SERVER-42916 Reject non-inline mapReduce to an existing sharded collection that is not sharded by _id

2 files changed, 14 insertions, 6 deletions

diff --git a/src/mongo/db/commands/mr_common.cpp b/src/mongo/db/commands/mr_common.cpp
index 1621ac716cd..eab02894609 100644
--- a/src/mongo/db/commands/mr_common.cpp
+++ b/src/mongo/db/commands/mr_common.cpp
@@ -316,11 +316,22 @@ std::unique_ptr<Pipeline, PipelineDeleter> translateFromMR(
     if (outType == OutputType::Merge || outType == OutputType::Reduce) {
         uassert(ErrorCodes::InvalidOptions,
                 "Source collection cannot be the same as destination collection in MapReduce when "
-                "using merge or "
-                "reduce actions",
+                "using merge or reduce actions",
                 inNss != outNss);
     }
 
+    // If non-inline output, verify that the target collection is *not* sharded by anything other
+    // than _id.
+    if (outType != OutputType::InMemory) {
+        auto [shardKey, targetCollectionVersion] =
+            expCtx->mongoProcessInterface->ensureFieldsUniqueOrResolveDocumentKey(
+                expCtx, boost::none, boost::none, outNss);
+        uassert(31313,
+                "The mapReduce target collection must either be unsharded or sharded by {_id: 1} "
+                "or {_id: 'hashed'}",
+                shardKey == std::set<FieldPath>{FieldPath("_id"s)});
+    }
+
     // TODO: It would be good to figure out what kind of errors this would produce in the Status.
     // It would be better not to produce something incomprehensible out of an internal translation.
     return uassertStatusOK(Pipeline::create(
diff --git a/src/mongo/s/commands/cluster_map_reduce.cpp b/src/mongo/s/commands/cluster_map_reduce.cpp
index 9ae34eca3c5..0eadc1b8ee3 100644
--- a/src/mongo/s/commands/cluster_map_reduce.cpp
+++ b/src/mongo/s/commands/cluster_map_reduce.cpp
@@ -671,10 +671,7 @@ bool runMapReduce(OperationContext* opCtx,
                 outputRoutingInfo.cm());
     }
 
-    if (!ok) {
-        errmsg = str::stream() << "MR post processing failed: " << singleResult.toString();
-        return false;
-    }
+    uassert(31311, str::stream() << "MR post processing failed: " << singleResult.toString(), ok);
 
     // copy some elements from a single result
     // annoying that we have to copy all results for inline, but no way around it