SERVER-42177 Implement cheap metadata transfer between Document and WorkingSetMember.

Author: dstorch

src/mongo/db/exec/working_set.h

@@ -285,6 +285,23 @@ class WorkingSetMember {
         return _metadata;
     }
 
+    /**
+     * Clears all metadata fields inside this WorkingSetMember, and returns a structure containing
+     * that extracted metadata to the caller. The metadata can then be attached to a new
+     * WorkingSetMember or to another data structure that houses metadata.
+     */
+    DocumentMetadataFields releaseMetadata() {
+        return std::move(_metadata);
+    }
+
+    /**
+     * Transfers metadata fields to this working set member. By pairs of calls to releaseMetadata()
+     * and setMetadata(), callers can cheaply transfer metadata between WorkingSetMembers.
+     */
+    void setMetadata(DocumentMetadataFields&& metadata) {
+        _metadata = std::move(metadata);
+    }
+
 private:
     friend class WorkingSet;
 

src/mongo/db/exec/working_set_test.cpp

@@ -27,21 +27,17 @@
  *    it in the license file.
  */
 
-/**
- * This file contains tests for mongo/db/exec/working_set.cpp
- */
-
+#include "mongo/platform/basic.h"
 
 #include "mongo/db/exec/working_set.h"
 #include "mongo/db/jsobj.h"
 #include "mongo/db/json.h"
+#include "mongo/db/pipeline/document.h"
 #include "mongo/db/storage/snapshot.h"
 #include "mongo/unittest/unittest.h"
 #include "mongo/util/assert_util.h"
 
-using namespace mongo;
-
-namespace {
+namespace mongo {
 
 using std::string;
 
@@ -156,4 +152,42 @@ TEST_F(WorkingSetFixture, getDottedFieldFromIndex) {
     ASSERT_FALSE(member->getFieldDotted("y", &elt));
 }
 
-}  // namespace
+TEST_F(WorkingSetFixture, MetadataCanBeCorrectlyTransferredBackAndForthFromDocument) {
+    // Add some metadata to the WSM.
+    member->metadata().setTextScore(1.2);
+    member->metadata().setSearchScore(3.4);
+
+    // Test that the metadata can be extracted from the WSM.
+    auto releasedMetadata = member->releaseMetadata();
+    ASSERT_FALSE(member->metadata());
+    ASSERT_FALSE(member->metadata().hasTextScore());
+    ASSERT_FALSE(member->metadata().hasSearchScore());
+    ASSERT_TRUE(releasedMetadata);
+    ASSERT_TRUE(releasedMetadata.hasTextScore());
+    ASSERT_TRUE(releasedMetadata.hasSearchScore());
+
+    // Test that the extracted metadata can be added to a Document.
+    Document document;
+    MutableDocument md{std::move(document)};
+    md.setMetadata(std::move(releasedMetadata));
+    document = md.freeze();
+    ASSERT_FALSE(releasedMetadata);
+    ASSERT_FALSE(releasedMetadata.hasTextScore());
+    ASSERT_FALSE(releasedMetadata.hasSearchScore());
+    ASSERT_TRUE(document.metadata());
+    ASSERT_TRUE(document.metadata().hasTextScore());
+    ASSERT_TRUE(document.metadata().hasSearchScore());
+
+    // Test that metadata can be transferred back to the WSM.
+    MutableDocument md2{std::move(document)};
+    member->setMetadata(md2.releaseMetadata());
+    document = md2.freeze();
+    ASSERT_FALSE(document.metadata());
+    ASSERT_FALSE(document.metadata().hasTextScore());
+    ASSERT_FALSE(document.metadata().hasSearchScore());
+    ASSERT_TRUE(member->metadata());
+    ASSERT_TRUE(member->metadata().hasTextScore());
+    ASSERT_TRUE(member->metadata().hasSearchScore());
+}
+
+}  // namespace mongo

src/mongo/db/pipeline/document.h

@@ -524,6 +524,23 @@ class MutableDocument {
         return storage().metadata();
     }
 
+    /**
+     * Clears all metadata fields inside this Document, and returns a structure containing that
+     * extracted metadata to the caller. The metadata can then be attached to a new Document or to
+     * another data structure that houses metadata.
+     */
+    DocumentMetadataFields releaseMetadata() {
+        return storage().releaseMetadata();
+    }
+
+    /**
+     * Transfers metadata fields to this Document. By pairs of calls to releaseMetadata() and
+     * setMetadata(), callers can cheaply transfer metadata between Documents.
+     */
+    void setMetadata(DocumentMetadataFields&& metadata) {
+        storage().setMetadata(std::move(metadata));
+    }
+
     /** Convert to a read-only document and release reference.
      *
      *  Call this to indicate that you are done with this Document and will

src/mongo/db/pipeline/document_internal.h

@@ -407,6 +407,14 @@ class DocumentStorage : public RefCountable {
         return _metadataFields;
     }
 
+    DocumentMetadataFields releaseMetadata() {
+        return std::move(_metadataFields);
+    }
+
+    void setMetadata(DocumentMetadataFields&& metadata) {
+        _metadataFields = std::move(metadata);
+    }
+
     static unsigned hashKey(StringData name) {
         // TODO consider FNV-1a once we have a better benchmark corpus
         unsigned out;