SERVER-57403 Serialize 'let' variables by wrapping with $literal

3 files changed, 41 insertions, 3 deletions

diff --git a/jstests/core/command_let_variables.js b/jstests/core/command_let_variables.js
index 208a0801530..6a3de698291 100644
--- a/jstests/core/command_let_variables.js
+++ b/jstests/core/command_let_variables.js
@@ -536,4 +536,11 @@ assert.between(0, result, 1);
     const deduped = [...new Set(values)];
     assert.eq(1, deduped.length, `Expected all identical values: ${deduped}`);
 }
+
+// Test that expressions wrapped with $literal are serialized correctly when run in sharded cluster
+// environments.
+result = coll.aggregate([{$match: {$expr: {$eq: ["$_id", 2]}}}, {$project: {a: "$$b"}}],
+                        {let : {b: {$literal: "$notAFieldPath"}}})
+             .toArray();
+assert.eq(result, [{_id: 2, a: "$notAFieldPath"}]);
 }());
diff --git a/jstests/noPassthroughWithMongod/command_let_variables_merge_only.js b/jstests/noPassthroughWithMongod/command_let_variables_merge_only.js
index b6d5ceddbbf..ab913e01888 100644
--- a/jstests/noPassthroughWithMongod/command_let_variables_merge_only.js
+++ b/jstests/noPassthroughWithMongod/command_let_variables_merge_only.js
@@ -1,7 +1,7 @@
 // Tests that the aggregate command can use command-level let variables with $merge. Note $merge
 // tests must be run in a noPassthrough suite so the other operators are exercised in
 // jstests/core/command_let_variables.js.
-// @tags: [assumes_against_mongod_not_mongos, requires_fcv46]
+// @tags: [requires_fcv46]
 
 (function() {
 "use strict";
@@ -125,4 +125,35 @@ assert.commandWorked(db.runCommand({
     let : {variable: "OUTER"}
 }));
 assert.eq(targetColl.aggregate({$match: {$expr: {$eq: ["$var", "INNER"]}}}).toArray().length, 1);
+
+// Test that expressions wrapped with $literal are serialized correctly in combination with
+// pipelines containing $merge.
+prepMergeTargetColl();
+assert.commandWorked(db.runCommand({
+    aggregate: coll.getName(),
+    pipeline: [{$project: {var : "$$variable"}}, {$merge: {into: targetColl.getName()}}],
+    let : {variable: {$literal: "$notAFieldPath"}},
+    cursor: {}
+}));
+assert.eq(targetColl.aggregate({$match: {$expr: {$eq: ["$var", {$literal: "$notAFieldPath"}]}}})
+              .toArray()
+              .length,
+          4);
+
+prepMergeTargetColl();
+assert.commandWorked(db.runCommand({
+    aggregate: coll.getName(),
+    pipeline: [{
+        $merge: {
+            into: targetColl.getName(),
+            let : {variable: {$literal: "$notAFieldPath"}},
+            whenMatched: [{$addFields: {"var": "$$variable"}}]
+        }
+    }],
+    cursor: {},
+}));
+assert.eq(targetColl.aggregate({$match: {$expr: {$eq: ["$var", {$literal: "$notAFieldPath"}]}}})
+              .toArray()
+              .length,
+          1);
 }());
diff --git a/src/mongo/db/pipeline/variables.cpp b/src/mongo/db/pipeline/variables.cpp
index 8cabf0f3c52..6adc0e280c1 100644
--- a/src/mongo/db/pipeline/variables.cpp
+++ b/src/mongo/db/pipeline/variables.cpp
@@ -350,7 +350,7 @@ BSONObj VariablesParseState::serialize(const Variables& vars) const {
     auto bob = BSONObjBuilder{};
     for (auto&& [var_name, id] : _variables)
         if (vars.hasValue(id))
-            bob << var_name << vars.getValue(id);
+            bob << var_name << Value(DOC("$literal" << vars.getValue(id)));
 
     // System variables have to be added separately since the variable IDs are reserved and not
     // allocated like normal variables, and so not present in '_variables'.
@@ -363,7 +363,7 @@ std::pair<LegacyRuntimeConstants, BSONObj> VariablesParseState::transitionalComp
     auto bob = BSONObjBuilder{};
     for (auto&& [var_name, id] : _variables)
         if (vars.hasValue(id))
-            bob << var_name << vars.getValue(id);
+            bob << var_name << Value(DOC("$literal" << vars.getValue(id)));
 
     return {vars.transitionalExtractRuntimeConstants(), bob.obj()};
 }