SERVER-34798 Remove ServiceContext subclasses and use new ServiceContext in every unit test.

This patch does several loosely related and surprisingly hard to separate things.

1.) Make the ServiceContext class final

2.) Create a mechanism, called ConstructorActions, for running methods on
ServiceContexts immediately after they're built and immediately before they're
destroyed.

3.) Introduce / improve test fixture base classes for tests, giving them fresh
ServiceContext instances for each test case. There is one fixture for tests that
need a storage engine and another for those that do not.

4.) Make several remaining global variables SC decorations in support of (3)

5.) Replace many MONGO_INITIALIZERS that access getGlobalServiceContext with the
new constructor-actions system, which is needed for (3.)

6.) Fix up tests to use the fixtures from (3) and fix tests that silently used
different service contexts in together in a technically illegal fashion that now
breaks.

7.) Utilize (2) as necessary to simplify initialization of new ServiceContexts,
simplifying the fixtures in (3).

1 files changed, 32 insertions, 0 deletions

diff --git a/src/mongo/db/client.h b/src/mongo/db/client.h
index 99b172505e3..74130191b99 100644
--- a/src/mongo/db/client.h
+++ b/src/mongo/db/client.h
@@ -47,6 +47,7 @@
 #include "mongo/transport/session.h"
 #include "mongo/util/concurrency/spin_lock.h"
 #include "mongo/util/decorable.h"
+#include "mongo/util/invariant.h"
 #include "mongo/util/net/hostandport.h"
 
 namespace mongo {
@@ -242,6 +243,37 @@ private:
     PseudoRandom _prng;
 };
 
+/**
+ * Utility class to temporarily swap which client is bound to the running thread.
+ *
+ * Use this class to bind a client to the current thread for the duration of the
+ * AlternativeClientRegion's lifetime, restoring the prior client, if any, at the
+ * end of the block.
+ */
+class AlternativeClientRegion {
+public:
+    explicit AlternativeClientRegion(ServiceContext::UniqueClient& clientToUse)
+        : _alternateClient(&clientToUse) {
+        invariant(clientToUse);
+        if (Client::getCurrent()) {
+            _originalClient = Client::releaseCurrent();
+        }
+        Client::setCurrent(std::move(*_alternateClient));
+    }
+
+    ~AlternativeClientRegion() {
+        *_alternateClient = Client::releaseCurrent();
+        if (_originalClient) {
+            Client::setCurrent(std::move(_originalClient));
+        }
+    }
+
+private:
+    ServiceContext::UniqueClient _originalClient;
+    ServiceContext::UniqueClient* const _alternateClient;
+};
+
+
 /** get the Client object for this thread. */
 Client& cc();