From 274f2fddd6a0f975a678e8a953e704f7be6dda14 Mon Sep 17 00:00:00 2001
From: Alya Berciu <alyacarina@gmail.com>
Date: Wed, 16 Mar 2022 12:11:37 +0000
Subject: SERVER-63141 Ensure stages without dependency tracking are not cached
 in $lookup

---
 jstests/aggregation/lookup_let_optimization.js     | 45 ++++++++++++++++++++++
 .../document_source_sequential_document_cache.cpp  |  6 +--
 2 files changed, 48 insertions(+), 3 deletions(-)
 create mode 100644 jstests/aggregation/lookup_let_optimization.js

diff --git a/jstests/aggregation/lookup_let_optimization.js b/jstests/aggregation/lookup_let_optimization.js
new file mode 100644
index 00000000000..53bd5b7f3e1
--- /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 arrayEq.
+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}) => {
+        assert(arrayEq(coll.aggregate(pipeline).toArray(), expected, true));
+    };
+
+    // 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 c3c423af77f..49c038f828c 100644
--- a/src/mongo/db/pipeline/document_source_sequential_document_cache.cpp
+++ b/src/mongo/db/pipeline/document_source_sequential_document_cache.cpp
@@ -106,9 +106,9 @@ 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) ==
+             DocumentSource::GetDepsReturn::NOT_SUPPORTED) ||
+            deps.hasVariableReferenceTo(varIDs)) {
             break;
         }
     }
-- 
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