Avoid making references to null

In `compare_items` if `sort_order` is a top level expression, it has
no context and `get_context()` method returns the null pointer. To see
this, run

    $ ledger -f test/input/demo.ledger --sort display_amount reg --debug scope.symbols

and there will be many lines like

    [DEBUG] Binding scope 0 with ...

In such case making a reference to the context is an undefined
behavior (honestly the dereferencing itself feels quite problematic,
but many compilers just run without any complaints) and could
potentially cause segfaults.

Therefore, we change to use only the grandchild scope (`left` or
`right`) for `find_sort_values` here. Note that it may seem to be more
appropriate to use `report` here for the parent scope. However, in
`find_sort_values` which is called right after, the `report` scope is
always bound to this scope. So we only use grandchild scope to avoid
unnecessary operations.

Fixes #2069.

2 files changed, 40 insertions, 8 deletions

diff --git a/src/compare.cc b/src/compare.cc
index 63b85154..121f3014 100644
--- a/src/compare.cc
+++ b/src/compare.cc
@@ -88,15 +88,23 @@ bool compare_items<post_t>::operator()(post_t * left, post_t * right)
 
   post_t::xdata_t& lxdata(left->xdata());
   if (! lxdata.has_flags(POST_EXT_SORT_CALC)) {
-    bind_scope_t bound_scope(*sort_order.get_context(), *left);
-    find_sort_values(lxdata.sort_values, bound_scope);
+    if (sort_order.get_context()) {
+      bind_scope_t bound_scope(*sort_order.get_context(), *left);
+      find_sort_values(lxdata.sort_values, bound_scope);
+    } else {
+      find_sort_values(lxdata.sort_values, *left);
+    }
     lxdata.add_flags(POST_EXT_SORT_CALC);
   }
 
   post_t::xdata_t& rxdata(right->xdata());
   if (! rxdata.has_flags(POST_EXT_SORT_CALC)) {
-    bind_scope_t bound_scope(*sort_order.get_context(), *right);
-    find_sort_values(rxdata.sort_values, bound_scope);
+    if (sort_order.get_context()) {
+      bind_scope_t bound_scope(*sort_order.get_context(), *right);
+      find_sort_values(rxdata.sort_values, bound_scope);
+    } else {
+      find_sort_values(rxdata.sort_values, *right);
+    }
     rxdata.add_flags(POST_EXT_SORT_CALC);
   }
 
@@ -111,15 +119,23 @@ bool compare_items<account_t>::operator()(account_t * left, account_t * right)
 
   account_t::xdata_t& lxdata(left->xdata());
   if (! lxdata.has_flags(ACCOUNT_EXT_SORT_CALC)) {
-    bind_scope_t bound_scope(*sort_order.get_context(), *left);
-    find_sort_values(lxdata.sort_values, bound_scope);
+    if (sort_order.get_context()) {
+      bind_scope_t bound_scope(*sort_order.get_context(), *left);
+      find_sort_values(lxdata.sort_values, bound_scope);
+    } else {
+      find_sort_values(lxdata.sort_values, *left);
+    }
     lxdata.add_flags(ACCOUNT_EXT_SORT_CALC);
   }
 
   account_t::xdata_t& rxdata(right->xdata());
   if (! rxdata.has_flags(ACCOUNT_EXT_SORT_CALC)) {
-    bind_scope_t bound_scope(*sort_order.get_context(), *right);
-    find_sort_values(rxdata.sort_values, bound_scope);
+    if (sort_order.get_context()) {
+      bind_scope_t bound_scope(*sort_order.get_context(), *right);
+      find_sort_values(rxdata.sort_values, bound_scope);
+    } else {
+      find_sort_values(rxdata.sort_values, *right);
+    }
     rxdata.add_flags(ACCOUNT_EXT_SORT_CALC);
   }
 
diff --git a/test/regress/2069.test b/test/regress/2069.test
new file mode 100644
index 00000000..1029644c
--- /dev/null
+++ b/test/regress/2069.test
@@ -0,0 +1,16 @@
+P 2021-01-01 EUR 1.15 USD
+
+2021-01-01 Test 1
+    A             100 USD
+    B
+
+2021-01-01 Test 2
+    A             100 EUR
+    B
+
+test -X USD --sort display_amount reg
+21-Jan-01 Test 2                B                          -115 USD     -115 USD
+21-Jan-01 Test 1                B                          -100 USD     -215 USD
+                                A                           100 USD     -115 USD
+21-Jan-01 Test 2                A                           115 USD            0
+end test