SERVER-62322 Allow an empty object to be projected in $setWindowFields

jstests/aggregation/sources/setWindowFields/parse.js

@@ -213,8 +213,7 @@ assert.commandWorked(run({
 }));
 
 // Not every accumulator is automatically a window function.
-
-var err = assert.commandFailedWithCode(run({$setWindowFields: {output: {a: {b: {$sum: "$a"}}}}}),
+let err = assert.commandFailedWithCode(run({$setWindowFields: {output: {a: {b: {$sum: "$a"}}}}}),
                                        ErrorCodes.FailedToParse);
 assert.includes(err.errmsg, 'Expected a $-prefixed window function, b');
 
@@ -250,4 +249,14 @@ err = assert.commandFailedWithCode(
     }),
     ErrorCodes.FailedToParse);
 assert.includes(err.errmsg, 'Unrecognized window function, $summ');
+
+// Test that an empty object is a valid projected field.
+assert.commandWorked(coll.insert({}));
+assert.commandWorked(run({$setWindowFields: {output: {v: {$max: {mergeObjects: {}}}}}}));
+
+// However conflicting field paths is always an error.
+err = assert.commandFailedWithCode(
+    run({$setWindowFields: {output: {a: {$sum: 1}, 'a.b': {$sum: 1}}}}), 40176);
+assert.includes(err.errmsg, 'specification contains two conflicting paths');
+
 })();

src/mongo/db/pipeline/document_source_set_window_fields.cpp

@@ -461,10 +461,12 @@ DocumentSource::GetNextResult DocumentSourceInternalSetWindowFields::doGetNext()
     MutableDocument addFieldsSpec;
     for (auto&& [fieldName, function] : _executableOutputs) {
         try {
+            // Wrap the projected value in a $literal since there are limitations on a user-facing
+            // $addFields that we don't want to enforce here (e.g. empty object).
+            addFieldsSpec.addField(fieldName, Value(DOC("$literal" << function->getNext())));
+        } catch (const DBException&) {
             // If we hit a uassert while evaluating expressions on user data, delete the temporary
             // table before aborting the operation.
-            addFieldsSpec.addField(fieldName, function->getNext());
-        } catch (const DBException&) {
             _iterator.finalize();
             throw;
         }
@@ -506,6 +508,7 @@ DocumentSource::GetNextResult DocumentSourceInternalSetWindowFields::doGetNext()
             _iterator.finalize();
             break;
     }
+
     auto projExec = projection_executor::AddFieldsProjectionExecutor::create(
         pExpCtx, addFieldsSpec.freeze().toBson());