SERVER-20267 Move runExplain() to Strategy to fix circular dependency

libmongoscore intentionally depends on libcluster_query, but prior
work for SERVER-20267 introduced a call to Strategy::commandOp()
(libmongoscore) from ClusterFind::runExplain() (libcluster_query),
which created a two-cycle.

This issue is addressed by moving ClusterFind::runExplain() to
Strategy::explainFind().

5 files changed, 57 insertions, 49 deletions

diff --git a/src/mongo/s/commands/cluster_find_cmd.cpp b/src/mongo/s/commands/cluster_find_cmd.cpp
index 0b0e60a8a13..445e0870bfe 100644
--- a/src/mongo/s/commands/cluster_find_cmd.cpp
+++ b/src/mongo/s/commands/cluster_find_cmd.cpp
@@ -36,6 +36,7 @@
 #include "mongo/db/query/cursor_response.h"
 #include "mongo/db/stats/counters.h"
 #include "mongo/s/query/cluster_find.h"
+#include "mongo/s/strategy.h"
 
 namespace mongo {
 namespace {
@@ -115,7 +116,7 @@ public:
             return lpq.getStatus();
         }
 
-        return ClusterFind::runExplain(
+        return Strategy::explainFind(
             txn, cmdObj, *lpq.getValue(), verbosity, serverSelectionMetadata, out);
     }
 
diff --git a/src/mongo/s/query/cluster_find.cpp b/src/mongo/s/query/cluster_find.cpp
index 289f8b29797..abc496f7a37 100644
--- a/src/mongo/s/query/cluster_find.cpp
+++ b/src/mongo/s/query/cluster_find.cpp
@@ -428,37 +428,6 @@ StatusWith<CursorResponse> ClusterFind::runGetMore(OperationContext* txn,
     return CursorResponse(request.nss, idToReturn, std::move(batch), startingFrom);
 }
 
-Status ClusterFind::runExplain(OperationContext* txn,
-                               const BSONObj& findCommand,
-                               const LiteParsedQuery& lpq,
-                               ExplainCommon::Verbosity verbosity,
-                               const rpc::ServerSelectionMetadata& serverSelectionMetadata,
-                               BSONObjBuilder* out) {
-    BSONObjBuilder explainCmdBob;
-    int options = 0;
-    ClusterExplain::wrapAsExplain(
-        findCommand, verbosity, serverSelectionMetadata, &explainCmdBob, &options);
-
-    // We will time how long it takes to run the commands on the shards.
-    Timer timer;
-
-    std::vector<Strategy::CommandResult> shardResults;
-    Strategy::commandOp(txn,
-                        lpq.nss().db().toString(),
-                        explainCmdBob.obj(),
-                        options,
-                        lpq.nss().toString(),
-                        lpq.getFilter(),
-                        &shardResults);
-
-    long long millisElapsed = timer.millis();
-
-    const char* mongosStageName = ClusterExplain::getStageNameForReadOp(shardResults, findCommand);
-
-    return ClusterExplain::buildExplainResult(
-        txn, shardResults, mongosStageName, millisElapsed, out);
-}
-
 StatusWith<ReadPreferenceSetting> ClusterFind::extractUnwrappedReadPref(const BSONObj& cmdObj,
                                                                         const bool isSlaveOk) {
     BSONElement queryOptionsElt;
diff --git a/src/mongo/s/query/cluster_find.h b/src/mongo/s/query/cluster_find.h
index 7a7d3deac69..64821512ded 100644
--- a/src/mongo/s/query/cluster_find.h
+++ b/src/mongo/s/query/cluster_find.h
@@ -76,21 +76,6 @@ public:
                                                  const GetMoreRequest& request);
 
