SERVER-18198 fix incorrect logic for applying read pref to aggregate command

2 files changed, 45 insertions, 1 deletions

diff --git a/jstests/aggregation/bugs/server18198.js b/jstests/aggregation/bugs/server18198.js
new file mode 100644
index 00000000000..6d8d827e416
--- /dev/null
+++ b/jstests/aggregation/bugs/server18198.js
@@ -0,0 +1,42 @@
+// SERVER-18198 check read pref is only applied when there is no $out stage
+// in aggregate shell helper
+(function() {
+    "use strict";
+    var t = db.server18198;
+    t.drop();
+
+    var mongo = db.getMongo();
+
+    try {
+        var commandsRan = [];
+        // hook in our patched mongo
+        var mockMongo = {
+            getSlaveOk: function() { return true; },
+            runCommand: function(db, cmd, opts) {
+              commandsRan.push({db: db, cmd: cmd, opts: opts});
+              return {ok: 1.0};
+            },
+            getReadPref: function() { return {mode: "secondaryPreferred"}; },
+            getReadPrefMode: function() { return "secondaryPreferred"; }
+        };
+
+        db._mongo = mockMongo;
+
+        // this query should not get a read pref
+        t.aggregate([{$sort: {"x" : 1}}, {$out: "foo"}]);
+        assert.eq(commandsRan.length, 1);
+        // check that it doesn't have a read preference
+        assert(!commandsRan[0].cmd.hasOwnProperty("$readPreference"));
+
+        commandsRan = [];
+
+        t.aggregate([{$sort: {"x" : 1}}]);
+        // check another command was run
+        assert.eq(commandsRan.length, 1);
+        // check that it has a read preference
+        assert(commandsRan[0].cmd.hasOwnProperty("$readPreference"));
+    }
+    finally {
+        db._mongo = mongo;
+    }
+})();
diff --git a/src/mongo/shell/collection.js b/src/mongo/shell/collection.js
index 6bf21087887..5ecfd01cc31 100644
--- a/src/mongo/shell/collection.js
+++ b/src/mongo/shell/collection.js
@@ -1272,13 +1272,15 @@ DBCollection.prototype.aggregate = function(pipeline, extraOpts) {
         cmd.cursor = {};
     }
 
+    // in a well formed pipeline, $out must be the last stage. If it isn't then the server
+    // will reject the pipeline anyway.
     var hasOutStage = pipeline.length >= 1 && pipeline[pipeline.length - 1].hasOwnProperty("$out");
 
     var doAgg = function(cmd) {
         // if we don't have an out stage, we could run on a secondary
         // so we need to attach readPreference
         return hasOutStage ?
-            this.runReadCommand("aggregate", cmd) : this.runCommand("aggregate", cmd);
+            this.runCommand("aggregate", cmd) : this.runReadCommand("aggregate", cmd);
     }.bind(this);
 
     var res = doAgg(cmd);