1 files changed, 68 insertions, 33 deletions

diff --git a/src/mongo/db/pipeline/document_source.cpp b/src/mongo/db/pipeline/document_source.cpp
index 55d400af60b..ea1cf479480 100644
--- a/src/mongo/db/pipeline/document_source.cpp
+++ b/src/mongo/db/pipeline/document_source.cpp
@@ -36,6 +36,7 @@
 #include "mongo/db/commands/feature_compatibility_version_documentation.h"
 #include "mongo/db/exec/document_value/value.h"
 #include "mongo/db/matcher/expression_algo.h"
+#include "mongo/db/pipeline/change_stream_constants.h"
 #include "mongo/db/pipeline/document_source_add_fields.h"
 #include "mongo/db/pipeline/document_source_group.h"
 #include "mongo/db/pipeline/document_source_internal_shard_filter.h"
@@ -171,45 +172,77 @@ bool groupMatchSwapVerified(const DocumentSourceMatch& nextMatch,
     return !expression::hasExistencePredicateOnPath(*(nextMatch.getMatchExpression()), "_id"_sd);
 }
 
+/**
+ * Returns 'true' if the given stage is an internal change stream stage that can appear in a router
+ * (mongoS) pipeline, or 'false' otherwise.
+ */
+bool isChangeStreamRouterPipelineStage(StringData stageName) {
+    return change_stream_constants::kChangeStreamRouterPipelineStages.contains(stageName);
+}
 }  // namespace
 
 bool DocumentSource::pushMatchBefore(Pipeline::SourceContainer::iterator itr,
                                      Pipeline::SourceContainer* container) {
-    auto nextMatch = dynamic_cast<DocumentSourceMatch*>((*std::next(itr)).get());
-    auto thisGroup = dynamic_cast<DocumentSourceGroup*>(this);
-    if (constraints().canSwapWithMatch && nextMatch && !nextMatch->isTextQuery() &&
-        (!thisGroup || groupMatchSwapVerified(*nextMatch, *thisGroup))) {
-        // We're allowed to swap with a $match and the stage after us is a $match. Furthermore, the
-        // $match does not contain a text search predicate, which we do not attempt to optimize
-        // because such a $match must already be the first stage in the pipeline. We can attempt to
-        // swap the $match or part of the $match before ourselves.
-        auto splitMatch =
-            DocumentSourceMatch::splitMatchByModifiedFields(nextMatch, getModifiedPaths());
-        invariant(splitMatch.first || splitMatch.second);
-
-        if (splitMatch.first) {
-            // At least part of the $match can be moved before this stage. Erase the original $match
-            // and put the independent part before this stage. If splitMatch.second is not null,
-            // then there is a new $match stage to insert after ourselves which is dependent on the
-            // modified fields.
-            LOGV2_DEBUG(
-                5943503,
+    if (!constraints().canSwapWithMatch) {
+        return false;
+    }
+
+    auto nextStageAsMatch = dynamic_cast<DocumentSourceMatch*>((*std::next(itr)).get());
+    if (!nextStageAsMatch || nextStageAsMatch->isTextQuery()) {
+        // We do not need to attempt this optimization if the $match contains a text search
+        // predicate because, in that scenario, $match is already required to be the first stage in
+        // the pipeline.
+        return false;
+    }
+
+    // At this point:
+    // 1) The next stage after 'this' is $match.
+    // 2) The $match stage does not contain a text search predicate.
+
+    // TODO SERVER-55492: Remove the following workaround when there are rename checks for 'other'
+    // match expressions.
+    if (isChangeStreamRouterPipelineStage(this->getSourceName())) {
+        // Always move the $match stage ahead of internal change stream stages appearing in the
+        // router (mongoS) pipeline, because they do not access or modify any paths in the input
+        // document.
+        container->splice(itr, *container, std::next(itr));
+        return true;
+    }
+
+    auto thisStageAsGroup = dynamic_cast<DocumentSourceGroup*>(this);
+    if (thisStageAsGroup && !groupMatchSwapVerified(*nextStageAsMatch, *thisStageAsGroup)) {
+        return false;
+    }
+
+    auto [renameableMatchPart, nonRenameableMatchPart] =
+        DocumentSourceMatch::splitMatchByModifiedFields(nextStageAsMatch, getModifiedPaths());
+    invariant(renameableMatchPart || nonRenameableMatchPart);
+    if (!renameableMatchPart) {
+        return false;
+    }
+
+    LOGV2_DEBUG(5943503,
                 5,
                 "Swapping all or part of a $match stage in front of another stage: ",
-                "matchMovingBefore"_attr = redact(splitMatch.first->serializeToBSONForDebug()),
+                "matchMovingBefore"_attr = redact(renameableMatchPart->serializeToBSONForDebug()),
                 "thisStage"_attr = redact(serializeToBSONForDebug()),
                 "matchLeftAfter"_attr = redact(
-                    splitMatch.second ? splitMatch.second->serializeToBSONForDebug() : BSONObj()));
-            container->erase(std::next(itr));
-            container->insert(itr, std::move(splitMatch.first));
-            if (splitMatch.second) {
-                container->insert(std::next(itr), std::move(splitMatch.second));
-            }
-
-            return true;
-        }
+                    nonRenameableMatchPart ? nonRenameableMatchPart->serializeToBSONForDebug()
+                                           : BSONObj()));
+
+    // At this point we know that at least part of the $match expression can be moved ahead of
+    // 'this'. So, we erase the original $match and move that renameable part ahead of 'this' stage.
+    container->erase(std::next(itr));
+    container->insert(itr, std::move(renameableMatchPart));
+
+    // If 'nonRenameableMatchPart' is not null, the 'renameableMatchPart' of the $match expression
+    // was only one component of the original $match. So, we need to create a new $match stage for
+    // the remaining 'nonRenameableMatchPart' and insert it after 'this' - effectively keeping it in
+    // its original position in the pipeline.
+    if (nonRenameableMatchPart) {
+        container->insert(std::next(itr), std::move(nonRenameableMatchPart));
     }
-    return false;
+    return true;
 }
 
 bool DocumentSource::pushSampleBefore(Pipeline::SourceContainer::iterator itr,
@@ -227,7 +260,9 @@ bool DocumentSource::pushSampleBefore(Pipeline::SourceContainer::iterator itr,
 
 BSONObj DocumentSource::serializeToBSONForDebug() const {
     std::vector<Value> serialized;
-    serializeToArray(serialized, ExplainOptions::Verbosity::kQueryPlanner);
+    auto opts =
+        SerializationOptions{boost::make_optional(ExplainOptions::Verbosity::kQueryPlanner)};
+    serializeToArray(serialized, opts);
     if (serialized.empty()) {
         LOGV2_DEBUG(5943501,
                     5,
@@ -278,8 +313,8 @@ Pipeline::SourceContainer::iterator DocumentSource::optimizeAt(
 }
 
 void DocumentSource::serializeToArray(vector<Value>& array,
-                                      boost::optional<ExplainOptions::Verbosity> explain) const {
-    Value entry = serialize(explain);
+                                      const SerializationOptions& opts) const {
+    Value entry = serialize(opts);
     if (!entry.missing()) {
         array.push_back(entry);
     }