SERVER-40508 Remove hash from OplogSlot and make OplogSlot an alias for OpTime.r4.1.10

2 files changed, 16 insertions, 25 deletions

diff --git a/src/mongo/db/repl/oplog.cpp b/src/mongo/db/repl/oplog.cpp
index f421603e99f..129f7ab16db 100644
--- a/src/mongo/db/repl/oplog.cpp
+++ b/src/mongo/db/repl/oplog.cpp
@@ -137,9 +137,6 @@ struct LocalOplogInfo {
     // Synchronizes the section where a new Timestamp is generated and when it is registered in the
     // storage engine.
     stdx::mutex newOpMutex;
-
-    // Used to generate "h" fields in pv0. Synchronized by newOpMutex.
-    PseudoRandom hashGenerator{std::unique_ptr<SecureRandom>(SecureRandom::create())->nextInt64()};
 };
 
 const auto localOplogInfo = ServiceContext::declareDecoration<LocalOplogInfo>();
@@ -185,8 +182,7 @@ void _getNextOpTimes(OperationContext* opCtx,
     }
 
     for (std::size_t i = 0; i < count; i++) {
-        slotsOut[i].opTime = {Timestamp(ts.asULL() + i), term};
-        slotsOut[i].hash = oplogInfo.hashGenerator.nextInt64();
+        slotsOut[i] = {Timestamp(ts.asULL() + i), term};
     }
 }
 
@@ -449,7 +445,6 @@ OplogDocWriter _logOpWriter(OperationContext* opCtx,
                             const BSONObj* o2,
                             bool fromMigrate,
                             OpTime optime,
-                            long long hashNew,
                             Date_t wallTime,
                             const OperationSessionInfo& sessionInfo,
                             StmtId statementId,
@@ -461,7 +456,10 @@ OplogDocWriter _logOpWriter(OperationContext* opCtx,
     b.append("ts", optime.getTimestamp());
     if (optime.getTerm() != -1)
         b.append("t", optime.getTerm());
-    b.append("h", hashNew);
+
+    // Always write zero hash instead of using FCV to gate this for retryable writes
+    // and change stream, who expect to be able to read oplog across FCV's.
+    b.append("h", 0LL);
     b.append("v", OplogEntry::kOplogVersion);
     b.append("op", opstr);
     b.append("ns", nss.ns());
@@ -608,7 +606,7 @@ OpTime logOp(OperationContext* opCtx,
     auto const oplog = oplogInfo.oplog;
     OplogSlot slot;
     WriteUnitOfWork wuow(opCtx);
-    if (oplogSlot.opTime.isNull()) {
+    if (oplogSlot.isNull()) {
         _getNextOpTimes(opCtx, oplog, 1, &slot);
     } else {
         slot = oplogSlot;
@@ -621,8 +619,7 @@ OpTime logOp(OperationContext* opCtx,
                                obj,
                                o2,
                                fromMigrate,
-                               slot.opTime,
-                               slot.hash,
+                               slot,
                                wallClockTime,
                                sessionInfo,
                                statementId,
@@ -630,10 +627,10 @@ OpTime logOp(OperationContext* opCtx,
                                prepare,
                                inTxn);
     const DocWriter* basePtr = &writer;
-    auto timestamp = slot.opTime.getTimestamp();
-    _logOpsInner(opCtx, nss, &basePtr, &timestamp, 1, oplog, slot.opTime, wallClockTime);
+    auto timestamp = slot.getTimestamp();
+    _logOpsInner(opCtx, nss, &basePtr, &timestamp, 1, oplog, slot, wallClockTime);
     wuow.commit();
-    return slot.opTime;
+    return slot;
 }
 
 std::vector<OpTime> logInsertOps(OperationContext* opCtx,
@@ -686,7 +683,7 @@ std::vector<OpTime> logInsertOps(OperationContext* opCtx,
         // Make a mutable copy.
         auto insertStatementOplogSlot = begin[i].oplogSlot;
         // Fetch optime now, if not already fetched.
-        if (insertStatementOplogSlot.opTime.isNull()) {
+        if (insertStatementOplogSlot.isNull()) {
             _getNextOpTimes(opCtx, oplog, 1, &insertStatementOplogSlot);
         }
         // Only 'applyOps' oplog entries can be prepared.
@@ -698,17 +695,16 @@ std::vector<OpTime> logInsertOps(OperationContext* opCtx,
                                           begin[i].doc,
                                           NULL,
                                           fromMigrate,
-                                          insertStatementOplogSlot.opTime,
-                                          insertStatementOplogSlot.hash,
+                                          insertStatementOplogSlot,
                                           wallClockTime,
                                           sessionInfo,
                                           begin[i].stmtId,
                                           oplogLink,
                                           prepare,
                                           false /* inTxn */));
-        oplogLink.prevOpTime = insertStatementOplogSlot.opTime;
+        oplogLink.prevOpTime = insertStatementOplogSlot;
         timestamps[i] = oplogLink.prevOpTime.getTimestamp();
-        opTimes.push_back(insertStatementOplogSlot.opTime);
+        opTimes.push_back(insertStatementOplogSlot);
     }
 
     MONGO_FAIL_POINT_BLOCK(sleepBetweenInsertOpTimeGenerationAndLogOp, customWait) {
diff --git a/src/mongo/db/repl/oplog.h b/src/mongo/db/repl/oplog.h
index 694d5c38152..d9fa2c8bf74 100644
--- a/src/mongo/db/repl/oplog.h
+++ b/src/mongo/db/repl/oplog.h
@@ -51,12 +51,7 @@ class OperationContext;
 class OperationSessionInfo;
 class Session;
 
-struct OplogSlot {
-    OplogSlot() {}
-    OplogSlot(repl::OpTime opTime, std::int64_t hash) : opTime(opTime), hash(hash) {}
-    repl::OpTime opTime;
-    std::int64_t hash = 0;
-};
+using OplogSlot = repl::OpTime;
 
 struct InsertStatement {
 public:
@@ -67,7 +62,7 @@ public:
     InsertStatement(StmtId statementId, BSONObj toInsert, OplogSlot os)
         : stmtId(statementId), oplogSlot(os), doc(toInsert) {}
     InsertStatement(BSONObj toInsert, Timestamp ts, long long term)
-        : oplogSlot(repl::OpTime(ts, term), 0), doc(toInsert) {}
+        : oplogSlot(repl::OpTime(ts, term)), doc(toInsert) {}
 
     StmtId stmtId = kUninitializedStmtId;
     OplogSlot oplogSlot;