SERVER-63141 Ensure stages without dependency tracking are not cached in $lookup

3 files changed, 50 insertions, 3 deletions

diff --git a/etc/backports_required_for_multiversion_tests.yml b/etc/backports_required_for_multiversion_tests.yml
index 0fa0e4b083f..c62d42aab5f 100644
--- a/etc/backports_required_for_multiversion_tests.yml
+++ b/etc/backports_required_for_multiversion_tests.yml
@@ -130,6 +130,9 @@ all:
     test_file: jstests/sharding/cancel_coordinate_txn_commit_with_tickets_exhausted.js
   - ticket: SERVER-62147
     test_file: jstests/core/exhaust.js
+  - ticket: SERVER-63141
+    test_file: jstests/aggregation/lookup_let_optimization.js
+
 
 # Tests that should only be excluded from particular suites should be listed under that suite.
 suites:
diff --git a/jstests/aggregation/lookup_let_optimization.js b/jstests/aggregation/lookup_let_optimization.js
new file mode 100644
index 00000000000..8431fb434c2
--- /dev/null
+++ b/jstests/aggregation/lookup_let_optimization.js
@@ -0,0 +1,45 @@
+/**
+ * This test ensures that stages dependent on a let variable optimizing to a constant in a $lookup
+ * pipeline are evaluated correctly.
+ * @tags: [assumes_unsharded_collection]
+ */
+
+load('jstests/aggregation/extras/utils.js');  // For assertArrayEq.
+load("jstests/libs/fixture_helpers.js");      // For FixtureHelpers.
+
+(function() {
+"use strict";
+
+const collName = "lookup_let_redact";
+const coll = db[collName];
+coll.drop();
+assert.commandWorked(coll.insert([
+    {_id: "true", test: true},
+    {_id: "false", test: false},
+]));
+
+const verifyAggregationResults = ({pipeline, expected}) => {
+    assertArrayEq({actual: coll.aggregate(pipeline).toArray(), expected});
+};
+
+// Verify $redact.
+verifyAggregationResults({
+    pipeline: [
+        {$lookup: {
+            from: collName,
+            let: {iShouldPrune: "$test"},
+            pipeline: [
+                {$redact: {$cond: {if: "$$iShouldPrune", then: "$$PRUNE", else: "$$DESCEND"}}}
+            ],
+            as: "redacted"
+        }}
+    ],
+    expected: [
+        {_id: "true", test: true, redacted: []}, // Expect that documents were pruned.
+        {_id: "false", test: false, redacted: [ // Expect that $redact descended instead.
+            {_id: "true", test: true},
+            {_id: "false", test: false} 
+        ]}
+    ]
+});
+}());
diff --git a/src/mongo/db/pipeline/document_source_sequential_document_cache.cpp b/src/mongo/db/pipeline/document_source_sequential_document_cache.cpp
index 15627744247..0fb88369117 100644
--- a/src/mongo/db/pipeline/document_source_sequential_document_cache.cpp
+++ b/src/mongo/db/pipeline/document_source_sequential_document_cache.cpp
@@ -112,9 +112,8 @@ Pipeline::SourceContainer::iterator DocumentSourceSequentialDocumentCache::doOpt
 
     // Iterate through the pipeline stages until we find one which references an external variable.
     for (; prefixSplit != container->end(); ++prefixSplit) {
-        (*prefixSplit)->getDependencies(&deps);
-
-        if (deps.hasVariableReferenceTo(varIDs)) {
+        if (((*prefixSplit)->getDependencies(&deps) == DepsTracker::State::NOT_SUPPORTED) ||
+            deps.hasVariableReferenceTo(varIDs)) {
             break;
         }
     }