SERVER-61846 redact document using null instead of default mask to stay under BSON limit

(cherry picked from commit 886e09ce457d5cdbef16710678c386ec6765db3c)

2 files changed, 21 insertions, 8 deletions

diff --git a/src/mongo/bson/bsonobj.cpp b/src/mongo/bson/bsonobj.cpp
index 53c7f313289..33ce012139b 100644
--- a/src/mongo/bson/bsonobj.cpp
+++ b/src/mongo/bson/bsonobj.cpp
@@ -143,25 +143,38 @@ BSONObj BSONObj::redact() const {
 
     // Helper to get an "internal function" to be able to do recursion
     struct redactor {
-        void operator()(BSONObjBuilder& builder, const BSONObj& obj) {
+        void operator()(BSONObjBuilder& builder, const BSONObj& obj, bool appendMask) {
             for (BSONElement e : obj) {
                 if (e.type() == Object) {
                     BSONObjBuilder subBuilder = builder.subobjStart(e.fieldNameStringData());
-                    operator()(subBuilder, e.Obj());
+                    operator()(subBuilder, e.Obj(), appendMask);
                     subBuilder.done();
                 } else if (e.type() == Array) {
                     BSONObjBuilder subBuilder = builder.subarrayStart(e.fieldNameStringData());
-                    operator()(subBuilder, e.Obj());
+                    operator()(subBuilder, e.Obj(), appendMask);
                     subBuilder.done();
-                } else {
+                } else if (appendMask) {
                     builder.append(e.fieldNameStringData(), "###"_sd);
+                } else {
+                    builder.appendNull(e.fieldNameStringData());
                 }
             }
         }
     };
 
+    try {
+        BSONObjBuilder builder;
+        redactor()(builder, *this, /*appendMask=*/true);
+        return builder.obj();
+    } catch (const ExceptionFor<ErrorCodes::BSONObjectTooLarge>&) {
+    }
+
+    // For some BSONObj with lots of small fields, replacing each element's value with the default
+    // redaction mask "###" may cause us to exceed the maximum allowed BSON size. In this case,
+    // we use BSONType::jstNull, which ensures the redacted object will not be larger than the
+    // original.
     BSONObjBuilder builder;
-    redactor()(builder, *this);
+    redactor()(builder, *this, /*appendMask=*/false);
     return builder.obj();
 }
 
diff --git a/src/mongo/logv2/redaction_test.cpp b/src/mongo/logv2/redaction_test.cpp
index 8ab293822c2..bc22f60b4ae 100644
--- a/src/mongo/logv2/redaction_test.cpp
+++ b/src/mongo/logv2/redaction_test.cpp
@@ -155,7 +155,7 @@ TEST(RedactBSONTest, BSONWithArrays) {
     testBSONCases(testCases);
 }
 
-TEST(RedactBSONTest, RedactCausesBSONTooLarge) {
+TEST(RedactBSONTest, RedactedObjectShouldBeSmallerOrEqualInSizeToOriginal) {
     logv2::setShouldRedactLogs(true);
     BSONObjBuilder bob;
     for (int i = 0; i < 1024 * 1024; i++) {
@@ -164,8 +164,8 @@ TEST(RedactBSONTest, RedactCausesBSONTooLarge) {
         bob.append(fieldName, 1);
     }
     const auto obj = bob.obj();
-    // Demonstrates it is possible to grow a BSON too large by redacting fields.
-    ASSERT_THROWS_CODE(redact(obj), DBException, ErrorCodes::BSONObjectTooLarge);
+    const auto redactedObj = redact(obj);
+    ASSERT_LTE(redactedObj.objsize(), obj.objsize());
 }
 }  // namespace
 }  // namespace mongo