From 1b475ef190b29cb62d9a205a05bde1d2ddb6332b Mon Sep 17 00:00:00 2001
From: Gregory Wlodarek <gregory.wlodarek@mongodb.com>
Date: Mon, 6 Feb 2023 22:51:09 +0000
Subject: SERVER-71353 Add diagnostics for when the durable catalog is scanned
 due to missing catalogId mapping

---
 src/mongo/db/catalog/collection_catalog.cpp | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/src/mongo/db/catalog/collection_catalog.cpp b/src/mongo/db/catalog/collection_catalog.cpp
index 40b45b404d8..ed75b200986 100644
--- a/src/mongo/db/catalog/collection_catalog.cpp
+++ b/src/mongo/db/catalog/collection_catalog.cpp
@@ -33,6 +33,7 @@
 
 #include "mongo/db/catalog/database.h"
 #include "mongo/db/catalog/uncommitted_catalog_updates.h"
+#include "mongo/db/commands/server_status.h"
 #include "mongo/db/concurrency/exception_util.h"
 #include "mongo/db/concurrency/lock_manager_defs.h"
 #include "mongo/db/concurrency/resource_catalog.h"
@@ -60,6 +61,8 @@ static RecordId kUnknownRangeMarkerId = RecordId::minLong();
 // reached we will fall back to more durable catalog scans.
 static constexpr int kMaxCatalogIdMappingLengthForMissingInsert = 1000;
 
+constexpr auto kNumDurableCatalogScansDueToMissingMapping = "numScansDueToMissingMapping"_sd;
+
 struct LatestCollectionCatalog {
     std::shared_ptr<CollectionCatalog> catalog = std::make_shared<CollectionCatalog>();
 };
@@ -100,6 +103,27 @@ const auto minUuid = UUID::parse("00000000-0000-0000-0000-000000000000").getValu
 
 }  // namespace
 
+/**
+ * Defines a new serverStatus section "collectionCatalog".
+ */
+class CollectionCatalogSection final : public ServerStatusSection {
+public:
+    CollectionCatalogSection() : ServerStatusSection("collectionCatalog") {}
+
+    bool includeByDefault() const override {
+        return true;
+    }
+
+    BSONObj generateSection(OperationContext* opCtx, const BSONElement&) const override {
+        BSONObjBuilder section;
+        section.append(kNumDurableCatalogScansDueToMissingMapping,
+                       numScansDueToMissingMapping.loadRelaxed());
+        return section.obj();
+    }
+
+    AtomicWord<long long> numScansDueToMissingMapping;
+} gCollectionCatalogSection;
+
 class IgnoreExternalViewChangesForDatabase {
 public:
     IgnoreExternalViewChangesForDatabase(OperationContext* opCtx, const DatabaseName& dbName)
@@ -1036,6 +1060,7 @@ boost::optional<DurableCatalogEntry> CollectionCatalog::_fetchPITCatalogEntry(
         invariant(readTimestamp);
 
         // Scan durable catalog when we don't have accurate catalogId mapping for this timestamp.
+        gCollectionCatalogSection.numScansDueToMissingMapping.fetchAndAdd(1);
         auto catalogEntry = DurableCatalog::get(opCtx)->scanForCatalogEntryByNss(opCtx, nss);
         writeCatalogIdAfterScan(catalogEntry);
         return catalogEntry;
-- 
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