Import wiredtiger-wiredtiger-2.8.0-137-g94c171d.tar.gz from wiredtige…

…r branch mongodb-3.4

ref: 5047aab..94c171d

WT-2534       Invalid transaction snapshots on PowerPC
SERVER-23517  WiredTiger changes for MongoDB 3.3.5

src/third_party/wiredtiger/src/include/txn.i

@@ -357,8 +357,13 @@ __wt_txn_id_alloc(WT_SESSION_IMPL *session, bool publish)
 		WT_PUBLISH(WT_SESSION_TXN_STATE(session)->id, id);
 	}
 
-	++txn_global->current;
-	 __wt_spin_unlock(session, &txn_global->id_lock);
+	/*
+	 * Even though we are in a spinlock, readers are not.  We rely on
+	 * atomic reads of the current ID to create snapshots, so for unlocked
+	 * reads to be well defined, we must use an atomic increment here.
+	 */
+	(void)__wt_atomic_addv64(&txn_global->current, 1);
+	__wt_spin_unlock(session, &txn_global->id_lock);
 	return (id);
 }
 

src/third_party/wiredtiger/src/txn/txn_ckpt.c

@@ -441,14 +441,15 @@ __txn_checkpoint(WT_SESSION_IMPL *session, const char *cfg[])
 	 * Start the checkpoint for real.
 	 *
 	 * Bump the global checkpoint generation, used to figure out whether
-	 * checkpoint has visited a tree.  There is no need for this to be
-	 * atomic: it is only written while holding the checkpoint lock.
+	 * checkpoint has visited a tree.  Use an atomic increment even though
+	 * we are single-threaded because readers of the checkpoint generation
+	 * don't hold the checkpoint lock.
 	 *
 	 * We do need to update it before clearing the checkpoint's entry out
 	 * of the transaction table, or a thread evicting in a tree could
 	 * ignore the checkpoint's transaction.
 	 */
-	++txn_global->checkpoint_gen;
+	(void)__wt_atomic_addv64(&txn_global->checkpoint_gen, 1);
 	WT_STAT_FAST_CONN_SET(session,
 	    txn_checkpoint_generation, txn_global->checkpoint_gen);