Print traceback when encountering errors

Uses the new command line option --alwaystrace. This lets GAP
also print tracebacks when the break loop is disabled by -T.

This still overwrites the library functions Where, WHERE, and
ErrorInner. This can be avoided once there is a central GAP
error stream where these functions send their output, see
gap-system/gap#1815.

Since redirecting error messages was not available in 4.9
this also sets the required GAP version to >= 4.10.

Fixes #38.

PackageInfo.g

@@ -65,7 +65,7 @@ PackageDoc := rec(
 ),
 
 Dependencies := rec(
-  GAP := ">= 4.9",
+  GAP := ">= 4.10",
   NeededOtherPackages := [ [ "GAPDoc", ">= 1.5" ]
                          , [ "io",     ">= 4.4.6" ]
                          , [ "json",   ">= 1.1.0" ]

bin/jupyter-kernel-gap

@@ -7,7 +7,7 @@ elif [ -z "$GAP" ] ; then
 fi
 
 echo "GAP Jupyter Kernel Starting using $GAP"
-exec $GAP -q -T <<EOF
+exec $GAP -q -T --alwaystrace <<EOF
    LoadPackage("JupyterKernel");
    JUPYTER_KernelStart_GAP("$1");
    QUIT_GAP(0);

gap/JupyterError.gi

@@ -4,16 +4,64 @@
 
 GAP_ERROR_STREAM := "*errout*";
 
+MakeReadWriteGlobal("WHERE");
+Unbind(WHERE);
+
+BIND_GLOBAL("WHERE", function( context, depth, outercontext)
+    local   bottom,  lastcontext,  f;
+    if depth <= 0 then
+        return;
+    fi;
+    bottom := GetBottomLVars();
+    lastcontext := outercontext;
+    while depth > 0  and context <> bottom do
+        PRINT_CURRENT_STATEMENT(GAP_ERROR_STREAM, context);
+        PrintTo(GAP_ERROR_STREAM, " called from\n");
+        lastcontext := context;
+        context := ParentLVars(context);
+        depth := depth-1;
+    od;
+    if depth = 0 then
+        PrintTo(GAP_ERROR_STREAM, "...  ");
+    else
+        f := ContentsLVars(lastcontext).func;
+        PrintTo(GAP_ERROR_STREAM, "<function \"",NAME_FUNC(f)
+              ,"\">( <arguments> )\n called from read-eval loop ");
+    fi;
+end);
+
+MakeReadWriteGlobal("Where");
+Unbind(Where);
+
+BIND_GLOBAL("Where", function(arg)
+    local   depth;
+    if LEN_LIST(arg) = 0 then
+        depth := 5;
+    else
+        depth := arg[1];
+    fi;
+
+    if ErrorLVars = fail or ErrorLVars = GetBottomLVars() then
+        PrintTo(GAP_ERROR_STREAM, "not in any function ");
+    else
+        WHERE(ParentLVars(ErrorLVars),depth, ErrorLVars);
+    fi;
+    PrintTo(GAP_ERROR_STREAM, "at ",INPUT_FILENAME(),":",INPUT_LINENUMBER(),"\n");
+end);
+
+OnBreak := Where;
+
 MakeReadWriteGlobal("ErrorInner");
 Unbind(ErrorInner);
 
 BIND_GLOBAL("ErrorInner",
         function( arg )
     local   context, mayReturnVoid,  mayReturnObj,  lateMessage,  earlyMessage,
             x,  prompt,  res, errorLVars, justQuit, printThisStatement,
-            location;
+            printEarlyMessage, printEarlyTraceback, lastErrorStream,
+            shellOut, shellIn;
 
