Intercept goal_failed (within web_api module)

include/common/error_handling.pl

@@ -39,9 +39,10 @@ prolog:message(missing_key(Key)) -->
  *  it expected the value to be of another type.
  *
  *  N.B.: We throw this as Bad Request. However, we cannot check the
- *  type of the value prior to parsing. Thus, we are comparing prolog 
- *  term with expected prolog types. So in principle this exception 
- *  might also be thrown due to a parsing mistake, i.e. a server error. 
+ *        type of the value prior to parsing. Thus, we are comparing 
+ *        prolog term with expected prolog types. So in principle this 
+ *        exception might also be thrown due to a parsing mistake, 
+ *        i.e. a server error. 
  */
 http:map_exception_to_http_status_hook(wrong_type(Key, ExpectedType, Value),
     bad_request(wrong_type(Key, ExpectedType, Value)), [], []).
@@ -136,4 +137,34 @@ item_not_relevant(Item) -->
  *  Server Errors *
  ******************/
 
+/* error(goal_failed(web_api:Goal),C)
+ * 
+ * Thrown by http_dispatch:call_action/2 if one of our handlers fails.
+ * Rethrown as error(api_error(goal_failed(PredIndicator)),C).
+ *
+ * N.B.: The SWI-Prolog documentation states "The hook is not allowed to modify
+ *       ExceptionOut in such a way that it no longer unifies with the catching 
+ *       frame." The Exception is caught in httpd_wrapper:handler_with_output_to/5,
+ *       where the catcher is unbound. Thus, we do not violate this constraint.
+ */
+user:prolog_exception_hook(error(goal_failed(web_api:Goal),Context),
+        error(api_error(Hash, goal_failed),Context), Frame, _) :- 
+    Goal =.. [PredName | Args],
+    last(Args, Request),
+    error_hash(Request, Hash),
+
+    get_prolog_backtrace(20, Backtrace, [frame(Frame), clause_references(false)]),
+    
+    http_log('incident(~a, ~q, ~q).~n',[Hash, goal_failed, [goal(Goal), prolog_stack(Backtrace), original_context(Context)]]).
+
+
+%! error_hash(+Request, --Hash) is det
+error_hash(Request, Hash) :-
+    get_time(TimeStamp),
+    random_between(0,1000000,RandInt),
+    variant_sha1( error_hash(TimeStamp, RandInt, Request), FullHash),
+    atom_chars(FullHash,HashChars),
+    length(HashStartChars, 16), 
+    append(HashStartChars,_,HashChars), 
+    format(atom(Hash), '~s~s~s~s-~s~s~s~s-~s~s~s~s-~s~s~s~s', HashStartChars).