From ef74db9b48a13e1f902ce0eea3c0b5a24ba45ecc Mon Sep 17 00:00:00 2001
From: Martin Neupauer <martin.neupauer@mongodb.com>
Date: Fri, 1 Mar 2019 12:28:39 -0500
Subject: SERVER-39903 notablescan parameter should be ignored for internally
 used namespaces

(cherry picked from commit 18593fe22c55b7a51c44f04a39d30573b0b01873)
---
 jstests/noPassthrough/ignore_notablescan.js | 36 +++++++++++++++++++++++++++++
 src/mongo/db/query/get_executor.cpp         |  6 ++---
 2 files changed, 39 insertions(+), 3 deletions(-)
 create mode 100644 jstests/noPassthrough/ignore_notablescan.js

diff --git a/jstests/noPassthrough/ignore_notablescan.js b/jstests/noPassthrough/ignore_notablescan.js
new file mode 100644
index 00000000000..9b7b0f8d379
--- /dev/null
+++ b/jstests/noPassthrough/ignore_notablescan.js
@@ -0,0 +1,36 @@
+// Test that 'notablescan' parameter does not affect queries internal namespaces.
+// @tags: [uses_transactions]
+(function() {
+    "use strict";
+
+    const dbName = "test";
+    const collName = "coll";
+
+    const rst = new ReplSetTest({nodes: 1, nodeOptions: {setParameter: {notablescan: true}}});
+    rst.startSet();
+    rst.initiate();
+
+    const configDB = rst.getPrimary().getDB("config");
+    const session = rst.getPrimary().getDB(dbName).getMongo().startSession();
+    const primaryDB = session.getDatabase(dbName);
+
+    // Implicitly create the collection outside of the transaction.
+    assert.writeOK(primaryDB.getCollection(collName).insert({x: 1}));
+
+    // Run a transaction so the 'config.transactions' collection is implicitly created.
+    session.startTransaction();
+    assert.writeOK(primaryDB.getCollection(collName).insert({x: 2}));
+    session.commitTransaction();
+
+    // Run a predicate query that would fail if we did not ignore the 'notablescan' flag.
+    assert.eq(configDB.transactions.find({any_nonexistent_field: {$exists: true}}).itcount(), 0);
+
+    // Run the same query against the user created collection honoring the 'notablescan' flag.
+    // This will cause the query to fail as there is no viable query plan. Unfortunately,
+    // the reported query error code is the cryptic 'BadValue'.
+    assert.commandFailedWithCode(
+        primaryDB.runCommand({find: collName, filter: {any_nonexistent_field: {$exists: true}}}),
+        ErrorCodes.BadValue);
+
+    rst.stopSet();
+}());
diff --git a/src/mongo/db/query/get_executor.cpp b/src/mongo/db/query/get_executor.cpp
index fc44f7fd459..4f7f4477081 100644
--- a/src/mongo/db/query/get_executor.cpp
+++ b/src/mongo/db/query/get_executor.cpp
@@ -167,10 +167,10 @@ void fillOutPlannerParams(OperationContext* opCtx,
     // overrides this behavior by not outputting a collscan even if there are no indexed
     // solutions.
     if (storageGlobalParams.noTableScan.load()) {
-        const string& ns = canonicalQuery->ns();
+        const auto& nss = canonicalQuery->nss();
         // There are certain cases where we ignore this restriction:
-        bool ignore = canonicalQuery->getQueryObj().isEmpty() ||
-            (string::npos != ns.find(".system.")) || (0 == ns.find("local."));
+        bool ignore =
+            canonicalQuery->getQueryObj().isEmpty() || nss.isSystem() || nss.isOnInternalDb();
         if (!ignore) {
             plannerParams->options |= QueryPlannerParams::NO_TABLE_SCAN;
         }
-- 
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