SERVER-28814 Clean up ClusterExplain::wrapAsExplain()

8 files changed, 46 insertions, 92 deletions

diff --git a/src/mongo/s/commands/cluster_count_cmd.cpp b/src/mongo/s/commands/cluster_count_cmd.cpp
index d100c4c6f91..2389a7c9b97 100644
--- a/src/mongo/s/commands/cluster_count_cmd.cpp
+++ b/src/mongo/s/commands/cluster_count_cmd.cpp
@@ -265,8 +265,7 @@ public:
             return status;
         }
 
-        BSONObjBuilder explainCmdBob;
-        ClusterExplain::wrapAsExplain(cmdObj, verbosity, &explainCmdBob);
+        const auto explainCmd = ClusterExplain::wrapAsExplain(cmdObj, verbosity);
 
         // We will time how long it takes to run the commands on the shards
         Timer timer;
@@ -274,8 +273,8 @@ public:
         BSONObj viewDefinition;
         auto swShardResponses = scatterGatherForNamespace(opCtx,
                                                           nss,
-                                                          explainCmdBob.obj(),
-                                                          getReadPref(ssm),
+                                                          explainCmd,
+                                                          getReadPref(explainCmd),
                                                           targetingQuery,
                                                           targetingCollation,
                                                           true,  // do shard versioning
diff --git a/src/mongo/s/commands/cluster_explain.cpp b/src/mongo/s/commands/cluster_explain.cpp
index 3add2b0e580..59f4ca83cc5 100644
--- a/src/mongo/s/commands/cluster_explain.cpp
+++ b/src/mongo/s/commands/cluster_explain.cpp
@@ -31,7 +31,6 @@
 #include "mongo/bson/bsonmisc.h"
 #include "mongo/db/commands.h"
 #include "mongo/rpc/get_status_from_command_result.h"
-#include "mongo/rpc/metadata/server_selection_metadata.h"
 #include "mongo/s/client/shard_registry.h"
 #include "mongo/s/commands/cluster_explain.h"
 #include "mongo/s/grid.h"
@@ -130,35 +129,20 @@ std::vector<Strategy::CommandResult> ClusterExplain::downconvert(
 }
 
 // static
-void ClusterExplain::wrapAsExplain(const BSONObj& cmdObj,
-                                   ExplainOptions::Verbosity verbosity,
-                                   BSONObjBuilder* explainBuilder) {
-    explainBuilder->append("explain", cmdObj);
-    explainBuilder->append("verbosity", ExplainOptions::verbosityString(verbosity));
-
-    // Propagate readConcern
-    if (auto readConcern = cmdObj["readConcern"]) {
-        explainBuilder->append(readConcern);
+BSONObj ClusterExplain::wrapAsExplain(const BSONObj& cmdObj, ExplainOptions::Verbosity verbosity) {
+    BSONObjBuilder out;
+    out.append("explain", cmdObj);
+    out.append("verbosity", ExplainOptions::verbosityString(verbosity));
+
+    // Propagate all generic arguments out of the inner command since the shards will only process
+    // them at the top level.
+    for (auto elem : cmdObj) {
+        if (Command::isGenericArgument(elem.fieldNameStringData())) {
+            out.append(elem);
+        }
     }
-}
 
-// static
-void ClusterExplain::wrapAsExplainDeprecated(
-    const BSONObj& cmdObj,
-    ExplainOptions::Verbosity verbosity,
-    const rpc::ServerSelectionMetadata& serverSelectionMetadata,
-    BSONObjBuilder* out,
-    int* optionsOut) {
-    BSONObjBuilder explainBuilder;
-    wrapAsExplain(cmdObj, verbosity, &explainBuilder);
-    const BSONObj explainCmdObj = explainBuilder.done();
-
-    // Attach metadata to the explain command in legacy format.
-    BSONObjBuilder metadataBuilder;
-    serverSelectionMetadata.writeToMetadata(&metadataBuilder);
-    const BSONObj metadataObj = metadataBuilder.done();
-    uassertStatusOK(
-        serverSelectionMetadata.downconvert(explainCmdObj, metadataObj, out, optionsOut));
+    return out.obj();
 }
 
 // static
diff --git a/src/mongo/s/commands/cluster_explain.h b/src/mongo/s/commands/cluster_explain.h
index 1bc72aacd6e..01542b2b16f 100644
--- a/src/mongo/s/commands/cluster_explain.h
+++ b/src/mongo/s/commands/cluster_explain.h
@@ -39,10 +39,6 @@ namespace mongo {
 
 class OperationContext;
 
-namespace rpc {
-class ServerSelectionMetadata;
-}  // namespace rpc
-
 /**
  * Namespace for the collection of static methods used by commands in the implementation of
  * explain on mongos.
@@ -61,33 +57,10 @@ public:
         OperationContext* opCtx, const std::vector<AsyncRequestsSender::Response>& responses);
 
