octave.cxx: fix exception raising for newer Octave versions

- Since (at least) Octave 5.1.0, the Octave error() function now raises a C++
  exception, which if uncaught immediately exits a SWIG wrapper function,
  bypassing any cleanup code that may appear after a "fail:" label.
- This patch adds a "try { ... } catch(...) { }" block around the contents of
  SWIG wrapper functions to first execute the cleanup code before rethrowing any
  exception raised.
- It is backward compatible with earlier versions of Octave where error() does
  not raise an exception, which will still branch to the "fail:" block to
  execute cleanup code if an error is encountered.

Author: kwwette

CHANGES.current

@@ -16,6 +16,22 @@ Version 4.0.2 (in progress)
 2020-05-24: vapier
             [JS] #1796 Fix pkg-config invocation in configure.
 
+2020-04-30: kwwette
+            [Octave] Fix exception raising for newer Octave versions
+            Since (at least) Octave 5.1.0, the Octave error() function now raises a C++ exception,
+            which if uncaught immediately exits a SWIG wrapper function, bypassing any cleanup code
+            that may appear after a "fail:" label. This patch adds a "try { ... } catch(...) { }"
+            block around the contents of SWIG wrapper functions to first execute the cleanup code
+            before rethrowing any exception raised. It is backward compatible with earlier versions
+            of Octave where error() does not raise an exception, which will still branch to the
+            "fail:" block to execute cleanup code if an error is encountered.
+
+            Note that the new "try { ... } catch(...) { }" block will localise any local variables
+            used in typemaps that were NOT declared through SWIG's %typemap(...) syntax, so it's
+            possible this could break existing SWIG wrappers which were implicitly sharing local
+            variables between typemaps. This can be fixed, however, by declaring local variables
+            which need to be shared between typemaps through SWIG's %typemap(...) syntax.
+
 2020-02-18: ryannevell
             [Lua] #1728 Add support for LUA lightuserdata to SWIG_Lua_ConvertPtr.
 

Source/Modules/octave.cxx

@@ -567,6 +567,10 @@ class OCTAVE:public Language {
     Wrapper *f = NewWrapper();
     Octave_begin_function(n, f->def, iname, overname, !overloaded);
 
+    // Start default try block to execute
+    // cleanup code if exception is thrown
+    Printf(f->code, "try {\n");
+
     emit_parameter_variables(l, f);
     emit_attach_parmmaps(l, f);
     Setattr(n, "wrap:parms", l);
@@ -754,9 +758,20 @@ class OCTAVE:public Language {
     }
 
     Printf(f->code, "return _out;\n");
-    Printf(f->code, "fail:\n");	// we should free locals etc if this happens
+
+    // Execute cleanup code if branched to fail: label
+    Printf(f->code, "fail:\n");
     Printv(f->code, cleanup, NIL);
     Printf(f->code, "return octave_value_list();\n");
+
+    // Execute cleanup code if exception was thrown
+    Printf(f->code, "}\n");
+    Printf(f->code, "catch(...) {\n");
+    Printv(f->code, cleanup, NIL);
+    Printf(f->code, "throw;\n");
+    Printf(f->code, "}\n");
+
+    // End wrapper function
     Printf(f->code, "}\n");
 
     /* Substitute the cleanup code */