WT-2804 Don't read values in a tree without a snapshot. (#2924)

* Improve two recent assertions, one from WT-2798 relating to writing metadata updates to disk that are part of a running transaction, and another from WT-2802 that checks that we don't try to copy values from a cursor without a transaction pinned.  The latter doesn't apply to cursors on checkpoints (including chunk cursors in an LSM tree).
  
* Copy cursor values before rollback in autocommit.
  
  If an autocommit operation such as WT_CURSOR::update touches multiple trees (e.g., multiple column groups in a table, or index updates, or multiple chunks in an LSM tree), then some cursors may have consumed the application's key/value pair when the operation has to roll back.  Take a copy of any such values before attempting to retry the operation.

3 files changed, 15 insertions, 7 deletions

diff --git a/src/include/api.h b/src/include/api.h
index 50b2eab83b8..0a4593178dc 100644
--- a/src/include/api.h
+++ b/src/include/api.h
@@ -66,6 +66,8 @@
 		else if (ret == 0 && !F_ISSET(&(s)->txn, WT_TXN_ERROR))	\
 			ret = __wt_txn_commit((s), NULL);		\
 		else {							\
+			if (retry)					\
+				WT_TRET(__wt_session_copy_values(s));	\
 			WT_TRET(__wt_txn_rollback((s), NULL));		\
 			if ((ret == 0 || ret == WT_ROLLBACK) &&		\
 			    (retry)) {					\
diff --git a/src/reconcile/rec_write.c b/src/reconcile/rec_write.c
index 0a14c027da7..b96b34594b0 100644
--- a/src/reconcile/rec_write.c
+++ b/src/reconcile/rec_write.c
@@ -1152,15 +1152,17 @@ __rec_txn_read(WT_SESSION_IMPL *session, WT_RECONCILE *r,
 	if (!skipped &&
 	    (F_ISSET(btree, WT_BTREE_LOOKASIDE) ||
 	    __wt_txn_visible_all(session, max_txn))) {
+#ifdef HAVE_DIAGNOSTIC
 		/*
 		 * The checkpoint transaction is special.  Make sure we never
 		 * write (metadata) updates from a checkpoint in a concurrent
 		 * session.
 		 */
-		WT_ASSERT(session, *updp == NULL ||
-		    (txnid = (*updp)->txnid) == WT_TXN_NONE ||
+		txnid = *updp == NULL ? WT_TXN_NONE : (*updp)->txnid;
+		WT_ASSERT(session, txnid == WT_TXN_NONE ||
 		    txnid != S2C(session)->txn_global.checkpoint_txnid ||
 		    WT_SESSION_IS_CHECKPOINT(session));
+#endif
 		return (0);
 	}
 
diff --git a/src/session/session_api.c b/src/session/session_api.c
index dd5acdbbf56..752fafffda5 100644
--- a/src/session/session_api.c
+++ b/src/session/session_api.c
@@ -66,11 +66,15 @@ __wt_session_copy_values(WT_SESSION_IMPL *session)
 
 	TAILQ_FOREACH(cursor, &session->cursors, q)
 		if (F_ISSET(cursor, WT_CURSTD_VALUE_INT)) {
-#if 0 /* !!! LSM updates can violate this assertion. */
-			/* We have to do this with a transaction ID pinned. */
-			WT_ASSERT(session,
-			   WT_SESSION_TXN_STATE(session)->snap_min !=
-			   WT_TXN_NONE);
+#ifdef HAVE_DIAGNOSTIC
+			/*
+			 * We have to do this with a transaction ID pinned
+			 * unless the cursor is reading from a checkpoint.
+			 */
+			WT_TXN_STATE *txn_state = WT_SESSION_TXN_STATE(session);
+			WT_ASSERT(session, txn_state->snap_min != WT_TXN_NONE ||
+			   (WT_PREFIX_MATCH(cursor->uri, "file:") &&
+			   F_ISSET((WT_CURSOR_BTREE *)cursor, WT_CBT_NO_TXN)));
 #endif
 
 			F_CLR(cursor, WT_CURSTD_VALUE_INT);