     /**
-     * Given the BSON specification for a command, 'cmdObj', wraps the object in order to produce
-     * the BSON for an explain of that command, at the given verbosity level 'verbosity.'
-     *
-     * Adds the result to the BSONObjBuidler 'out'.
-     *
-     * Unlike wrapAsExplain, does not downconvert the command to OP_QUERY. Should be used for paths
-     * that send the command over the NetworkInterfaceASIO rather than DBClient.
-     */
-    static void wrapAsExplain(const BSONObj& cmdObj,
-                              ExplainOptions::Verbosity verbosity,
-                              BSONObjBuilder* explainBuilder);
-
-    /**
-     * Given the BSON specification for a command, 'cmdObj', wraps the object in order to
-     * produce the BSON for an explain of that command, at the given verbosity level
-     * 'verbosity' and according to the metadata in 'serverSelectionMetadata'.
-     *
-     * Adds the result to the BSONObj builder 'out'.
-     *
-     * Also uses 'serverSelectionMetdata' to set 'optionsOut' to the options bit vector that should
-     * be forwarded to the shards.
+     * Returns an explain command request wrapping the passed in command at the given verbosity
+     * level, propagating generic top-level command arguments.
      */
-    static void wrapAsExplainDeprecated(const BSONObj& cmdObj,
-                                        ExplainOptions::Verbosity verbosity,
-                                        const rpc::ServerSelectionMetadata& serverSelectionMetadata,
-                                        BSONObjBuilder* out,
-                                        int* optionsOut);
+    static BSONObj wrapAsExplain(const BSONObj& cmdObj, ExplainOptions::Verbosity verbosity);
 
     /**
      * Determines the kind of "execution stage" that mongos would use in order to collect
diff --git a/src/mongo/s/commands/cluster_explain_cmd.cpp b/src/mongo/s/commands/cluster_explain_cmd.cpp
index fed93b5247e..db053de01a0 100644
--- a/src/mongo/s/commands/cluster_explain_cmd.cpp
+++ b/src/mongo/s/commands/cluster_explain_cmd.cpp
@@ -116,8 +116,23 @@ public:
             return appendCommandStatus(result, verbosity.getStatus());
         }
 
-        // This is the nested command which we are explaining.
-        BSONObj explainObj = cmdObj.firstElement().Obj();
+        // This is the nested command which we are explaining. We need to propagate generic
+        // arguments into the inner command since it is what is passed to the virtual
+        // Command::explain() method.
+        const BSONObj explainObj = ([&] {
+            const auto innerObj = cmdObj.firstElement().Obj();
+            BSONObjBuilder bob;
+            bob.appendElements(innerObj);
+            for (auto outerElem : cmdObj) {
+                // If the argument is in both the inner and outer command, we currently let the
+                // inner version take precedence.
+                const auto name = outerElem.fieldNameStringData();
+                if (Command::isGenericArgument(name) && !innerObj.hasField(name)) {
+                    bob.append(outerElem);
+                }
+            }
+            return bob.obj();
+        }());
 
         const std::string cmdName = explainObj.firstElementFieldName();
         Command* commToExplain = Command::findCommand(cmdName);
diff --git a/src/mongo/s/commands/cluster_find_and_modify_cmd.cpp b/src/mongo/s/commands/cluster_find_and_modify_cmd.cpp
index befc371efdb..f16c508ca59 100644
--- a/src/mongo/s/commands/cluster_find_and_modify_cmd.cpp
+++ b/src/mongo/s/commands/cluster_find_and_modify_cmd.cpp
@@ -128,16 +128,13 @@ public:
             shard = shardStatus.getValue();
         }
 
-        BSONObjBuilder explainCmd;
-        int options = 0;
-        ClusterExplain::wrapAsExplainDeprecated(
-            cmdObj, verbosity, serverSelectionMetadata, &explainCmd, &options);
+        const auto explainCmd = ClusterExplain::wrapAsExplain(cmdObj, verbosity);
 
         // Time how long it takes to run the explain command on the shard.
         Timer timer;
 
         BSONObjBuilder result;
-        bool ok = _runCommand(opCtx, chunkMgr, shard->getId(), nss, explainCmd.obj(), result);
+        bool ok = _runCommand(opCtx, chunkMgr, shard->getId(), nss, explainCmd, result);
         long long millisElapsed = timer.millis();
 
         if (!ok) {
diff --git a/src/mongo/s/commands/cluster_write_cmd.cpp b/src/mongo/s/commands/cluster_write_cmd.cpp
index 38cf186155c..dc9ab0c27b9 100644
--- a/src/mongo/s/commands/cluster_write_cmd.cpp
+++ b/src/mongo/s/commands/cluster_write_cmd.cpp
@@ -110,8 +110,7 @@ public:
             return Status(ErrorCodes::InvalidLength, "explained write batches must be of size 1");
         }
 
