, &result);
+        auto status = ClusterAggregate::runAggregate(
+            opCtx, nsStruct, request.getValue(), cmdObj, options, &result);
         appendCommandStatus(result, status);
         return status.isOK();
     }
+
+    Status explain(OperationContext* opCtx,
+                   const std::string& dbName,
+                   const BSONObj& cmdObj,
+                   ExplainOptions::Verbosity verbosity,
+                   const rpc::ServerSelectionMetadata& serverSelectionMetadata,
+                   BSONObjBuilder* out) const override {
+        const NamespaceString nss(parseNsCollectionRequired(dbName, cmdObj));
+
+        auto request = AggregationRequest::parseFromBSON(nss, cmdObj, verbosity);
+        if (!request.isOK()) {
+            return request.getStatus();
+        }
+
+        // Add the server selection metadata to the aggregate command in the "unwrapped" format that
+        // runAggregate() expects: {aggregate: ..., $queryOptions: {$readPreference: ...}}.
+        BSONObjBuilder aggCmdBuilder;
+        aggCmdBuilder.appendElements(cmdObj);
+        if (auto readPref = serverSelectionMetadata.getReadPreference()) {
+            auto readPrefObj = readPref->toBSON();
+            aggCmdBuilder.append(QueryRequest::kUnwrappedReadPrefField,
+                                 BSON("$readPreference" << readPrefObj));
+        }
+
+        int options = 0;
+        if (serverSelectionMetadata.isSecondaryOk()) {
+            options |= QueryOption_SlaveOk;
+        }
+
+        ClusterAggregate::Namespaces nsStruct;
+        nsStruct.requestedNss = nss;
+        nsStruct.executionNss = std::move(nss);
+
+        return ClusterAggregate::runAggregate(
+            opCtx, nsStruct, request.getValue(), cmdObj, options, out);
+    }
 } clusterPipelineCmd;
 
 }  // namespace