     /**
-     * Helper to run an explain of a find operation on the shards. Fills 'out' with the result of
-     * the of the explain command on success. On failure, returns a non-OK status and does not
-     * modify 'out'.
-     *
-     * Used both if mongos receives an explain command and if it receives an OP_QUERY find with the
-     * $explain modifier.
-     */
-    static Status runExplain(OperationContext* txn,
-                             const BSONObj& findCommand,
-                             const LiteParsedQuery& lpq,
-                             ExplainCommon::Verbosity verbosity,
-                             const rpc::ServerSelectionMetadata& serverSelectionMetadata,
-                             BSONObjBuilder* out);
-
-    /**
      * Extracts the read preference from 'cmdObj', or determines the read pref based on 'isSlaveOk'
      * if 'cmdObj' does not contain a read preference.
      *
diff --git a/src/mongo/s/strategy.cpp b/src/mongo/s/strategy.cpp
index 4cd7ec26101..88b3dd35f64 100644
--- a/src/mongo/s/strategy.cpp
+++ b/src/mongo/s/strategy.cpp
@@ -53,6 +53,7 @@
 #include "mongo/s/bson_serializable.h"
 #include "mongo/s/catalog/catalog_cache.h"
 #include "mongo/s/client/shard_registry.h"
+#include "mongo/s/cluster_explain.h"
 #include "mongo/s/chunk_manager.h"
 #include "mongo/s/chunk_version.h"
 #include "mongo/s/config.h"
@@ -212,8 +213,8 @@ void Strategy::queryOp(OperationContext* txn, Request& request) {
             rpc::ServerSelectionMetadata metadata(secondaryOk, readPreference);
 
             BSONObjBuilder explainBuilder;
-            uassertStatusOK(ClusterFind::runExplain(
-                txn, findCommand, lpq, verbosity, metadata, &explainBuilder));
+            uassertStatusOK(
+                Strategy::explainFind(txn, findCommand, lpq, verbosity, metadata, &explainBuilder));
 
             BSONObj explainObj = explainBuilder.done();
             replyToQuery(0,  // query result flags
@@ -799,4 +800,35 @@ void Strategy::writeOp(OperationContext* txn, int op, Request& request) {
             break;
     }
 }
+
+Status Strategy::explainFind(OperationContext* txn,
+                             const BSONObj& findCommand,
+                             const LiteParsedQuery& lpq,
+                             ExplainCommon::Verbosity verbosity,
+                             const rpc::ServerSelectionMetadata& serverSelectionMetadata,
+                             BSONObjBuilder* out) {
+    BSONObjBuilder explainCmdBob;
+    int options = 0;
+    ClusterExplain::wrapAsExplain(
+        findCommand, verbosity, serverSelectionMetadata, &explainCmdBob, &options);
+
+    // We will time how long it takes to run the commands on the shards.
+    Timer timer;
+
+    std::vector<Strategy::CommandResult> shardResults;
+    Strategy::commandOp(txn,
+                        lpq.nss().db().toString(),
+                        explainCmdBob.obj(),
+                        options,
+                        lpq.nss().toString(),
+                        lpq.getFilter(),
+                        &shardResults);
+
+    long long millisElapsed = timer.millis();
+
+    const char* mongosStageName = ClusterExplain::getStageNameForReadOp(shardResults, findCommand);
+
+    return ClusterExplain::buildExplainResult(
+        txn, shardResults, mongosStageName, millisElapsed, out);
+}
 }
diff --git a/src/mongo/s/strategy.h b/src/mongo/s/strategy.h
index 8ac55de81f0..75416719999 100644
--- a/src/mongo/s/strategy.h
+++ b/src/mongo/s/strategy.h
@@ -28,14 +28,20 @@
 
 #pragma once
 
+#include "mongo/db/query/explain_common.h"
 #include "mongo/client/connection_string.h"
 #include "mongo/s/client/shard.h"
 #include "mongo/s/request.h"
 
 namespace mongo {
 
+class LiteParsedQuery;
 class OperationContext;
 
+namespace rpc {
+class ServerSelectionMetadata;
+}  // namespace rpc
+
 // A spigot to enable the ClusterClientCursor codepath.
 extern bool useClusterClientCursor;
 
@@ -52,6 +58,21 @@ public:
 
     static void writeOp(OperationContext* txn, int op, Request& request);
 
+    /**
+     * Helper to run an explain of a find operation on the shards. Fills 'out' with the result of
+     * the of the explain command on success. On failure, returns a non-OK status and does not
+     * modify 'out'.
+     *
+     * Used both if mongos receives an explain command and if it receives an OP_QUERY find with the
+     * $explain modifier.
+     */
+    static Status explainFind(OperationContext* txn,
+                              const BSONObj& findCommand,
+                              const LiteParsedQuery& lpq,
+                              ExplainCommon::Verbosity verbosity,
+                              const rpc::ServerSelectionMetadata& serverSelectionMetadata,
+                              BSONObjBuilder* out);
+
     struct CommandResult {
         ShardId shardTargetId;
         ConnectionString target;