SERVER-27244 Status usage compile-time facilities.

There are numerous places in the codebase where `mongo::Status` or
`mongo::StatusWith< T >` objects are returned and never checked.
Many of these are innocuous, but many of them are potentially
severe bugs.  This change introduces facilities to permit
compile-time warning of unchecked `Status` and `StatusWith` usage
on clang compilers.  It introduces an `ignore` function which is
useful to state that a specific "ignored status" case was
intentional.  It not presently an error, in clang builds, to
forget to check a `Status` -- this will come in a later commit.

This also introduces a `transitional_ignore` function, which allows
for easy continual auditing of the codebase for current "whitelisted"
unchecked-status instances.  All present "ignored status" cases
have been marked `transitional_ignore`.

3 files changed, 6 insertions, 6 deletions

diff --git a/src/mongo/rpc/metadata/oplog_query_metadata_test.cpp b/src/mongo/rpc/metadata/oplog_query_metadata_test.cpp
index cc08bf832fb..06784a65ee1 100644
--- a/src/mongo/rpc/metadata/oplog_query_metadata_test.cpp
+++ b/src/mongo/rpc/metadata/oplog_query_metadata_test.cpp
@@ -47,7 +47,7 @@ TEST(ReplResponseMetadataTest, OplogQueryMetadataRoundtrip) {
     ASSERT_EQ(opTime2, metadata.getLastOpApplied());
 
     BSONObjBuilder builder;
-    metadata.writeToMetadata(&builder);
+    metadata.writeToMetadata(&builder).transitional_ignore();
 
     BSONObj expectedObj(BSON(kOplogQueryMetadataFieldName << BSON(
                                  "lastOpCommitted"
@@ -75,7 +75,7 @@ TEST(ReplResponseMetadataTest, OplogQueryMetadataRoundtrip) {
     ASSERT_EQ(metadata.getSyncSourceIndex(), clonedMetadata.getSyncSourceIndex());
 
     BSONObjBuilder clonedBuilder;
-    clonedMetadata.writeToMetadata(&clonedBuilder);
+    clonedMetadata.writeToMetadata(&clonedBuilder).transitional_ignore();
 
     BSONObj clonedSerializedObj = clonedBuilder.obj();
     ASSERT_BSONOBJ_EQ(expectedObj, clonedSerializedObj);
diff --git a/src/mongo/rpc/metadata/repl_set_metadata_test.cpp b/src/mongo/rpc/metadata/repl_set_metadata_test.cpp
index 6d5b7be1a2f..b6285e63342 100644
--- a/src/mongo/rpc/metadata/repl_set_metadata_test.cpp
+++ b/src/mongo/rpc/metadata/repl_set_metadata_test.cpp
@@ -52,7 +52,7 @@ TEST(ReplResponseMetadataTest, Roundtrip) {
     ASSERT_TRUE(metadata.hasReplicaSetId());
 
     BSONObjBuilder builder;
-    metadata.writeToMetadata(&builder);
+    metadata.writeToMetadata(&builder).transitional_ignore();
 
     BSONObj expectedObj(
         BSON(kReplSetMetadataFieldName
@@ -82,7 +82,7 @@ TEST(ReplResponseMetadataTest, Roundtrip) {
     ASSERT_EQ(metadata.getReplicaSetId(), clonedMetadata.getReplicaSetId());
 
     BSONObjBuilder clonedBuilder;
-    clonedMetadata.writeToMetadata(&clonedBuilder);
+    clonedMetadata.writeToMetadata(&clonedBuilder).transitional_ignore();
 
     BSONObj clonedSerializedObj = clonedBuilder.obj();
     ASSERT_BSONOBJ_EQ(expectedObj, clonedSerializedObj);
diff --git a/src/mongo/rpc/metadata/sharding_metadata.cpp b/src/mongo/rpc/metadata/sharding_metadata.cpp
index b1c4b47b8e5..384c75f2679 100644
--- a/src/mongo/rpc/metadata/sharding_metadata.cpp
+++ b/src/mongo/rpc/metadata/sharding_metadata.cpp
@@ -116,7 +116,7 @@ Status ShardingMetadata::downconvert(const BSONObj& commandReply,
         // We can reuse the same logic to write the sharding metadata out to the legacy
         // command as the element has the same format whether it is there or on the metadata
         // object.
-        swShardingMetadata.getValue().writeToMetadata(legacyCommandReplyBob);
+        swShardingMetadata.getValue().writeToMetadata(legacyCommandReplyBob).transitional_ignore();
     } else if (swShardingMetadata.getStatus() == ErrorCodes::NoSuchKey) {
         // It is valid to not have a $gleStats field.
     } else {
@@ -132,7 +132,7 @@ Status ShardingMetadata::upconvert(const BSONObj& legacyCommand,
     // as it has the same format whether it is there or on the metadata object.
     auto swShardingMetadata = readFromMetadata(legacyCommand);
     if (swShardingMetadata.isOK()) {
-        swShardingMetadata.getValue().writeToMetadata(metadataBob);
+        swShardingMetadata.getValue().writeToMetadata(metadataBob).transitional_ignore();
 
         // Write out the command excluding the $gleStats subobject.
         for (const auto& elem : legacyCommand) {