1 files changed, 82 insertions, 0 deletions
diff --git a/jstests/core/query/explain/explain_multi_plan.js b/jstests/core/query/explain/explain_multi_plan.js
new file mode 100644
index 00000000000..45484510196
--- /dev/null
+++ b/jstests/core/query/explain/explain_multi_plan.js
@@ -0,0 +1,82 @@
+// @tags: [
+//   # Cannot implicitly shard accessed collections because of following errmsg: A single
+//   # update/delete on a sharded collection must contain an exact match on _id or contain the shard
+//   # key.
+//   assumes_unsharded_collection,
+//   # TODO SERVER-30466
+//   does_not_support_causal_consistency,
+// ]
+
+/**
+ * Tests running explain on a variety of explainable commands (find, update, remove, etc.) when
+ * there are multiple plans available. This is a regression test for SERVER-20849 and SERVER-21376.
+ */
+(function() {
+"use strict";
+var coll = db.explainMultiPlan;
+coll.drop();
+
+// Create indices to ensure there are multiple plans available.
+assert.commandWorked(coll.createIndex({a: 1, b: 1}));
+assert.commandWorked(coll.createIndex({a: 1, b: -1}));
+
+// Insert some data to work with.
+var bulk = coll.initializeOrderedBulkOp();
+var nDocs = 100;
+for (var i = 0; i < nDocs; ++i) {
+    bulk.insert({a: i, b: nDocs - i});
+}
+bulk.execute();
+
+// SERVER-20849: The following commands should not crash the server.
+assert.doesNotThrow(function() {
+    coll.explain("allPlansExecution").update({a: {$gte: 1}}, {$set: {x: 0}});
+});
+
+assert.doesNotThrow(function() {
+    coll.explain("allPlansExecution").remove({a: {$gte: 1}});
+});
+
+assert.doesNotThrow(function() {
+    coll.explain("allPlansExecution").findAndModify({query: {a: {$gte: 1}}, remove: true});
+});
+
+assert.doesNotThrow(function() {
+    coll.explain("allPlansExecution").findAndModify({query: {a: {$gte: 1}}, update: {y: 1}});
+});
+
+assert.doesNotThrow(function() {
+    coll.explain("allPlansExecution").find({a: {$gte: 1}}).finish();
+});
+
+assert.doesNotThrow(function() {
+    coll.explain("allPlansExecution").count({a: {$gte: 1}});
+});
+
+assert.doesNotThrow(function() {
+    coll.explain("allPlansExecution").distinct("a", {a: {$gte: 1}});
+});
+
+// SERVER-21376: Make sure the 'rejectedPlans' field is filled in appropriately.
+function assertHasRejectedPlans(explainOutput) {
+    var queryPlannerOutput = explainOutput.queryPlanner;
+
+    // The 'rejectedPlans' section will be in a different place if passed through a mongos.
+    if ("SINGLE_SHARD" == queryPlannerOutput.winningPlan.stage) {
+        var shards = queryPlannerOutput.winningPlan.shards;
+        shards.forEach(function assertShardHasRejectedPlans(shard) {
+            assert.gt(shard.rejectedPlans.length, 0);
+        });
+    } else {
+        assert.gt(queryPlannerOutput.rejectedPlans.length, 0);
+    }
+}
+
+var res = coll.explain("queryPlanner").find({a: {$gte: 1}}).finish();
+assert.commandWorked(res);
+assertHasRejectedPlans(res);
+
+res = coll.explain("executionStats").find({a: {$gte: 1}}).finish();
+assert.commandWorked(res);
+assertHasRejectedPlans(res);
+}());