From 5fd3e8b800b43c25d92530fc1b3e0b9bbec7bf6b Mon Sep 17 00:00:00 2001
From: Anna Wawrzyniak <anna.wawrzyniak@mongodb.com>
Date: Wed, 12 Jan 2022 18:42:57 +0000
Subject: SERVER-59960 Make ContextCtx::root optional

---
 src/mongo/db/exec/sbe/expressions/expression.cpp | 4 +++-
 src/mongo/db/exec/sbe/expressions/expression.h   | 6 ++++++
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/mongo/db/exec/sbe/expressions/expression.cpp b/src/mongo/db/exec/sbe/expressions/expression.cpp
index 2072f793492..4f9ad6ce653 100644
--- a/src/mongo/db/exec/sbe/expressions/expression.cpp
+++ b/src/mongo/db/exec/sbe/expressions/expression.cpp
@@ -107,7 +107,9 @@ vm::CodeFragment EVariable::compileDirect(CompileCtx& ctx) const {
         int offset = -_var - 1;
         code.appendLocalVal(*_frameId, offset, _moveFrom);
     } else {
-        auto accessor = ctx.root->getAccessor(ctx, _var);
+        // ctx.root is optional. If root stage is not specified, then resolve the variable using
+        // default context rules.
+        auto accessor = ctx.root ? ctx.root->getAccessor(ctx, _var) : ctx.getAccessor(_var);
         if (_moveFrom) {
             code.appendMoveVal(accessor);
         } else {
diff --git a/src/mongo/db/exec/sbe/expressions/expression.h b/src/mongo/db/exec/sbe/expressions/expression.h
index 71ee0251d8a..29055c7bc96 100644
--- a/src/mongo/db/exec/sbe/expressions/expression.h
+++ b/src/mongo/db/exec/sbe/expressions/expression.h
@@ -237,7 +237,13 @@ struct CompileCtx {
     CompileCtx makeCopyForParallelUse();
     CompileCtx makeCopy() const;
 
+    /**
+     * Root plan stage is used to resolve slot accessors introduced by PlanStage (optional).
+     * - if specified, the root plan stage will be used to resolve the slot accessor.
+     * - otherwise, if null, default context accessor resolution rules will be used.
+     */
     PlanStage* root{nullptr};
+
     value::SlotAccessor* accumulator{nullptr};
     std::vector<std::pair<value::SlotId, value::SlotAccessor*>> correlated;
     stdx::unordered_map<SpoolId, std::shared_ptr<SpoolBuffer>> spoolBuffers;
-- 
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