SERVER-38691 Include serverInfo in explain output for aggregation and sharding

10 files changed, 129 insertions, 22 deletions

diff --git a/buildscripts/resmokeconfig/suites/sharding_last_stable_mongos_and_mixed_shards.yml b/buildscripts/resmokeconfig/suites/sharding_last_stable_mongos_and_mixed_shards.yml
index 570dcfd1c78..99104cfca00 100644
--- a/buildscripts/resmokeconfig/suites/sharding_last_stable_mongos_and_mixed_shards.yml
+++ b/buildscripts/resmokeconfig/suites/sharding_last_stable_mongos_and_mixed_shards.yml
@@ -111,6 +111,8 @@ selector:
   - jstests/sharding/server_status_crud_metrics.js
   # Enable after SERVER-31083 is backported and available on 4.2 and 4.0 binaries
   - jstests/sharding/enable_sharding_with_primary.js
+  # Enable after SERVER-38691 gets backported to 4.2 and becomes the last stable.
+  - jstests/sharding/explain_cmd.js
 
 executor:
   config:
diff --git a/jstests/aggregation/explain.js b/jstests/aggregation/explain.js
index 3e446afe43a..a9c78170afb 100644
--- a/jstests/aggregation/explain.js
+++ b/jstests/aggregation/explain.js
@@ -1,5 +1,6 @@
-// Tests the behavior of explain() when used with the aggregation
-// pipeline.  Explain() should not read or modify the plan cache.
+// Tests the behavior of explain() when used with the aggregation pipeline.
+// - Explain() should not read or modify the plan cache.
+// - The result should always include serverInfo.
 (function() {
     "use strict";
 
@@ -13,6 +14,8 @@
 
     let result = coll.explain().aggregate([{$match: {x: 1, y: 1}}]);
     assert.eq(null, getAggPlanStage(result, "CACHED_PLAN"));
+    assert(result.hasOwnProperty('serverInfo'), result);
+    assert.hasFields(result.serverInfo, ['host', 'port', 'version', 'gitVersion']);
 
     // At this point, there should be no entries in the plan cache.
     result = coll.explain().aggregate([{$match: {x: 1, y: 1}}]);
@@ -25,4 +28,11 @@
     result = coll.explain().aggregate([{$match: {x: 1, y: 1}}]);
     assert.eq(null, getAggPlanStage(result, "CACHED_PLAN"));
 
+    // At the time of this writing there are times when an entire aggregation pipeline can be
+    // absorbed into the query layer. In these cases we use a different explain mechanism. Using
+    // $lookup will prevent this optimization and stress an explain implementation in the
+    // aggregation layer. Test that this implementation also includes serverInfo.
+    result = coll.explain().aggregate([{$lookup: {from: 'other_coll', pipeline: [], as: 'docs'}}]);
+    assert(result.hasOwnProperty('serverInfo'), result);
+    assert.hasFields(result.serverInfo, ['host', 'port', 'version', 'gitVersion']);
 })();
diff --git a/jstests/sharding/explain_cmd.js b/jstests/sharding/explain_cmd.js
index 0b5dcf8bf30..6cf9b78d82a 100644
--- a/jstests/sharding/explain_cmd.js
+++ b/jstests/sharding/explain_cmd.js
@@ -51,6 +51,8 @@
     assert("executionStats" in explain);
     assert.eq(2, explain.queryPlanner.winningPlan.shards.length);
     assert.eq(2, explain.executionStats.executionStages.shards.length);
+    assert("serverInfo" in explain, explain);
+    assert.hasFields(explain.serverInfo, ['host', 'port', 'version', 'gitVersion']);
 
     // An explain of a command that doesn't exist should fail gracefully.
     explain = db.runCommand({
diff --git a/src/mongo/db/query/SConscript b/src/mongo/db/query/SConscript
index f4a37ab200d..30f8745cb9e 100644
--- a/src/mongo/db/query/SConscript
+++ b/src/mongo/db/query/SConscript
@@ -63,6 +63,7 @@ env.Library(
     target="query_common",
     source=[
         "collation/collator_factory_icu_decoration.cpp",
+        "explain_common.cpp",
         "find_common.cpp",
         "parsed_distinct.cpp",
     ],
diff --git a/src/mongo/db/query/explain.cpp b/src/mongo/db/query/explain.cpp
index 1c9612833de..ac8f29d0bf6 100644
--- a/src/mongo/db/query/explain.cpp
+++ b/src/mongo/db/query/explain.cpp
@@ -45,6 +45,7 @@
 #include "mongo/db/exec/text.h"
 #include "mongo/db/exec/working_set_common.h"
 #include "mongo/db/keypattern.h"
+#include "mongo/db/query/explain_common.h"
 #include "mongo/db/query/get_executor.h"
 #include "mongo/db/query/plan_executor.h"
 #include "mongo/db/query/plan_summary_stats.h"
@@ -732,17 +733,6 @@ void Explain::generateSinglePlanExecutionInfo(const PlanStageStats* stats,
     stagesBob.doneFast();
 }
 
-// static
-void Explain::generateServerInfo(BSONObjBuilder* out) {
-    BSONObjBuilder serverBob(out->subobjStart("serverInfo"));
-    out->append("host", getHostNameCached());
-    out->appendNumber("port", serverGlobalParams.port);
-    auto&& vii = VersionInfoInterface::instance();
-    out->append("version", vii.version());
-    out->append("gitVersion", vii.gitVersion());
-    serverBob.doneFast();
-}
-
 std::unique_ptr<PlanStageStats> Explain::getWinningPlanTrialStats(PlanExecutor* exec) {
     // Inspect the tree to see if there is a MultiPlanStage. Plan selection has already happened at
     // this point, since we have a PlanExecutor.
@@ -851,6 +841,8 @@ void Explain::explainPipelineExecutor(PlanExecutor* exec,
     }
 
