Add a callback to FuncJUMP_TO_CATCH

This is another bit of the GAP dynamic library code. Again without tests
and without a guarantee that this will work in HPC-GAP

src/gap.c

@@ -1103,11 +1103,14 @@ Obj FuncCALL_WITH_CATCH( Obj self, Obj func, volatile Obj args )
     return res;
 }
 
-Obj FuncJUMP_TO_CATCH( Obj self, Obj payload)
+Obj FuncJUMP_TO_CATCH(Obj self, Obj payload)
 {
-  STATE(ThrownObject) = payload;
-  syLongjmp(&(STATE(ReadJmpError)), 1);
-  return 0;
+    STATE(ThrownObject) = payload;
+    if (STATE(JumpToCatchCallback) != 0) {
+        (*STATE(JumpToCatchCallback))();
+    }
+    syLongjmp(&(STATE(ReadJmpError)), 1);
+    return 0;
 }
 
 Obj FuncSetUserHasQuit( Obj Self, Obj value)
@@ -3047,10 +3050,12 @@ void InitializeGap (
     STATE(ThrownObject) = 0;
     STATE(UserHasQUIT) = 0;
     STATE(UserHasQuit) = 0;
+    STATE(JumpToCatchCallback) = 0;
 
     NrImportedGVars = 0;
     NrImportedFuncs = 0;
 
+
     sysenviron = environ;
 
     /* get info structures for the built in modules                        */

src/gapstate.h

@@ -123,6 +123,9 @@ typedef struct GAPState {
     Obj  BaseShellContext;
     Obj  ErrorLVars;        // ErrorLVars as modified by DownEnv / UpEnv
     Int  ErrorLLevel;       // record where on the stack ErrorLVars is relative to the top, i.e. BaseShellContext
+    void (*JumpToCatchCallback)(); // This callback is called in FuncJUMP_TO_CATCH,
+                                   // this is not used by GAP itself but by programs
+                                   // that use GAP as a library to handle errors
 
     /* From objects.c */
     Int PrintObjIndex;