From bbdf42ae03fdab670dbee8d5a154156aae5342a9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20G=C3=B3mez=20Ferro?=
 <daniel.gomezferro@mongodb.com>
Date: Tue, 15 Feb 2022 14:08:54 +0000
Subject: SERVER-63585 Allow negative values in WTRecordStore::numRecords

---
 .../storage/wiredtiger/wiredtiger_record_store.cpp | 10 ++---
 .../wiredtiger/wiredtiger_record_store_test.cpp    | 50 ++++++++++++++++++++++
 2 files changed, 54 insertions(+), 6 deletions(-)

(limited to 'src')

diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp b/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp
index 2f17cb04310..bb92b0c88c0 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp
@@ -1050,7 +1050,8 @@ long long WiredTigerRecordStore::dataSize(OperationContext* opCtx) const {
 }
 
 long long WiredTigerRecordStore::numRecords(OperationContext* opCtx) const {
-    return _sizeInfo->numRecords.load();
+    auto numRecords = _sizeInfo->numRecords.load();
+    return numRecords > 0 ? numRecords : 0;
 }
 
 int64_t WiredTigerRecordStore::storageSize(OperationContext* opCtx,
@@ -1966,12 +1967,9 @@ void WiredTigerRecordStore::_changeNumRecords(OperationContext* opCtx, int64_t d
     opCtx->recoveryUnit()->onRollback([this, diff]() {
         LOGV2_DEBUG(
             22404, 3, "WiredTigerRecordStore: rolling back NumRecordsChange", "diff"_attr = -diff);
-        if (_sizeInfo->numRecords.addAndFetch(-diff) < 0) {
-            _sizeInfo->numRecords.store(0);
-        }
+        _sizeInfo->numRecords.addAndFetch(-diff);
     });
-    if (_sizeInfo->numRecords.addAndFetch(diff) < 0)
-        _sizeInfo->numRecords.store(0);
+    _sizeInfo->numRecords.addAndFetch(diff);
 }
 
 void WiredTigerRecordStore::_increaseDataSize(OperationContext* opCtx, int64_t amount) {
diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_record_store_test.cpp b/src/mongo/db/storage/wiredtiger/wiredtiger_record_store_test.cpp
index 9d40f13b878..c02a8ca55c2 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_record_store_test.cpp
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_record_store_test.cpp
@@ -48,6 +48,7 @@
 #include "mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.h"
 #include "mongo/db/storage/wiredtiger/wiredtiger_size_storer.h"
 #include "mongo/db/storage/wiredtiger/wiredtiger_util.h"
+#include "mongo/unittest/barrier.h"
 #include "mongo/unittest/temp_dir.h"
 #include "mongo/unittest/unittest.h"
 #include "mongo/util/fail_point.h"
@@ -1141,5 +1142,54 @@ TEST(WiredTigerRecordStoreTest, ClusteredRecordStore) {
     ASSERT_EQ(0, memcmp(dataUpdated, rd.data(), strlen(dataUpdated)));
 }
 
+// Make sure numRecords is accurate after a delete rolls back and some other transaction deletes the
+// same rows before we have a chance of patching up the metadata.
+TEST(WiredTigerRecordStoreTest, NumRecordsAccurateAfterRollbackWithDelete) {
+    const auto harnessHelper(newRecordStoreHarnessHelper());
+    unique_ptr<RecordStore> rs(harnessHelper->newRecordStore());
+
+    RecordId rid;  // This record will be deleted by two transactions.
+
+    ServiceContext::UniqueOperationContext ctx(harnessHelper->newOperationContext());
+    {
+        WriteUnitOfWork uow(ctx.get());
+        rid = rs->insertRecord(ctx.get(), "a", 2, Timestamp()).getValue();
+        uow.commit();
+    }
+
+    ASSERT_EQ(1, rs->numRecords(ctx.get()));
+
+    WriteUnitOfWork uow(ctx.get());
+
+    auto aborted = std::make_shared<unittest::Barrier>(2);
+    auto deleted = std::make_shared<unittest::Barrier>(2);
+
+    // This thread will delete the record and then rollback. We'll block the roll back process after
+    // rolling back the WT transaction and before running the rest of the registered changes,
+    // allowing the main thread to delete the same rows again.
+    stdx::thread abortedThread([&harnessHelper, &rs, &rid, aborted, deleted]() {
+        auto client = harnessHelper->serviceContext()->makeClient("c1");
+        auto ctx = harnessHelper->newOperationContext(client.get());
+        WriteUnitOfWork txn(ctx.get());
+        // Registered changes are executed in reverse order.
+        rs->deleteRecord(ctx.get(), rid);
+        ctx.get()->recoveryUnit()->onRollback([&]() { deleted->countDownAndWait(); });
+        ctx.get()->recoveryUnit()->onRollback([&]() { aborted->countDownAndWait(); });
+    });
+
+    // Wait for the other thread to abort.
+    aborted->countDownAndWait();
+
+    rs->deleteRecord(ctx.get(), rid);
+
+    // Notify the other thread we have deleted, let it complete the rollback.
+    deleted->countDownAndWait();
+
+    uow.commit();
+
+    abortedThread.join();
+    ASSERT_EQ(0, rs->numRecords(ctx.get()));
+}
+
 }  // namespace
 }  // namespace mongo
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