branches/zip: Add comments that clarify why the remaining calls to

row_build(), row_upd_index_replace_new_col_vals_index_pos(), and
row_upd_index_replace_new_col_vals() are safe.

btr_cur_optimistic_update(), btr_cur_pessimistic_update(): Note that
the B-tree page of the clustered index record is latched in mtr.

trx_undo_prev_version_build(): Add const qualifiers to index_rec
and rec.  Note that the page of index_rec is latched in index_mtr.

row_vers_impl_x_locked_off_kernel(), row_vers_old_has_index_entry():
Note that the stack of versions is locked by mtr and thus it is
safe to call row_build().

btr/btr0cur.c

@@ -1863,6 +1863,9 @@ any_extern:
 	/* We checked above that there are no externally stored fields. */
 	ut_a(!n_ext);
 
+	/* The page containing the clustered index record
+	corresponding to new_entry is latched in mtr.
+	Thus the following call is safe. */
 	row_upd_index_replace_new_col_vals_index_pos(new_entry, index, update,
 						     FALSE, NULL, heap);
 	old_rec_size = rec_offs_size(offsets);
@@ -2134,6 +2137,9 @@ btr_cur_pessimistic_update(
 	ut_ad(rec_offs_validate(NULL, index, offsets));
 	rec_offs_make_valid(rec, index, offsets);
 
+	/* The page containing the clustered index record
+	corresponding to new_entry is latched in mtr.
+	Thus the following call is safe. */
 	row_upd_index_replace_new_col_vals_index_pos(new_entry, index, update,
 						     FALSE, *heap, *heap);
 	if (!(flags & BTR_KEEP_SYS_FLAG)) {

include/trx0rec.h

@@ -251,11 +251,11 @@ trx_undo_prev_version_build(
 				the previous version is not >= purge_view,
 				which means that it may have been removed,
 				DB_ERROR if corrupted record */
-	rec_t*		index_rec,/* in: clustered index record in the
+	const rec_t*	index_rec,/* in: clustered index record in the
 				index tree */
 	mtr_t*		index_mtr,/* in: mtr which contains the latch to
 				index_rec page and purge_view */
-	rec_t*		rec,	/* in: version of a clustered index record */
+	const rec_t*	rec,	/* in: version of a clustered index record */
 	dict_index_t*	index,	/* in: clustered index */
 	ulint*		offsets,/* in: rec_get_offsets(rec, index) */
 	mem_heap_t*	heap,	/* in: memory heap from which the memory

row/row0vers.c

@@ -163,6 +163,9 @@ row_vers_impl_x_locked_off_kernel(
 			clust_offsets = rec_get_offsets(
 				prev_version, clust_index, NULL,
 				ULINT_UNDEFINED, &heap);
+			/* The stack of versions is locked by mtr.
+			Thus, it is safe to fetch the prefixes for
+			externally stored columns. */
 			row = row_build(ROW_COPY_POINTERS, clust_index,
 					prev_version, clust_offsets,
 					NULL, &ext, heap);
@@ -339,6 +342,9 @@ row_vers_old_has_index_entry(
 	if (also_curr && !rec_get_deleted_flag(rec, comp)) {
 		row_ext_t*	ext;
 
+		/* The stack of versions is locked by mtr.
+		Thus, it is safe to fetch the prefixes for
+		externally stored columns. */
 		row = row_build(ROW_COPY_POINTERS, clust_index,
 				rec, clust_offsets, NULL, &ext, heap);
 		entry = row_build_index_entry(row, ext, index, heap);
@@ -396,6 +402,9 @@ row_vers_old_has_index_entry(
 		if (!rec_get_deleted_flag(prev_version, comp)) {
 			row_ext_t*	ext;
 
+			/* The stack of versions is locked by mtr.
+			Thus, it is safe to fetch the prefixes for
+			externally stored columns. */
 			row = row_build(ROW_COPY_POINTERS, clust_index,
 					prev_version, clust_offsets,
 					NULL, &ext, heap);

trx/trx0rec.c

@@ -1317,12 +1317,12 @@ trx_undo_prev_version_build(
 				the previous version is not >= purge_view,
 				which means that it may have been removed,
 				DB_ERROR if corrupted record */
-	rec_t*		index_rec,/* in: clustered index record in the
+	const rec_t*	index_rec,/* in: clustered index record in the
 				index tree */
 	mtr_t*		index_mtr __attribute__((unused)),
 				/* in: mtr which contains the latch to
 				index_rec page and purge_view */
-	rec_t*		rec,	/* in: version of a clustered index record */
+	const rec_t*	rec,	/* in: version of a clustered index record */
 	dict_index_t*	index,	/* in: clustered index */
 	ulint*		offsets,/* in: rec_get_offsets(rec, index) */
 	mem_heap_t*	heap,	/* in: memory heap from which the memory
@@ -1467,6 +1467,9 @@ trx_undo_prev_version_build(
 		entry = row_rec_to_index_entry(ROW_COPY_DATA, rec, index,
 					       offsets, &n_ext, heap);
 		n_ext += btr_push_update_extern_fields(entry, update);
+		/* The page containing the clustered index record
+		corresponding to entry is latched in mtr.  Thus the
+		following call is safe. */
 		row_upd_index_replace_new_col_vals(entry, index, update,
 						   heap, heap);