SERVER-10026 use server parameter for triggering query system

Author: Hari Khalsa

src/mongo/db/ops/query.cpp

@@ -43,11 +43,6 @@
 
 namespace mongo {
 
-    /**
-     * Set in db/query/new_find.cpp.  If true, use the new query system to handle queries.
-     */
-    extern bool useNewQuerySystem;
-
     /* We cut off further objects once we cross this threshold; thus, you might get
        a little bit more than this, it is a threshold rather than a limit.
     */
@@ -115,7 +110,7 @@ namespace mongo {
                                 int pass,
                                 bool& exhaust,
                                 bool* isCursorAuthorized ) {
-        if (useNewQuerySystem) {
+        if (isNewQueryFrameworkEnabled()) {
             return newGetMore(ns, ntoreturn, cursorid, curop, pass, exhaust, isCursorAuthorized);
         }
 
@@ -1065,7 +1060,7 @@ namespace mongo {
             uassert( 10110 , "bad query object", false);
         }
 
-        if (useNewQuerySystem) {
+        if (isNewQueryFrameworkEnabled()) {
             // TODO: Copy prequel curop debugging into runNewQuery
             return newRunQuery(m, q, curop, result);
         }

src/mongo/db/query/new_find.cpp

@@ -28,18 +28,21 @@
 #include "mongo/db/query/single_solution_runner.h"
 #include "mongo/db/query/stage_builder.h"
 #include "mongo/db/repl/repl_reads_ok.h"
+#include "mongo/db/server_parameters.h"
 #include "mongo/s/chunk_version.h"
 #include "mongo/s/d_logic.h"
 #include "mongo/s/stale_exception.h"
 
 namespace mongo {
 
+    // Server parameter
+    MONGO_EXPORT_SERVER_PARAMETER(newQueryFrameworkEnabled, bool, false);
+
+    bool isNewQueryFrameworkEnabled() { return newQueryFrameworkEnabled; }
+
     // Copied from db/ops/query.cpp.
     static const int32_t MaxBytesToReturnToClientAtOnce = 4 * 1024 * 1024;
 
-    // Used in db/ops/query.cpp.
-    bool useNewQuerySystem = false;
-
     /**
      * For a given query, get a runner.  The runner could be a SingleSolutionRunner, a
      * CachedQueryRunner, or a MultiPlanRunner, depending on the cache/query solver/etc.

src/mongo/db/query/new_find.h

@@ -24,9 +24,20 @@
 
 namespace mongo {
 
+    /**
+     * A switch to choose between old Cursor-based code and new Runner-based code.
+     */
+    bool isNewQueryFrameworkEnabled();
+
+    /**
+     * Called from the getMore entry point in ops/query.cpp.
+     */
     QueryResult* newGetMore(const char* ns, int ntoreturn, long long cursorid, CurOp& curop,
                             int pass, bool& exhaust, bool* isCursorAuthorized);
 
+    /**
+     * Called from the runQuery entry point in ops/query.cpp.
+     */
     string newRunQuery(Message& m, QueryMessage& q, CurOp& curop, Message &result);
 
 }  // namespace mongo