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, 23 insertions, 0 deletions

diff --git a/gcc/testsuite/gfortran.dg/interface_procedure_1.f90 b/gcc/testsuite/gfortran.dg/interface_procedure_1.f90
new file mode 100644
index 0000000..6a58b6a
--- /dev/null
+++ b/gcc/testsuite/gfortran.dg/interface_procedure_1.f90
@@ -0,0 +1,23 @@
+! { dg-do compile }
+! { dg-additional-options "-std=f95" }
+!
+! PR fortran/48776
+! The following used to generate a segmentation fault in the front-end,
+! because a pointer to the get1 symbol was remaining in the get interface
+! after the procedure statement was rejected and the symbol freed.
+
+  interface get
+    procedure get1  ! { dg-error "Fortran 2003: PROCEDURE statement" }
+  end interface
+
+  integer :: h
+  call set1 (get (h))  ! { dg-error "no specific function for the generic 'get'" }
+contains
+  subroutine set1 (a)
+    integer, intent(in) :: a
+  end subroutine
+
+  integer function get1 (s)
+    integer :: s
+  end function
+end