fortran: Restore interface to its previous state on error [PR48776]

Keep memory of the content of the current interface body being parsed
and restore it to its previous state if it has been modified at the time
a parse attempt fails.

This fixes memory errors and random segmentation faults caused by
dangling symbol pointers kept in interfaces' linked lists of symbols.
If a parsing attempt fails and symbols are freed, they should also be
removed from the current interface linked list.

As the list of symbol is a linked list, and parsing only adds new
symbols to the head of the list, all that is needed to track the
previous content of the list is a pointer to its previous head.
This adds such a pointer, and the restoration of the list of symbols
to that pointer on error.

	PR fortran/48776

gcc/fortran/ChangeLog:

	* gfortran.h (gfc_drop_interface_elements_before): New prototype.
	(gfc_current_interface_head): Return a reference to the pointer.
	* interface.cc (gfc_current_interface_head): Ditto.
	(free_interface_elements_until): New function, generalizing
	gfc_free_interface.
	(gfc_free_interface): Use free_interface_elements_until.
	(gfc_drop_interface_elements_before): New function.
	* parse.cc
	(current_interface_ptr, previous_interface_head): New static variables.
	(current_interface_valid_p, get_current_interface_ptr): New functions.
	(decode_statement): Initialize previous_interface_head.
	(reject_statement): Restore current interface pointer to point to
	previous_interface_head.

gcc/testsuite/ChangeLog:

	* gfortran.dg/interface_procedure_1.f90: New test.

1 files changed, 34 insertions, 5 deletions

diff --git a/gcc/fortran/interface.cc b/gcc/fortran/interface.cc
index ea82056..c01df04 100644
--- a/gcc/fortran/interface.cc
+++ b/gcc/fortran/interface.cc
@@ -78,14 +78,17 @@ along with GCC; see the file COPYING3.  If not see
 gfc_interface_info current_interface;
 
 
-/* Free a singly linked list of gfc_interface structures.  */
+/* Free the leading members of the gfc_interface linked list given in INTR
+   up to the END element (exclusive: the END element is not freed).
+   If END is not nullptr, it is assumed that END is in the linked list starting
+   with INTR.  */
 
-void
-gfc_free_interface (gfc_interface *intr)
+static void
+free_interface_elements_until (gfc_interface *intr, gfc_interface *end)
 {
   gfc_interface *next;
 
-  for (; intr; intr = next)
+  for (; intr != end; intr = next)
     {
       next = intr->next;
       free (intr);
@@ -93,6 +96,32 @@ gfc_free_interface (gfc_interface *intr)
 }
 
 
+/* Free a singly linked list of gfc_interface structures.  */
+
+void
+gfc_free_interface (gfc_interface *intr)
+{
+  free_interface_elements_until (intr, nullptr);
+}
+
+
+/* Update the interface pointer given by IFC_PTR to make it point to TAIL.
+   It is expected that TAIL (if non-null) is in the list pointed to by
+   IFC_PTR, hence the tail of it.  The members of the list before TAIL are
+   freed before the pointer reassignment.  */
+
+void
+gfc_drop_interface_elements_before (gfc_interface **ifc_ptr,
+				    gfc_interface *tail)
+{
+  if (ifc_ptr == nullptr)
+    return;
+
+  free_interface_elements_until (*ifc_ptr, tail);
+  *ifc_ptr = tail;
+}
+
+
 /* Change the operators unary plus and minus into binary plus and
    minus respectively, leaving the rest unchanged.  */
 
@@ -4953,7 +4982,7 @@ gfc_add_interface (gfc_symbol *new_sym)
 }
 
 
-gfc_interface *
+gfc_interface *&
 gfc_current_interface_head (void)
 {
   switch (current_interface.type)