     *out << "stages" << Value(pps->writeExplainOps(verbosity));
+
+    explain_common::generateServerInfo(out);
 }
 
 // static
@@ -875,7 +867,7 @@ void Explain::explainStages(PlanExecutor* exec,
 
     explainStages(exec, collection, verbosity, executePlanStatus, winningPlanTrialStats.get(), out);
 
-    generateServerInfo(out);
+    explain_common::generateServerInfo(out);
 }
 
 // static
diff --git a/src/mongo/db/query/explain.h b/src/mongo/db/query/explain.h
index 2d736c7e3d0..4f267291757 100644
--- a/src/mongo/db/query/explain.h
+++ b/src/mongo/db/query/explain.h
@@ -225,14 +225,6 @@ private:
                                                 ExplainOptions::Verbosity verbosity,
                                                 boost::optional<long long> totalTimeMillis,
                                                 BSONObjBuilder* out);
-
-    /**
-     * Adds the 'serverInfo' explain section to the BSON object being build
-     * by 'out'.
-     *
-     * This is a helper for generating explain BSON. It is used by explainStages(...).
-     */
-    static void generateServerInfo(BSONObjBuilder* out);
 };
 
 }  // namespace
diff --git a/src/mongo/db/query/explain_common.cpp b/src/mongo/db/query/explain_common.cpp
new file mode 100644
index 00000000000..de9f87be068
--- /dev/null
+++ b/src/mongo/db/query/explain_common.cpp
@@ -0,0 +1,52 @@
+/**
+ *    Copyright (C) 2019-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#include "mongo/platform/basic.h"
+
+#include "mongo/db/query/explain_common.h"
+
+#include "mongo/db/server_options.h"
+#include "mongo/util/net/socket_utils.h"
+#include "mongo/util/version.h"
+
+namespace mongo {
+namespace explain_common {
+
+void generateServerInfo(BSONObjBuilder* out) {
+    BSONObjBuilder serverBob(out->subobjStart("serverInfo"));
+    out->append("host", getHostNameCached());
+    out->appendNumber("port", serverGlobalParams.port);
+    auto&& vii = VersionInfoInterface::instance();
+    out->append("version", vii.version());
+    out->append("gitVersion", vii.gitVersion());
+    serverBob.doneFast();
+}
+
+}  // namespace explain_common
+}  // namespace mongo
diff --git a/src/mongo/db/query/explain_common.h b/src/mongo/db/query/explain_common.h
new file mode 100644
index 00000000000..abff0edecc3
--- /dev/null
+++ b/src/mongo/db/query/explain_common.h
@@ -0,0 +1,49 @@
+/**
+ *    Copyright (C) 2019-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#pragma once
+
+#include "mongo/bson/bsonobjbuilder.h"
+
+namespace mongo {
+
+/**
+ * Namespace for static methods that are shared between explain on mongod and on mongos.
+ */
+namespace explain_common {
+
+/**
+ * Adds the 'serverInfo' explain section to the BSON object being built by 'out'.
+ *
+ * This section include the host, port, version, and gitVersion.
+ */
+void generateServerInfo(BSONObjBuilder* out);
+
+}  // namespace explain_common
+}  // namespace mongo
diff --git a/src/mongo/s/commands/cluster_aggregate.cpp b/src/mongo/s/commands/cluster_aggregate.cpp
index d58ce76220a..0fab00f2d19 100644
--- a/src/mongo/s/commands/cluster_aggregate.cpp
+++ b/src/mongo/s/commands/cluster_aggregate.cpp
@@ -52,6 +52,7 @@
 #include "mongo/db/pipeline/pipeline.h"
 #include "mongo/db/query/collation/collator_factory_interface.h"
 #include "mongo/db/query/cursor_response.h"
+#include "mongo/db/query/explain_common.h"
 #include "mongo/db/query/find_common.h"
 #include "mongo/db/views/resolved_view.h"
 #include "mongo/db/views/view.h"
@@ -938,6 +939,10 @@ Status ClusterAggregate::runAggregate(OperationContext* opCtx,
                           << AggregationRequest::kMergeByPBRTName
                           << "] cannot be set to 'true' when sent to mongos",
             !request.needsMerge() && !request.isFromMongos() && !request.mergeByPBRT());
+    if (request.getExplain()) {
+        explain_common::generateServerInfo(result);
+    }
+
     auto executionNsRoutingInfoStatus = getExecutionNsRoutingInfo(opCtx, namespaces.executionNss);
     boost::optional<CachedCollectionRoutingInfo> routingInfo;
     LiteParsedPipeline litePipe(request);
diff --git a/src/mongo/s/commands/cluster_explain.cpp b/src/mongo/s/commands/cluster_explain.cpp
index 089b7a4221f..7d621438101 100644
--- a/src/mongo/s/commands/cluster_explain.cpp
+++ b/src/mongo/s/commands/cluster_explain.cpp
@@ -33,6 +33,7 @@
 #include "mongo/bson/bsonmisc.h"
 #include "mongo/db/command_generic_argument.h"
 #include "mongo/db/commands.h"
+#include "mongo/db/query/explain_common.h"
 #include "mongo/rpc/get_status_from_command_result.h"
 #include "mongo/s/client/shard_registry.h"
 #include "mongo/s/commands/cluster_explain.h"
@@ -373,6 +374,7 @@ Status ClusterExplain::buildExplainResult(OperationContext* opCtx,
 
     buildPlannerInfo(opCtx, shardResults, mongosStageName, out);
     buildExecStats(shardResults, mongosStageName, millisElapsed, out);
+    explain_common::generateServerInfo(out);
 
     return Status::OK();
 }