SERVER-40907 Fix pipeline_metadata_tree to handle empty pipelines

2 files changed, 33 insertions, 13 deletions

diff --git a/src/mongo/db/pipeline/pipeline_metadata_tree.h b/src/mongo/db/pipeline/pipeline_metadata_tree.h
index 212ff516a52..33c8ea619dc 100644
--- a/src/mongo/db/pipeline/pipeline_metadata_tree.h
+++ b/src/mongo/db/pipeline/pipeline_metadata_tree.h
@@ -258,10 +258,16 @@ inline void walk(Stage<T>* stage,
  * calling the 'propagator' function on the last source.
  */
 template <typename T>
-inline std::pair<Stage<T>, T> makeTree(
+inline std::pair<boost::optional<Stage<T>>, T> makeTree(
     std::deque<T>&& initialStageContents,
     const Pipeline& pipeline,
     const std::function<T(const T&, const std::vector<T>&, const DocumentSource&)>& propagator) {
+    // For empty pipelines, there's no Stage<T> to return and the output schema is the same as the
+    // input schema.
+    if (pipeline.getSources().empty()) {
+        return std::pair(boost::none, initialStageContents.front());
+    }
+
     auto && [ finalStage, reshaper ] =
         detail::makeTreeWithOffTheEndStage(std::move(initialStageContents), pipeline, propagator);
 
diff --git a/src/mongo/db/pipeline/pipeline_metadata_tree_test.cpp b/src/mongo/db/pipeline/pipeline_metadata_tree_test.cpp
index 4c405379c20..a0d85705579 100644
--- a/src/mongo/db/pipeline/pipeline_metadata_tree_test.cpp
+++ b/src/mongo/db/pipeline/pipeline_metadata_tree_test.cpp
@@ -127,15 +127,15 @@ TEST_F(PipelineMetadataTreeTest, LinearPipelinesConstructProperTrees) {
 
     ASSERT([&]() {
         auto pipePtr = jsonToPipeline("[{$project: {name : 1}}]");
-        return makeTree<TestThing>({initial}, *pipePtr, ignoreDocumentSourceAddOne).first;
-    }() == Stage(TestThing{23}, {}, {}));
+        return makeTree<TestThing>({initial}, *pipePtr, ignoreDocumentSourceAddOne);
+    }().first.get() == Stage(TestThing{23}, {}, {}));
 
     ASSERT([&]() {
         auto pipePtr = jsonToPipeline(
             "[{$project: {name: 1, status: 1}}, "
             "{$match: {status: \"completed\"}}]");
-        return makeTree<TestThing>({initial}, *pipePtr, ignoreDocumentSourceAddOne).first;
-    }() == Stage(TestThing{24}, makeUniqueStage(TestThing{23}, {}, {}), {}));
+        return makeTree<TestThing>({initial}, *pipePtr, ignoreDocumentSourceAddOne);
+    }().first.get() == Stage(TestThing{24}, makeUniqueStage(TestThing{23}, {}, {}), {}));
 
     ASSERT([&]() {
         auto pipePtr = jsonToPipeline(
@@ -145,8 +145,9 @@ TEST_F(PipelineMetadataTreeTest, LinearPipelinesConstructProperTrees) {
             "{$match: {status: \"completed\"}}, "
             "{$match: {status: \"completed\"}}, "
             "{$match: {status: \"completed\"}}]");
-        return makeTree<TestThing>({initial}, *pipePtr, ignoreDocumentSourceAddOne).first;
-    }() == Stage(TestThing{28},
+        return makeTree<TestThing>({initial}, *pipePtr, ignoreDocumentSourceAddOne);
+    }().first.get() ==
+           Stage(TestThing{28},
                  makeUniqueStage(
                      TestThing{27},
                      makeUniqueStage(
@@ -237,8 +238,9 @@ TEST_F(PipelineMetadataTreeTest, BranchingPipelinesConstructProperTrees) {
             "{$unwind: \"$instr\"}, "
             "{$group: {_id: {PositionID: \"$trade.mvtident\", \"InstrumentReference\": "
             "\"$instr.libelle\"}, NumberOfSecurities: {$sum:\"$trade.quantite\"}}}]");
-        return makeTree<TestThing>({{"1"}, {"2"}}, *pipePtr, buildRepresentativeString).first;
-    }() == Stage(TestThing{"1mpxul[2m]ulu"},
+        return makeTree<TestThing>({{"1"}, {"2"}}, *pipePtr, buildRepresentativeString);
+    }().first.get() ==
+           Stage(TestThing{"1mpxul[2m]ulu"},
                  makeUniqueStage(
                      TestThing{"1mpxul[2m]ul"},
                      makeUniqueStage(
@@ -271,8 +273,9 @@ TEST_F(PipelineMetadataTreeTest, BranchingPipelinesConstructProperTrees) {
             "{$bucket: {groupBy: \"$year\", boundaries: [ 2000, 2010, 2015, 2020]}}], "
             "\"categorizedByYears(Auto)\": [{$bucketAuto: {groupBy: \"$year\", buckets: 2}}]}}, "
             "{$limit: 12}]");
-        return makeTree<TestThing>({{""}}, *pipePtr, buildRepresentativeString).first;
-    }() == Stage(TestThing{"f[tugs, tmgs, tb]"},
+        return makeTree<TestThing>({{""}}, *pipePtr, buildRepresentativeString);
+    }().first.get() ==
+           Stage(TestThing{"f[tugs, tmgs, tb]"},
                  makeUniqueStage(
                      TestThing{""},
                      {},
@@ -344,7 +347,7 @@ TEST_F(PipelineMetadataTreeTest, ZipWalksAPipelineAndTreeInTandemAndInOrder) {
             "{$group: {_id: {PositionID: \"$trade.mvtident\", \"InstrumentReference\": "
             "\"$instr.libelle\"}, NumberOfSecurities: {$sum:\"$trade.quantite\"}}}]");
         auto tree = makeTree<TestThing>({{}, {}}, *pipePtr, takeTypeInfo).first;
-        zip<TestThing>(&tree, &*pipePtr, tookTypeInfoOrThrow);
+        zip<TestThing>(&tree.get(), &*pipePtr, tookTypeInfoOrThrow);
         previousStack.pop();
     }());
 
@@ -358,10 +361,21 @@ TEST_F(PipelineMetadataTreeTest, ZipWalksAPipelineAndTreeInTandemAndInOrder) {
             "\"categorizedByYears(Auto)\": [{$bucketAuto: {groupBy: \"$year\", buckets: 2}}]}}, "
             "{$limit: 12}]");
         auto tree = makeTree<TestThing>({{}, {}}, *pipePtr, takeTypeInfo).first;
-        zip<TestThing>(&tree, &*pipePtr, tookTypeInfoOrThrow);
+        zip<TestThing>(&tree.get(), &*pipePtr, tookTypeInfoOrThrow);
         previousStack.pop();
     }());
 }
 
+TEST_F(PipelineMetadataTreeTest, MakeTreeWithEmptyPipeline) {
+    auto pipeline = uassertStatusOK(Pipeline::parse({}, getExpCtx()));
+    auto result = makeTree<std::string>({std::string("input")},
+                                        *pipeline,
+                                        [](const auto&, const auto&, const DocumentSource& source) {
+                                            return std::string("not called");
+                                        });
+    ASSERT_FALSE(result.first);
+    ASSERT_EQ(result.second, "input"_sd);
+}
+
 }  // namespace
 }  // namespace mongo