SERVER-27438 Prevent mongos from dropping legacy $comment meta-operator

Closes #1135

Signed-off-by: James Wahlin <james.wahlin@10gen.com>

2 files changed, 88 insertions, 0 deletions

diff --git a/jstests/libs/profiler.js b/jstests/libs/profiler.js
index b7c4a3f89e7..c0fc210c928 100644
--- a/jstests/libs/profiler.js
+++ b/jstests/libs/profiler.js
@@ -14,4 +14,13 @@ function getProfilerProtocolStringForCommand(conn) {
     }
 
     return "op_command";
+}
+
+// Throws an assertion if the profiler does not contain exactly one entry matching <filter>.
+// Optional arguments <errorMsgFilter> and <errorMsgProj> limit profiler output if this asserts.
+function profilerHasSingleMatchingEntryOrThrow(inputDb, filter, errorMsgFilter, errorMsgProj) {
+    assert.eq(inputDb.system.profile.find(filter).itcount(),
+              1,
+              "Expected exactly one op matching: " + tojson(filter) + " in profiler " +
+                  tojson(inputDb.system.profile.find(errorMsgFilter, errorMsgProj).toArray()));
 }
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diff --git a/jstests/sharding/mongos_query_comment.js b/jstests/sharding/mongos_query_comment.js
new file mode 100644
index 00000000000..af035c5de30
--- /dev/null
+++ b/jstests/sharding/mongos_query_comment.js
@@ -0,0 +1,79 @@
+/**
+ * Test that a legacy query via mongos retains the $comment query meta-operator when transformed
+ * into a find command for the shards. In addition, verify that the find command comment parameter
+ * and query operator are passed to the shards correctly, and that an attempt to attach a non-string
+ * comment to the find command fails.
+ */
+(function() {
+    "use strict";
+
+    // For profilerHasSingleMatchingEntryOrThrow.
+    load("jstests/libs/profiler.js");
+
+    const st = new ShardingTest({name: "mongos_comment_test", mongos: 1, shards: 1});
+
+    const shard = st.shard0;
+    const mongos = st.s;
+
+    // Need references to the database via both mongos and mongod so that we can enable profiling &
+    // test queries on the shard.
+    const mongosDB = mongos.getDB("mongos_comment");
+    const shardDB = shard.getDB("mongos_comment");
+
+    assert.commandWorked(mongosDB.dropDatabase());
+
+    const mongosColl = mongosDB.test;
+    const shardColl = shardDB.test;
+
+    const collNS = mongosColl.getFullName();
+
+    for (let i = 0; i < 5; ++i) {
+        assert.writeOK(mongosColl.insert({_id: i, a: i}));
+    }
+
+    // The profiler will be used to verify that comments are present on the shard.
+    assert.commandWorked(shardDB.setProfilingLevel(2));
+    const profiler = shardDB.system.profile;
+
+    //
+    // Set legacy read mode for the mongos and shard connections.
+    //
+    mongosDB.getMongo().forceReadMode("legacy");
+    shardDB.getMongo().forceReadMode("legacy");
+
+    // TEST CASE: A legacy string $comment meta-operator is propagated to the shards via mongos.
+    assert.eq(mongosColl.find({$query: {a: 1}, $comment: "TEST"}).itcount(), 1);
+    profilerHasSingleMatchingEntryOrThrow(shardDB,
+                                          {op: "query", ns: collNS, "query.comment": "TEST"});
+
+    // TEST CASE: A legacy BSONObj $comment is converted to a string and propagated via mongos.
+    assert.eq(mongosColl.find({$query: {a: 1}, $comment: {c: 2, d: {e: "TEST"}}}).itcount(), 1);
+    profilerHasSingleMatchingEntryOrThrow(
+        shardDB, {op: "query", ns: collNS, "query.comment": "{ c: 2.0, d: { e: \"TEST\" } }"});
+
+    // TEST CASE: Legacy BSONObj $comment is NOT converted to a string when issued on the mongod.
+    assert.eq(shardColl.find({$query: {a: 1}, $comment: {c: 3, d: {e: "TEST"}}}).itcount(), 1);
+    profilerHasSingleMatchingEntryOrThrow(
+        shardDB, {op: "query", ns: collNS, "query.comment": {c: 3, d: {e: "TEST"}}});
+
+    //
+    // Revert to "commands" read mode for the find command test cases below.
+    //
+    mongosDB.getMongo().forceReadMode("commands");
+    shardDB.getMongo().forceReadMode("commands");
+
+    // TEST CASE: Verify that string find.comment and non-string find.filter.$comment propagate.
+    assert.eq(mongosColl.find({a: 1, $comment: {b: "TEST"}}).comment("TEST").itcount(), 1);
+    profilerHasSingleMatchingEntryOrThrow(
+        shardDB,
+        {op: "query", ns: collNS, "query.comment": "TEST", "query.filter.$comment": {b: "TEST"}});
+
+    // TEST CASE: Verify that find command with a non-string comment parameter is rejected.
+    assert.commandFailedWithCode(
+        mongosDB.runCommand(
+            {"find": mongosColl.getName(), "filter": {a: 1}, "comment": {b: "TEST"}}),
+        9,
+        "Non-string find command comment did not return an error.");
+
+    st.stop();
+})();
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