SERVER-58104 Prevent the balancer from calculating split points in sessions collection if shard key is not _id

(cherry picked from commit 9a12a55346e0da77acc94797bf5a558b001188e0)

3 files changed, 64 insertions, 0 deletions

diff --git a/buildscripts/resmokeconfig/suites/sharding_last_stable_mongos_and_mixed_shards.yml b/buildscripts/resmokeconfig/suites/sharding_last_stable_mongos_and_mixed_shards.yml
index a63bac55d57..49e017e2514 100644
--- a/buildscripts/resmokeconfig/suites/sharding_last_stable_mongos_and_mixed_shards.yml
+++ b/buildscripts/resmokeconfig/suites/sharding_last_stable_mongos_and_mixed_shards.yml
@@ -134,6 +134,8 @@ selector:
   - jstests/sharding/time_zone_info_mongos.js
   # Requires behavior that does not and will never exist in 3.6.
   - jstests/sharding/retryable_mongos_write_errors.js
+  # Requires the fix for SERVER-58104 which was not backported to 3.6.
+  - jstests/sharding/balancing_sessions_collection_reshard.js
 executor:
   config:
     shell_options:
diff --git a/jstests/sharding/balancing_sessions_collection_reshard.js b/jstests/sharding/balancing_sessions_collection_reshard.js
new file mode 100644
index 00000000000..fe34ed34116
--- /dev/null
+++ b/jstests/sharding/balancing_sessions_collection_reshard.js
@@ -0,0 +1,55 @@
+/*
+ * Tests that the balancer does not split the chunks for the sessions collection
+ * if the shard key is not _id.
+ */
+(function() {
+    "use strict";
+
+    let numShards = 2;
+    const kMinNumChunks = 2;
+    const clusterName = jsTest.name();
+    const st = new ShardingTest({
+        name: clusterName,
+        shards: numShards,
+        other:
+            {configOptions: {setParameter: {minNumChunksForSessionsCollection: kMinNumChunks}}}
+    });
+
+    let waitForBalancerToRun = function() {
+        let lastRoundNumber =
+            assert.commandWorked(st.s.adminCommand({balancerStatus: 1})).numBalancerRounds;
+        st.startBalancer();
+
+        assert.soon(function() {
+            let res = assert.commandWorked(st.s.adminCommand({balancerStatus: 1}));
+            return res.mode == "full" && res.numBalancerRounds - lastRoundNumber > 1;
+        });
+
+        st.stopBalancer();
+    };
+
+    const kSessionsNs = "config.system.sessions";
+    let configDB = st.s.getDB("config");
+
+    jsTest.log("Verify that the sessions collection is successfully dropped.");
+
+    assert.commandWorked(configDB.runCommand({drop: "system.sessions"}));
+
+    jsTest.log("Verify that the sessions collection is successfully recreated and resharded.");
+
+    assert.commandWorked(st.s.adminCommand({enableSharding: "config"}));
+    assert.commandWorked(st.s.adminCommand({shardCollection: kSessionsNs, key: {oldRoles: 1}}));
+
+    jsTest.log("Verify that balancer does not fail after resharding.");
+    waitForBalancerToRun();
+
+    jsTest.log("Verify that there is a single chunk for the collection " +
+               "and the bounds are the resharded key.");
+    assert.eq(1, configDB.chunks.count({ns: kSessionsNs}));
+
+    let doc = configDB.chunks.findOne({ns: kSessionsNs});
+    assert(doc.min.hasOwnProperty("oldRoles"));
+    assert(doc.max.hasOwnProperty("oldRoles"));
+
+    st.stop();
+}());
diff --git a/src/mongo/db/s/balancer/balancer_chunk_selection_policy_impl.cpp b/src/mongo/db/s/balancer/balancer_chunk_selection_policy_impl.cpp
index b113735802f..15bf7093fbf 100644
--- a/src/mongo/db/s/balancer/balancer_chunk_selection_policy_impl.cpp
+++ b/src/mongo/db/s/balancer/balancer_chunk_selection_policy_impl.cpp
@@ -234,6 +234,13 @@ void getSplitCandidatesToEnforceTagRanges(const ChunkManager* cm,
 void getSplitCandidatesForSessionsCollection(OperationContext* opCtx,
                                              const ChunkManager* cm,
                                              SplitCandidatesBuffer* splitCandidates) {
+    // Do not compute split points if shard key is not _id,
+    // since the split points depend on _id.id.
+    const auto& keyPattern = cm->getShardKeyPattern().getKeyPattern();
+    if (!KeyPattern::isIdKeyPattern(keyPattern.toBSON())) {
+        return;
+    }
+
     const auto minNumChunks = minNumChunksForSessionsCollection.load();
 
     if (cm->numChunks() >= minNumChunks) {