SERVER-63867 Allow zero plan cache maximum size

(cherry picked from commit 6f38fe9484d8ac4814416962a7584e5e5353ed76)

2 files changed, 15 insertions, 2 deletions

diff --git a/src/mongo/db/query/plan_cache.h b/src/mongo/db/query/plan_cache.h
index 889cd5dc1dd..5d92735f117 100644
--- a/src/mongo/db/query/plan_cache.h
+++ b/src/mongo/db/query/plan_cache.h
@@ -282,8 +282,7 @@ public:
     explicit PlanCacheBase(size_t cacheSize, size_t numPartitions = 1)
         : _numPartitions(numPartitions) {
         invariant(numPartitions > 0);
-        invariant(cacheSize / numPartitions > 0);
-        auto lru = Lru(cacheSize / numPartitions);
+        Lru lru{cacheSize / numPartitions};
         _partitionedCache = std::make_unique<Partitioned<Lru, Partitioner>>(numPartitions, lru);
     }
 
diff --git a/src/mongo/db/query/plan_cache_test.cpp b/src/mongo/db/query/plan_cache_test.cpp
index 72efd3f6e67..5e0bfc55806 100644
--- a/src/mongo/db/query/plan_cache_test.cpp
+++ b/src/mongo/db/query/plan_cache_test.cpp
@@ -2080,4 +2080,18 @@ TEST(PlanCacheTest, PlanCacheSizeWithMultiplePlanCaches) {
     ASSERT_EQ(planCacheTotalSizeEstimateBytes.get(), originalSize);
 }
 
+TEST(PlanCacheTest, PlanCacheMaxSizeParameterCanBeZero) {
+    PlanCache planCache{0U};
+    unique_ptr<CanonicalQuery> query(canonicalize("{a: 1, c: 1}"));
+    auto qs = getQuerySolutionForCaching();
+    auto decision = createDecision(1U);
+    auto decisionPtr = decision.get();
+
+    ASSERT_OK(planCache.set(makeKey(*query),
+                            qs->cacheData->clone(),
+                            *decisionPtr,
+                            Date_t{},
+                            plan_cache_util::buildDebugInfo(*query, std::move(decision))));
+    ASSERT_EQ(0U, planCache.size());
+}
 }  // namespace