From 907bf93c57b1f689ecf690df6beb7cb07fd9fa5e Mon Sep 17 00:00:00 2001
From: Ben Caimano <ben.caimano@10gen.com>
Date: Tue, 6 Oct 2020 21:59:59 +0000
Subject: SERVER-49109 Move to the dedicated executor after blocking commands

---
 src/mongo/db/commands.h                     | 28 ++++++++++++++++++++++++++++
 src/mongo/db/service_entry_point_common.cpp | 17 +++++++++++++++++
 src/mongo/s/commands/strategy.cpp           | 13 +++++++++++++
 3 files changed, 58 insertions(+)

diff --git a/src/mongo/db/commands.h b/src/mongo/db/commands.h
index 790c6123f8e..60fb3036daf 100644
--- a/src/mongo/db/commands.h
+++ b/src/mongo/db/commands.h
@@ -629,6 +629,34 @@ public:
                 {kDefaultReadConcernNotPermitted}};
     }
 
+    /**
+     * Returns if this invocation is safe to run on a borrowed threading model.
+     *
+     * In practice, this is attempting to predict if the operation will do network or storage reads
+     * and writes. It will allow auth commands for the most part, since while they do involve
+     * network or storage operations, they are not targeting the storage engine or remote
+     * mongo-server nodes.
+     */
+    virtual bool isSafeForBorrowedThreads() const {
+        if (definition()->maintenanceMode() || !definition()->maintenanceOk()) {
+            // If the command has maintenance implications, it has storage implications.
+            return false;
+        }
+
+        if (supportsWriteConcern()) {
+            // If the command supports write concern, it has storage and network implications.
+            return false;
+        }
+
+        if (auto result = supportsReadConcern(repl::ReadConcernLevel::kMajorityReadConcern);
+            result.readConcernSupport.isOK()) {
+            // If the command supports read concern, it has storage and newtork implications.
+            return false;
+        }
+
+        return true;
+    }
+
     /**
      * Return if this invocation can be mirrored to secondaries
      */
diff --git a/src/mongo/db/service_entry_point_common.cpp b/src/mongo/db/service_entry_point_common.cpp
index 2b20ed67bab..af22da58f4e 100644
--- a/src/mongo/db/service_entry_point_common.cpp
+++ b/src/mongo/db/service_entry_point_common.cpp
@@ -98,6 +98,7 @@
 #include "mongo/rpc/op_msg.h"
 #include "mongo/rpc/reply_builder_interface.h"
 #include "mongo/transport/ismaster_metrics.h"
+#include "mongo/transport/service_executor.h"
 #include "mongo/transport/session.h"
 #include "mongo/util/duration.h"
 #include "mongo/util/fail_point.h"
@@ -2253,6 +2254,22 @@ Future<DbResponse> ServiceEntryPointCommon::handleRequest(
                 [execContext = hr->executionContext](DbResponse response) -> void {
                     // Set the response upon successful execution
                     execContext->setResponse(std::move(response));
+
+                    auto opCtx = execContext->getOpCtx();
+
+                    auto seCtx = transport::ServiceExecutorContext::get(opCtx->getClient());
+                    if (!seCtx) {
+                        // We were run by a background worker.
+                        return;
+                    }
+
+                    if (auto invocation = CommandInvocation::get(opCtx);
+                        invocation && !invocation->isSafeForBorrowedThreads()) {
+                        // If the last command wasn't safe for a borrowed thread, then let's move
+                        // off of it.
+                        seCtx->setThreadingModel(
+                            transport::ServiceExecutor::ThreadingModel::kDedicated);
+                    }
                 });
         })
         .then([hr] { return hr->completeOperation(); })
diff --git a/src/mongo/s/commands/strategy.cpp b/src/mongo/s/commands/strategy.cpp
index f83b490d0ef..f3f27506da7 100644
--- a/src/mongo/s/commands/strategy.cpp
+++ b/src/mongo/s/commands/strategy.cpp
@@ -85,6 +85,7 @@
 #include "mongo/s/stale_exception.h"
 #include "mongo/s/transaction_router.h"
 #include "mongo/transport/ismaster_metrics.h"
+#include "mongo/transport/service_executor.h"
 #include "mongo/transport/session.h"
 #include "mongo/util/fail_point.h"
 #include "mongo/util/scopeguard.h"
@@ -287,6 +288,18 @@ void execCommandClient(OperationContext* opCtx,
 
     auto body = result->getBodyBuilder();
     appendRequiredFieldsToResponse(opCtx, &body);
+
+    auto seCtx = transport::ServiceExecutorContext::get(opCtx->getClient());
+    if (!seCtx) {
+        // We were run by a background worker.
+        return;
+    }
+
+    if (!invocation->isSafeForBorrowedThreads()) {
+        // If the last command wasn't safe for a borrowed thread, then let's move
+        // off of it.
+        seCtx->setThreadingModel(transport::ServiceExecutor::ThreadingModel::kDedicated);
+    }
 }
 
 MONGO_FAIL_POINT_DEFINE(doNotRefreshShardsOnRetargettingError);
-- 
cgit v1.2.1