From e95c1625acdaed28ddb9ebda5c293e307e8ee284 Mon Sep 17 00:00:00 2001
From: Ian Boros <ian.boros@mongodb.com>
Date: Wed, 1 Apr 2020 22:02:17 -0400
Subject: SERVER-47328 optimize MR by avoiding repeated creation of
 RuntimeConstants

---
 src/mongo/db/pipeline/expression_context.h |  4 ++--
 src/mongo/db/pipeline/variables.cpp        | 32 ++++++++++--------------------
 src/mongo/db/pipeline/variables.h          | 13 ++++++++----
 3 files changed, 21 insertions(+), 28 deletions(-)

(limited to 'src')

diff --git a/src/mongo/db/pipeline/expression_context.h b/src/mongo/db/pipeline/expression_context.h
index 96f9f538837..cbb0aab0c4b 100644
--- a/src/mongo/db/pipeline/expression_context.h
+++ b/src/mongo/db/pipeline/expression_context.h
@@ -257,7 +257,7 @@ public:
         _resolvedNamespaces = std::move(resolvedNamespaces);
     }
 
-    auto getRuntimeConstants() const {
+    const RuntimeConstants& getRuntimeConstants() const {
         return variables.getRuntimeConstants();
     }
 
@@ -273,7 +273,7 @@ public:
         uassert(31264,
                 "Cannot run server-side javascript without the javascript engine enabled",
                 getGlobalScriptEngine());
-        RuntimeConstants runtimeConstants = getRuntimeConstants();
+        const auto& runtimeConstants = getRuntimeConstants();
         const boost::optional<bool> isMapReduceCommand = runtimeConstants.getIsMapReduce();
         if (inMongos) {
             invariant(!forceLoadOfStoredProcedures);
diff --git a/src/mongo/db/pipeline/variables.cpp b/src/mongo/db/pipeline/variables.cpp
index 8a79c23f7ca..c76b12cabef 100644
--- a/src/mongo/db/pipeline/variables.cpp
+++ b/src/mongo/db/pipeline/variables.cpp
@@ -136,7 +136,7 @@ Value Variables::getValue(Id id, const Document& root) const {
                 return Value();
             case Variables::kNowId:
             case Variables::kClusterTimeId:
-                if (auto it = _runtimeConstants.find(id); it != _runtimeConstants.end()) {
+                if (auto it = _runtimeConstantsMap.find(id); it != _runtimeConstantsMap.end()) {
                     return it->second;
                 }
 
@@ -169,40 +169,28 @@ Document Variables::getDocument(Id id, const Document& root) const {
     return Document();
 }
 
-RuntimeConstants Variables::getRuntimeConstants() const {
-    RuntimeConstants constants;
-
-    if (auto it = _runtimeConstants.find(kNowId); it != _runtimeConstants.end()) {
-        constants.setLocalNow(it->second.getDate());
-    }
-    if (auto it = _runtimeConstants.find(kClusterTimeId); it != _runtimeConstants.end()) {
-        constants.setClusterTime(it->second.getTimestamp());
-    }
-    if (auto it = _runtimeConstants.find(kJsScopeId); it != _runtimeConstants.end()) {
-        constants.setJsScope(it->second.getDocument().toBson());
-    }
-    if (auto it = _runtimeConstants.find(kIsMapReduceId); it != _runtimeConstants.end()) {
-        constants.setIsMapReduce(it->second.getBool());
-    }
-
-    return constants;
+const RuntimeConstants& Variables::getRuntimeConstants() const {
+    invariant(_runtimeConstants);
+    return *_runtimeConstants;
 }
 
 void Variables::setRuntimeConstants(const RuntimeConstants& constants) {
-    _runtimeConstants[kNowId] = Value(constants.getLocalNow());
+    invariant(!_runtimeConstants);
+    _runtimeConstantsMap[kNowId] = Value(constants.getLocalNow());
     // We use a null Timestamp to indicate that the clusterTime is not available; this can happen if
     // the logical clock is not running. We do not use boost::optional because this would allow the
     // IDL to serialize a RuntimConstants without clusterTime, which should always be an error.
     if (!constants.getClusterTime().isNull()) {
-        _runtimeConstants[kClusterTimeId] = Value(constants.getClusterTime());
+        _runtimeConstantsMap[kClusterTimeId] = Value(constants.getClusterTime());
     }
 
     if (constants.getJsScope()) {
-        _runtimeConstants[kJsScopeId] = Value(constants.getJsScope().get());
+        _runtimeConstantsMap[kJsScopeId] = Value(constants.getJsScope().get());
     }
     if (constants.getIsMapReduce()) {
-        _runtimeConstants[kIsMapReduceId] = Value(constants.getIsMapReduce().get());
+        _runtimeConstantsMap[kIsMapReduceId] = Value(constants.getIsMapReduce().get());
     }
+    _runtimeConstants = constants;
 }
 
 void Variables::setDefaultRuntimeConstants(OperationContext* opCtx) {
diff --git a/src/mongo/db/pipeline/variables.h b/src/mongo/db/pipeline/variables.h
index b0035054c52..da9d6100518 100644
--- a/src/mongo/db/pipeline/variables.h
+++ b/src/mongo/db/pipeline/variables.h
@@ -136,12 +136,14 @@ public:
     }
 
     /**
-     * Serializes runtime constants. This is used to send the constants to shards.
+     * Return a reference to an object which represents the variables which are considered "runtime
+     * constants." It is a programming error to call this function without having called
+     * setRuntimeConstants().
      */
-    RuntimeConstants getRuntimeConstants() const;
+    const RuntimeConstants& getRuntimeConstants() const;
 
     /**
-     * Deserialize runtime constants.
+     * Set the runtime constants. It is a programming error to call this more than once.
      */
     void setRuntimeConstants(const RuntimeConstants& constants);
 
@@ -184,7 +186,10 @@ private:
 
     IdGenerator _idGenerator;
     stdx::unordered_map<Id, ValueAndState> _values;
-    stdx::unordered_map<Id, Value> _runtimeConstants;
+    stdx::unordered_map<Id, Value> _runtimeConstantsMap;
+
+    // Populated after construction. Should not be set more than once.
+    boost::optional<RuntimeConstants> _runtimeConstants;
 };
 
 /**
-- 
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