-        BSONObjBuilder explainCmdBob;
-        ClusterExplain::wrapAsExplain(cmdObj, verbosity, &explainCmdBob);
+        const auto explainCmd = ClusterExplain::wrapAsExplain(cmdObj, verbosity);
 
         // We will time how long it takes to run the commands on the shards.
         Timer timer;
@@ -120,7 +119,7 @@ public:
         BatchItemRef targetingBatchItem(&request, 0);
         vector<Strategy::CommandResult> shardResults;
         Status status =
-            _commandOpWrite(opCtx, dbname, explainCmdBob.obj(), targetingBatchItem, &shardResults);
+            _commandOpWrite(opCtx, dbname, explainCmd, targetingBatchItem, &shardResults);
         if (!status.isOK()) {
             return status;
         }
diff --git a/src/mongo/s/commands/commands_public.cpp b/src/mongo/s/commands/commands_public.cpp
index 5d006fb2c85..208539821b9 100644
--- a/src/mongo/s/commands/commands_public.cpp
+++ b/src/mongo/s/commands/commands_public.cpp
@@ -52,7 +52,6 @@
 #include "mongo/db/views/resolved_view.h"
 #include "mongo/executor/task_executor_pool.h"
 #include "mongo/rpc/get_status_from_command_result.h"
-#include "mongo/rpc/metadata/server_selection_metadata.h"
 #include "mongo/s/async_requests_sender.h"
 #include "mongo/s/catalog/sharding_catalog_client.h"
 #include "mongo/s/catalog_cache.h"
@@ -893,17 +892,7 @@ public:
                    BSONObjBuilder* out) const override {
         // We will time how long it takes to run the commands on the shards.
         Timer timer;
-
-        BSONObj command;
-        int options = 0;
-
-        {
-            BSONObjBuilder explainCmdBob;
-            ClusterExplain::wrapAsExplainDeprecated(
-                cmdObj, verbosity, serverSelectionMetadata, &explainCmdBob, &options);
-            command = explainCmdBob.obj();
-        }
-
+        BSONObj command = ClusterExplain::wrapAsExplain(cmdObj, verbosity);
         const NamespaceString nss(parseNs(dbname, cmdObj));
 
         auto routingInfo =
@@ -919,7 +908,7 @@ public:
             ShardConnection conn(routingInfo.primary()->getConnString(), "");
 
             // TODO: this can throw a stale config when mongos is not up-to-date -- fix.
-            if (!conn->runCommand(nss.db().toString(), command, shardResult, options)) {
+            if (!conn->runCommand(nss.db().toString(), command, shardResult)) {
                 conn.done();
                 return Status(ErrorCodes::OperationFailed,
                               str::stream() << "Passthrough command failed: " << command
@@ -1150,8 +1139,7 @@ public:
         // Extract the targeting collation.
         auto targetingCollation = uassertStatusOK(getCollation(cmdObj));
 
-        BSONObjBuilder explainCmdBob;
-        ClusterExplain::wrapAsExplain(cmdObj, verbosity, &explainCmdBob);
+        const auto explainCmd = ClusterExplain::wrapAsExplain(cmdObj, verbosity);
 
         // We will time how long it takes to run the commands on the shards.
         Timer timer;
@@ -1159,8 +1147,8 @@ public:
         BSONObj viewDefinition;
         auto swShardResponses = scatterGatherForNamespace(opCtx,
                                                           nss,
-                                                          explainCmdBob.obj(),
-                                                          getReadPref(ssm),
+                                                          explainCmd,
+                                                          getReadPref(explainCmd),
                                                           targetingQuery,
                                                           targetingCollation,
                                                           true,  // do shard versioning
diff --git a/src/mongo/s/commands/strategy.cpp b/src/mongo/s/commands/strategy.cpp
index fb7166c6a28..707b728efe4 100644
--- a/src/mongo/s/commands/strategy.cpp
+++ b/src/mongo/s/commands/strategy.cpp
@@ -619,8 +619,7 @@ Status Strategy::explainFind(OperationContext* opCtx,
                              ExplainOptions::Verbosity verbosity,
                              const rpc::ServerSelectionMetadata& ssm,
                              BSONObjBuilder* out) {
-    BSONObjBuilder explainCmdBob;
-    ClusterExplain::wrapAsExplain(findCommand, verbosity, &explainCmdBob);
+    const auto explainCmd = ClusterExplain::wrapAsExplain(findCommand, verbosity);
 
     // We will time how long it takes to run the commands on the shards.
     Timer timer;
@@ -628,7 +627,7 @@ Status Strategy::explainFind(OperationContext* opCtx,
     BSONObj viewDefinition;
     auto swShardResponses = scatterGatherForNamespace(opCtx,
                                                       qr.nss(),
-                                                      explainCmdBob.obj(),
+                                                      explainCmd,
                                                       getReadPref(ssm),
                                                       qr.getFilter(),
                                                       qr.getCollation(),