SERVER-64102 Ensure that unpacking measurements doesn't overwrite pushedown addFields that are computed on meta data

(cherry picked from commit 3a8cf3d2d1c6f668607756ab0b972f9ca2148f18)

3 files changed, 72 insertions, 0 deletions

diff --git a/jstests/core/timeseries/timeseries_project.js b/jstests/core/timeseries/timeseries_project.js
index 35c38da7426..9375f06240d 100644
--- a/jstests/core/timeseries/timeseries_project.js
+++ b/jstests/core/timeseries/timeseries_project.js
@@ -81,4 +81,36 @@ assert.docEq(result, [{_id: 0, time: docDate, a: {b: 1}, b: 4, c: [{}, {}]}]);
 // Test that an exclude does not overwrite meta field pushdown.
 result = coll.aggregate([{$unset: "b"}, {$set: {b: "$meta"}}]).toArray();
 assert.docEq(result, [{_id: 0, time: docDate, meta: 4, a: {b: 1}, b: 4, c: [{}, {}]}]);
+
+// Test that a field reference in a projection refers to the stage's input document
+// rather than another field with the same name in the projection.
+(function() {
+const regColl = db.timeseries_project_reg;
+regColl.drop();
+
+const tsColl = db.timeseries_project_ts;
+tsColl.drop();
+assert.commandWorked(
+    db.createCollection(tsColl.getName(), {timeseries: {timeField: 'time', metaField: 'x'}}));
+
+const doc = {
+    time: new Date("2019-10-11T14:39:18.670Z"),
+    x: 5,
+    a: 3,
+};
+assert.commandWorked(tsColl.insert(doc));
+assert.commandWorked(regColl.insert(doc));
+
+// Test $project.
+let pipeline = [{$project: {_id: 0, a: "$x", b: "$a"}}];
+let tsDoc = tsColl.aggregate(pipeline).toArray();
+let regDoc = regColl.aggregate(pipeline).toArray();
+assert.docEq(tsDoc, regDoc);
+
+// Test $addFields.
+pipeline = [{$addFields: {a: "$x", b: "$a"}}, {$project: {_id: 0}}];
+tsDoc = tsColl.aggregate(pipeline).toArray();
+regDoc = regColl.aggregate(pipeline).toArray();
+assert.docEq(tsDoc, regDoc);
+})();
 })();
diff --git a/src/mongo/db/exec/inclusion_projection_executor.cpp b/src/mongo/db/exec/inclusion_projection_executor.cpp
index 3e7bb26927b..d7384be8109 100644
--- a/src/mongo/db/exec/inclusion_projection_executor.cpp
+++ b/src/mongo/db/exec/inclusion_projection_executor.cpp
@@ -108,6 +108,10 @@ std::pair<BSONObj, bool> InclusionNode::extractComputedProjectionsInProject(
     if (_policies.computedFieldsPolicy != ComputedFieldsPolicy::kAllowComputedFields) {
         return {BSONObj{}, false};
     }
+
+    DepsTracker allDeps;
+    reportDependencies(&allDeps);
+
     // Auxiliary vector with extracted computed projections: <name, expression, replacement
     // strategy>. If the replacement strategy flag is true, the expression is replaced with a
     // projected field. If it is false - the expression is replaced with an identity projection.
@@ -120,6 +124,13 @@ std::pair<BSONObj, bool> InclusionNode::extractComputedProjectionsInProject(
             replaceWithProjField = false;
             continue;
         }
+        if (allDeps.fields.count(field) > 0) {
+            // Do not extract a computed projection if its name is the same as a dependent field. If
+            // the extracted $addFields were to be placed before this projection, the dependency
+            // with the common name would be shadowed by the computed projection.
+            replaceWithProjField = false;
+            continue;
+        }
         auto expressionIt = _expressions.find(field);
         if (expressionIt == _expressions.end()) {
             // After seeing the first dotted path expression we need to replace computed
@@ -185,6 +196,10 @@ std::pair<BSONObj, bool> InclusionNode::extractComputedProjectionsInAddFields(
     if (_policies.computedFieldsPolicy != ComputedFieldsPolicy::kAllowComputedFields) {
         return {BSONObj{}, false};
     }
+
+    DepsTracker allDeps;
+    reportDependencies(&allDeps);
+
     // Auxiliary vector with extracted computed projections: <name, expression>.
     // To preserve the original fields order, only projections at the beginning of the
     // _orderToProcessAdditionsAndChildren list can be extracted for pushdown.
@@ -194,6 +209,12 @@ std::pair<BSONObj, bool> InclusionNode::extractComputedProjectionsInAddFields(
         if (reservedNames.count(field) > 0) {
             break;
         }
+        if (allDeps.fields.count(field) > 0) {
+            // Do not extract a computed projection if its name is the same as a dependent field. If
+            // the extracted $addFields were to be placed before this $addFields, the dependency
+            // with the common name would be shadowed by the computed projection.
+            break;
+        }
         auto expressionIt = _expressions.find(field);
         if (expressionIt == _expressions.end()) {
             break;
diff --git a/src/mongo/db/exec/inclusion_projection_executor_test.cpp b/src/mongo/db/exec/inclusion_projection_executor_test.cpp
index a2d6932a87c..84ed31a9bb4 100644
--- a/src/mongo/db/exec/inclusion_projection_executor_test.cpp
+++ b/src/mongo/db/exec/inclusion_projection_executor_test.cpp
@@ -1071,6 +1071,25 @@ TEST_F(InclusionProjectionExecutionTestWithFallBackToDefault, ExtractComputedPro
     ASSERT_DOCUMENT_EQ(expectedProjection, inclusion->serializeTransformation(boost::none));
 }
 
+TEST_F(InclusionProjectionExecutionTestWithFallBackToDefault,
+       ExtractComputedProjectionInProjectShouldNotHideDependentFields) {
+    auto inclusion = makeInclusionProjectionWithDefaultPolicies(BSON("a"
+                                                                     << "$myMeta"
+                                                                     << "b"
+                                                                     << "$a"));
+
+    auto r = static_cast<InclusionProjectionExecutor*>(inclusion.get())->getRoot();
+    const std::set<StringData> reservedNames{};
+    auto [addFields, deleteFlag] =
+        r->extractComputedProjectionsInProject("myMeta", "meta", reservedNames);
+
+    ASSERT_EQ(addFields.nFields(), 0);
+    ASSERT_EQ(deleteFlag, false);
+
+    auto expectedProjection = Document(fromjson("{_id: true, a: '$myMeta', b: '$a'}"));
+    ASSERT_DOCUMENT_EQ(expectedProjection, inclusion->serializeTransformation(boost::none));
+}
+
 TEST_F(InclusionProjectionExecutionTestWithFallBackToDefault, ApplyProjectionAfterSplit) {
     auto inclusion = makeInclusionProjectionWithDefaultPolicies(
         BSON("a" << true << "computedMeta1"