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`.

2 files changed, 7 insertions, 4 deletions

diff --git a/src/mongo/logger/log_test.cpp b/src/mongo/logger/log_test.cpp
index 5be29f5d74b..122820f0fef 100644
--- a/src/mongo/logger/log_test.cpp
+++ b/src/mongo/logger/log_test.cpp
@@ -91,17 +91,20 @@ TEST_F(LogTestUnadornedEncoder, DetachAppender) {
     MessageLogDomain domain;
 
     // Appending to the domain before attaching the appender does not affect the appender.
-    domain.append(MessageEventEphemeral(Date_t(), LogSeverity::Log(), "", "1"));
+    domain.append(MessageEventEphemeral(Date_t(), LogSeverity::Log(), "", "1"))
+        .transitional_ignore();
     ASSERT_EQUALS(0, dynamic_cast<CountAppender*>(countAppender.get())->getCount());
 
     // Appending to the domain after attaching the appender does affect the appender.
     MessageLogDomain::AppenderHandle handle = domain.attachAppender(std::move(countAppender));
-    domain.append(MessageEventEphemeral(Date_t(), LogSeverity::Log(), "", "2"));
+    domain.append(MessageEventEphemeral(Date_t(), LogSeverity::Log(), "", "2"))
+        .transitional_ignore();
     countAppender = domain.detachAppender(handle);
     ASSERT_EQUALS(1, dynamic_cast<CountAppender*>(countAppender.get())->getCount());
 
     // Appending to the domain after detaching the appender does not affect the appender.
-    domain.append(MessageEventEphemeral(Date_t(), LogSeverity::Log(), "", "3"));
+    domain.append(MessageEventEphemeral(Date_t(), LogSeverity::Log(), "", "3"))
+        .transitional_ignore();
     ASSERT_EQUALS(1, dynamic_cast<CountAppender*>(countAppender.get())->getCount());
 }
 
diff --git a/src/mongo/logger/logstream_builder.cpp b/src/mongo/logger/logstream_builder.cpp
index 2378558bd4a..f0dfbaeebe9 100644
--- a/src/mongo/logger/logstream_builder.cpp
+++ b/src/mongo/logger/logstream_builder.cpp
@@ -104,7 +104,7 @@ LogstreamBuilder::~LogstreamBuilder() {
         MessageEventEphemeral message(
             Date_t::now(), _severity, _component, _contextName, _baseMessage);
         message.setIsTruncatable(_isTruncatable);
-        _domain->append(message);
+        _domain->append(message).transitional_ignore();
         if (_tee) {
             _os->str("");
             logger::MessageEventDetailsEncoder teeEncoder;