From af1a9dc12adcfa83cc19571cb3faba26eeddac92 Mon Sep 17 00:00:00 2001
From: Dianna Hohensee <dianna.hohensee@mongodb.com>
Date: Tue, 28 Dec 2021 22:41:48 +0000
Subject: SERVER-45953 Exempt internal replication oplog readers from acquiring
 read tickets in order to avoid deadlocks

(cherry picked from commit d1cb83d9199c1a25158d74e47b3aa88b5c33fe8b)
---
 src/mongo/db/commands/find_cmd.cpp    | 17 ++++++++++++++---
 src/mongo/db/commands/getmore_cmd.cpp | 13 ++++++++++++-
 src/mongo/db/query/query_request.cpp  |  2 +-
 src/mongo/db/query/query_request.h    |  2 +-
 4 files changed, 28 insertions(+), 6 deletions(-)

diff --git a/src/mongo/db/commands/find_cmd.cpp b/src/mongo/db/commands/find_cmd.cpp
index 4ce8c38e5f2..f59df4f4d8b 100644
--- a/src/mongo/db/commands/find_cmd.cpp
+++ b/src/mongo/db/commands/find_cmd.cpp
@@ -232,8 +232,8 @@ public:
         // Parse the command BSON to a QueryRequest.
         const bool isExplain = false;
         // Pass parseNs to makeFromFindCommand in case cmdObj does not have a UUID.
-        auto qrStatus = QueryRequest::makeFromFindCommand(
-            NamespaceString(parseNs(dbname, cmdObj)), cmdObj, isExplain);
+        const NamespaceString parsedNss(parseNs(dbname, cmdObj));
+        auto qrStatus = QueryRequest::makeFromFindCommand(parsedNss, cmdObj, isExplain);
         uassertStatusOK(qrStatus.getStatus());
 
         auto replCoord = repl::ReplicationCoordinator::get(opCtx);
@@ -245,12 +245,23 @@ public:
                     !(session->inActiveOrKilledMultiDocumentTransaction() && qr->isTailable()));
 
         // Validate term before acquiring locks, if provided.
-        if (auto term = qr->getReplicationTerm()) {
+        auto term = qr->getReplicationTerm();
+        if (term) {
             Status status = replCoord->updateTerm(opCtx, *term);
             // Note: updateTerm returns ok if term stayed the same.
             uassertStatusOK(status);
         }
 
+        // The presence of a term in the request indicates that this is an internal replication
+        // oplog read request.
+        if (term && parsedNss == NamespaceString::kRsOplogNamespace) {
+            // We do not want to take tickets for internal (replication) oplog reads. Stalling
+            // on ticket acquisition can cause complicated deadlocks. Primaries may depend on
+            // data reaching secondaries in order to proceed; and secondaries may get stalled
+            // replicating because of an inability to acquire a read ticket.
+            opCtx->lockState()->skipAcquireTicket();
+        }
+
         // Acquire locks. If the query is on a view, we release our locks and convert the query
         // request into an aggregation command.
         boost::optional<AutoGetCollectionForReadCommand> ctx;
diff --git a/src/mongo/db/commands/getmore_cmd.cpp b/src/mongo/db/commands/getmore_cmd.cpp
index 89d351baf86..886a6138b06 100644
--- a/src/mongo/db/commands/getmore_cmd.cpp
+++ b/src/mongo/db/commands/getmore_cmd.cpp
@@ -223,12 +223,23 @@ public:
         auto curOp = CurOp::get(opCtx);
         curOp->debug().cursorid = request.cursorid;
 
-        // Validate term before acquiring locks, if provided.
         if (request.term) {
+            // Validate term before acquiring locks, if provided.
             auto replCoord = repl::ReplicationCoordinator::get(opCtx);
             Status status = replCoord->updateTerm(opCtx, *request.term);
             // Note: updateTerm returns ok if term stayed the same.
             uassertStatusOK(status);
+
+            // If this is an oplog request, then this is a getMore for replication oplog
+            // fetching. The term field is only allowed for internal clients (see
+            // checkAuthForGetMore).
+            if (request.nss == NamespaceString::kRsOplogNamespace) {
+                // We do not want to take tickets for internal (replication) oplog reads.
+                // Stalling on ticket acquisition can cause complicated deadlocks. Primaries may
+                // depend on data reaching secondaries in order to proceed; and secondaries may
+                // get stalled replicating because of an inability to acquire a read ticket.
+                opCtx->lockState()->skipAcquireTicket();
+            }
         }
 
         // Cursors come in one of two flavors:
diff --git a/src/mongo/db/query/query_request.cpp b/src/mongo/db/query/query_request.cpp
index 0dd44bc7f3f..a2e098f11df 100644
--- a/src/mongo/db/query/query_request.cpp
+++ b/src/mongo/db/query/query_request.cpp
@@ -401,7 +401,7 @@ StatusWith<unique_ptr<QueryRequest>> QueryRequest::parseFromFindCommand(unique_p
     return std::move(qr);
 }
 
-StatusWith<unique_ptr<QueryRequest>> QueryRequest::makeFromFindCommand(NamespaceString nss,
+StatusWith<unique_ptr<QueryRequest>> QueryRequest::makeFromFindCommand(const NamespaceString& nss,
                                                                        const BSONObj& cmdObj,
                                                                        bool isExplain) {
     BSONElement first = cmdObj.firstElement();
diff --git a/src/mongo/db/query/query_request.h b/src/mongo/db/query/query_request.h
index 6ac6de5ce9b..88792140aca 100644
--- a/src/mongo/db/query/query_request.h
+++ b/src/mongo/db/query/query_request.h
@@ -74,7 +74,7 @@ public:
      * Returns a heap allocated QueryRequest on success or an error if 'cmdObj' is not well
      * formed.
      */
-    static StatusWith<std::unique_ptr<QueryRequest>> makeFromFindCommand(NamespaceString nss,
+    static StatusWith<std::unique_ptr<QueryRequest>> makeFromFindCommand(const NamespaceString& nss,
                                                                          const BSONObj& cmdObj,
                                                                          bool isExplain);
 
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