SERVER-27176 Improve performance of command writeConcern parsing

4 files changed, 38 insertions, 21 deletions

diff --git a/src/mongo/db/commands/dbcommands.cpp b/src/mongo/db/commands/dbcommands.cpp
index 9f52d6a91cf..4333c271f7a 100644
--- a/src/mongo/db/commands/dbcommands.cpp
+++ b/src/mongo/db/commands/dbcommands.cpp
@@ -1521,19 +1521,30 @@ bool Command::run(OperationContext* txn,
         replyBuilder->setMetadata(rpc::makeEmptyMetadata());
         return result;
     }
-    auto wcResult = extractWriteConcern(txn, cmd, db, supportsWriteConcern(cmd));
-    if (!wcResult.isOK()) {
-        auto result = appendCommandStatus(inPlaceReplyBob, wcResult.getStatus());
-        inPlaceReplyBob.doneFast();
-        replyBuilder->setMetadata(rpc::makeEmptyMetadata());
-        return result;
-    }
+
     std::string errmsg;
     bool result;
     if (!supportsWriteConcern(cmd)) {
+        if (commandSpecifiesWriteConcern(cmd)) {
+            auto result = appendCommandStatus(
+                inPlaceReplyBob,
+                {ErrorCodes::InvalidOptions, "Command does not support writeConcern"});
+            inPlaceReplyBob.doneFast();
+            replyBuilder->setMetadata(rpc::makeEmptyMetadata());
+            return result;
+        }
+
         // TODO: remove queryOptions parameter from command's run method.
         result = run(txn, db, cmd, 0, errmsg, inPlaceReplyBob);
     } else {
+        auto wcResult = extractWriteConcern(txn, cmd, db);
+        if (!wcResult.isOK()) {
+            auto result = appendCommandStatus(inPlaceReplyBob, wcResult.getStatus());
+            inPlaceReplyBob.doneFast();
+            replyBuilder->setMetadata(rpc::makeEmptyMetadata());
+            return result;
+        }
+
         // Change the write concern while running the command.
         const auto oldWC = txn->getWriteConcern();
         ON_BLOCK_EXIT([&] { txn->setWriteConcern(oldWC); });
diff --git a/src/mongo/db/write_concern.cpp b/src/mongo/db/write_concern.cpp
index f4a30f917f5..6886bed64ca 100644
--- a/src/mongo/db/write_concern.cpp
+++ b/src/mongo/db/write_concern.cpp
@@ -62,10 +62,13 @@ static ServerStatusMetricField<Counter64> gleWtimeoutsDisplay("getLastError.wtim
 
 MONGO_FP_DECLARE(hangBeforeWaitingForWriteConcern);
 
+bool commandSpecifiesWriteConcern(const BSONObj& cmdObj) {
+    return cmdObj.hasField(WriteConcernOptions::kWriteConcernField);
+}
+
 StatusWith<WriteConcernOptions> extractWriteConcern(OperationContext* txn,
                                                     const BSONObj& cmdObj,
-                                                    const std::string& dbName,
-                                                    const bool supportsWriteConcern) {
+                                                    const std::string& dbName) {
     // The default write concern if empty is {w:1}. Specifying {w:0} is/was allowed, but is
     // interpreted identically to {w:1}.
     auto wcResult = WriteConcernOptions::extractWCFromCommand(
@@ -84,16 +87,11 @@ StatusWith<WriteConcernOptions> extractWriteConcern(OperationContext* txn,
             writeConcern = {
                 WriteConcernOptions::kMajority, WriteConcernOptions::SyncMode::UNSET, Seconds(30)};
         }
-    } else if (supportsWriteConcern) {
-        // If it supports writeConcern and does not use the default, validate the writeConcern.
+    } else {
         Status wcStatus = validateWriteConcern(txn, writeConcern, dbName);
         if (!wcStatus.isOK()) {
             return wcStatus;
         }
-    } else {
-        // This command doesn't do writes so it should not be passed a writeConcern. If we did not
-        // use the default writeConcern, one was provided when it shouldn't have been by the user.
-        return {ErrorCodes::InvalidOptions, "Command does not support writeConcern"};
     }
 
     return writeConcern;
diff --git a/src/mongo/db/write_concern.h b/src/mongo/db/write_concern.h
index 9a9720f26ee..13bae072b05 100644
--- a/src/mongo/db/write_concern.h
+++ b/src/mongo/db/write_concern.h
@@ -28,6 +28,7 @@
 
 #pragma once
 
+#include "mongo/bson/bsonobj.h"
 #include "mongo/db/write_concern_options.h"
 #include "mongo/util/net/hostandport.h"
 
@@ -42,14 +43,18 @@ class OpTime;
 }
 
 /**
+ * Returns true if 'cmdObj' has a 'writeConcern' field.
+ */
+bool commandSpecifiesWriteConcern(const BSONObj& cmdObj);
+
+/**
  * Attempts to extract a writeConcern from cmdObj.
  * Verifies that the writeConcern is of type Object (BSON type) and
  * that the resulting writeConcern is valid for this particular host.
  */
 StatusWith<WriteConcernOptions> extractWriteConcern(OperationContext* txn,
                                                     const BSONObj& cmdObj,
-                                                    const std::string& dbName,
-                                                    const bool supportsWriteConcern);
+                                                    const std::string& dbName);
 
 /**
  * Verifies that a WriteConcern is valid for this particular host and database.
diff --git a/src/mongo/db/write_concern_options.cpp b/src/mongo/db/write_concern_options.cpp
index dd9694dcc61..a2ad573ff25 100644
--- a/src/mongo/db/write_concern_options.cpp
+++ b/src/mongo/db/write_concern_options.cpp
@@ -159,14 +159,17 @@ StatusWith<WriteConcernOptions> WriteConcernOptions::extractWCFromCommand(
         writeConcern.wNumNodes = 1;
     }
 
+    // Return the default write concern if no write concern is provided. We check for the existence
+    // of the write concern field up front in order to avoid the expense of constructing an error
+    // status in bsonExtractTypedField() below.
+    if (!cmdObj.hasField(kWriteConcernField)) {
+        return writeConcern;
+    }
+
     BSONElement writeConcernElement;
     Status wcStatus =
         bsonExtractTypedField(cmdObj, kWriteConcernField, Object, &writeConcernElement);
     if (!wcStatus.isOK()) {
-        if (wcStatus == ErrorCodes::NoSuchKey) {
-            // Return default write concern if no write concern is given.
-            return writeConcern;
-        }
         return wcStatus;
     }