SERVER-60218 add initialize helper function for document_source_group

2 files changed, 14 insertions, 3 deletions

diff --git a/src/mongo/db/pipeline/document_source_group.cpp b/src/mongo/db/pipeline/document_source_group.cpp
index 2ab40378383..73f3a76d81d 100644
--- a/src/mongo/db/pipeline/document_source_group.cpp
+++ b/src/mongo/db/pipeline/document_source_group.cpp
@@ -549,11 +549,16 @@ private:
 }  // namespace
 
 DocumentSource::GetNextResult DocumentSourceGroup::initialize() {
-    const size_t numAccumulators = _accumulatedFields.size();
-
-    // Barring any pausing, this loop exhausts 'pSource' and populates '_groups'.
     GetNextResult input = pSource->getNext();
+    return initializeSelf(input);
+}
 
+// This separate NOINLINE function is used here to decrease stack utilization of initialize() and
+// prevent stack overflows.
+MONGO_COMPILER_NOINLINE DocumentSource::GetNextResult DocumentSourceGroup::initializeSelf(
+    GetNextResult input) {
+    const size_t numAccumulators = _accumulatedFields.size();
+    // Barring any pausing, this loop exhausts 'pSource' and populates '_groups'.
     for (; input.isAdvanced(); input = pSource->getNext()) {
         if (shouldSpillWithAttemptToSaveMemory()) {
             _sortedFiles.push_back(spill());
diff --git a/src/mongo/db/pipeline/document_source_group.h b/src/mongo/db/pipeline/document_source_group.h
index 1e2e1c1d3d4..c9f635af90e 100644
--- a/src/mongo/db/pipeline/document_source_group.h
+++ b/src/mongo/db/pipeline/document_source_group.h
@@ -223,6 +223,12 @@ private:
     GetNextResult initialize();
 
     /**
+     * Initializes this $group after any children are potentially initialized see initialize() for
+     * more details.
+     */
+    GetNextResult initializeSelf(GetNextResult input);
+
+    /**
      * Spill groups map to disk and returns an iterator to the file. Note: Since a sorted $group
      * does not exhaust the previous stage before returning, and thus does not maintain as large a
      * store of documents at any one time, only an unsorted group can spill to disk.