SERVER-114196 Fix $unionWith system variables initialization (#44444)

GitOrigin-RevId: cacd16d51c46e07a2b1b427c155847a7856c0267

jstests/aggregation/sources/unionWith/unionWith_system_variables.js

@@ -0,0 +1,31 @@
+/**
+ * Test that $unionWith works when system variables are used after it and pushed in $unionWith
+ * during optimization.
+ * @tags: [
+ *   do_not_wrap_aggregations_in_facets,
+ *   assumes_unsharded_collection,
+ * ]
+ */
+const testDB = db.getSiblingDB(jsTestName());
+const coll = testDB.coll;
+coll.drop();
+assert.commandWorked(coll.insertOne({a: 1}));
+
+const date = new Date();
+const data =
+    coll.aggregate([{$unionWith: {coll: coll.getName(), pipeline: []}}, {$project: {now: "$$NOW"}}])
+        .toArray();
+
+assert.eq(data.length, 2);
+for (let doc of data) {
+    const diff = Math.abs(doc.now.getTime() - date.getTime());
+    assert.lte(
+        diff, 60000, "Expected $$NOW to be close to current time. Difference was " + diff + "ms");
+}
+
+assert.doesNotThrow(() => {
+    coll.explain("executionStats").aggregate([
+        {$unionWith: {coll: coll.getName(), pipeline: []}},
+        {$project: {now: "$$NOW"}},
+    ]);
+});

src/mongo/db/pipeline/document_source_union_with.cpp

@@ -324,6 +324,7 @@ DocumentSource::GetNextResult DocumentSourceUnionWith::doGetNext() {
             // context of the unionWith '_pipeline' as part of DocumentSourceUnionWith constructor.
             // Attach query settings to the '_pipeline->getContext()' by copying them from the
             // parent query ExpressionContext.
+            _pipeline->getContext()->initializeReferencedSystemVariables();
             _pipeline->getContext()->setQuerySettingsIfNotPresent(pExpCtx->getQuerySettings());
 
             LOGV2_DEBUG(9497002,
@@ -508,6 +509,7 @@ Value DocumentSourceUnionWith::serialize(const SerializationOptions& opts) const
             // NOTE: this is done here, as opposed to at the beginning of the serialize() method
             // because serialize() is called when generating query shape, however, at that
             // moment no query settings are present in the parent context.
+            _pipeline->getContext()->initializeReferencedSystemVariables();
             _pipeline->getContext()->setQuerySettingsIfNotPresent(pExpCtx->getQuerySettings());
 
             return pExpCtx->getMongoProcessInterface()->preparePipelineAndExplain(ownedPipeline,