-        context := arg[1].context;
+    context := arg[1].context;
     if not IsLVarsBag(context) then
         PrintTo(GAP_ERROR_STREAM, "ErrorInner:   option context must be a local variables bag\n");
         LEAVE_ALL_NAMESPACES();
@@ -74,58 +122,116 @@ BIND_GLOBAL("ErrorInner",
     else
         lateMessage := "";
     fi;
-    
-       earlyMessage := arg[2];
+
+    earlyMessage := arg[2];
     if Length(arg) <> 2 then
-        PrintTo(GAP_ERROR_STREAM,"ErrorInner: new format takes exactly two arguments\n");
+        PrintTo(GAP_ERROR_STREAM, "ErrorInner: new format takes exactly two arguments\n");
         LEAVE_ALL_NAMESPACES();
         JUMP_TO_CATCH(1);
     fi;
-
+
+    # Local functions that print the user feedback.
+    printEarlyMessage := function(stream, earlyMessage)
+        PrintTo(stream, "Error, ");
+        # earlyMessage usually contains information about what went wrong.
+        for x in earlyMessage do
+            PrintTo(stream, x);
+        od;
+    end;
+
+    printEarlyTraceback := function(stream, context, printThisStatement)
+        local location;
+        if printThisStatement then
+            if context <> GetBottomLVars() then
+                PrintTo(stream, " in\n  ");
+                PRINT_CURRENT_STATEMENT(stream, context);
+                PrintTo(stream, " called from ");
+            fi;
+        else
+            location := CURRENT_STATEMENT_LOCATION(context);
+            if location <> fail then
+              PrintTo(stream, " at ", location[1], ":", location[2]);
+            fi;
+            PrintTo(stream, " called from");
+        fi;
+        PrintTo(GAP_ERROR_STREAM, "\n");
+    end;
+
     ErrorLevel := ErrorLevel+1;
     ERROR_COUNT := ERROR_COUNT+1;
     errorLVars := ErrorLVars;
     ErrorLVars := context;
+    # Do we want to skip the break loop?
+    # BreakOnError is initialized by the `-T` command line flag in init.g
     if QUITTING or not BreakOnError then
-        PrintTo(GAP_ERROR_STREAM,"Error, ");
-        for x in earlyMessage do
-            PrintTo(GAP_ERROR_STREAM,x);
-        od;
-        PrintTo(GAP_ERROR_STREAM,"\n");
+        # If we skip the break loop, the standard behaviour is to print only
+        # the earlyMessage. If SilentNonInteractiveErrors is true we do not
+        # print any messages. If AlwaysPrintTracebackOnError is true we also
+        # call OnBreak(), which by default prints the traceback.
+        # SilentNonInteractiveErrors superseeds AlwaysPrintTracebackOnError.
+        # It is used by HPC-GAP to e.g. suppress error messages in worker
+        # threads.
+        if not SilentNonInteractiveErrors then
+            printEarlyMessage(GAP_ERROR_STREAM, earlyMessage);
+            if AlwaysPrintTracebackOnError then
+                printEarlyTraceback(GAP_ERROR_STREAM, context, printThisStatement);
+                if IsBound(OnBreak) and IsFunction(OnBreak) then
+                    OnBreak();
+                fi;
+            else
+                PrintTo(GAP_ERROR_STREAM, "\n");
+            fi;
+        fi;
+        if IsHPCGAP then
+            # In HPC-GAP we want to access error messages encountered in
+            # tasks via TaskError. To this end we store the error message
+            # in the thread local variable LastErrorMessage.
+            LastErrorMessage := "";
+            lastErrorStream := OutputTextString(LastErrorMessage, true);
+            printEarlyMessage(lastErrorStream, earlyMessage);
+            if AlwaysPrintTracebackOnError then
+                printEarlyTraceback(lastErrorStream, context, printThisStatement);
+                # FIXME: Also make HPCGAP work with OnBreak().
+                # If AlwaysPrintTracebackOnError is true, the output of
+                # OnBreak() should also be put into LastErrorMessage.
+                # To do this there needs to be a way to put its output
+                # into lastErrorStream.
+                # OnBreak() is documented to not take any arguments.
+                # One could work around that if there were e.g. a GAP error
+                # stream which all error functions print to.
+            fi;
+            CloseStream(lastErrorStream);
+            MakeImmutable(LastErrorMessage);
+        fi;
         ErrorLevel := ErrorLevel-1;
         ErrorLVars := errorLVars;
         if ErrorLevel = 0 then LEAVE_ALL_NAMESPACES(); fi;
         JUMP_TO_CATCH(0);
     fi;
-    PrintTo(GAP_ERROR_STREAM,"Error, ");
-    for x in earlyMessage do
-        PrintTo(GAP_ERROR_STREAM,x);
-    od;
-    if printThisStatement then 
-        if context <> GetBottomLVars() then
-            PrintTo(GAP_ERROR_STREAM," in\n  \c");
-            PRINT_CURRENT_STATEMENT(context);
-            Print("\c");
-            PrintTo(GAP_ERROR_STREAM," called from \n");
-        else
-            PrintTo(GAP_ERROR_STREAM,"\c\n");
-        fi;
-    else
-        location := CURRENT_STATEMENT_LOCATION(context);
-        if location <> fail then          PrintTo(GAP_ERROR_STREAM, " at ", location[1], ":", location[2]);
-        fi;
-        PrintTo(GAP_ERROR_STREAM," called from\c\n");
-    fi;
+
+    printEarlyMessage(GAP_ERROR_STREAM, earlyMessage);
+    printEarlyTraceback(GAP_ERROR_STREAM, context, printThisStatement);
 
     if SHOULD_QUIT_ON_BREAK() then
+        # Again, the default is to not print the rest of the traceback.
+        # If AlwaysPrintTracebackOnError is true we do so anyways.
+        if
+            AlwaysPrintTracebackOnError
+            and IsBound(OnBreak) and IsFunction(OnBreak)
+        then
+            OnBreak();
+        fi;
         FORCE_QUIT_GAP(1);
     fi;
 
+    # OnBreak() is set to Where() by default, which prints the traceback.
     if IsBound(OnBreak) and IsFunction(OnBreak) then
         OnBreak();
     fi;
+
+    # Now print lateMessage and OnBreakMessage a la "press return; to .."
     if IsString(lateMessage) then
-        PrintTo(GAP_ERROR_STREAM,lateMessage,"\n");
+        PrintTo(GAP_ERROR_STREAM, lateMessage,"\n");
     elif lateMessage then
         if IsBound(OnBreakMessage) and IsFunction(OnBreakMessage) then
             OnBreakMessage();
@@ -136,8 +242,16 @@ BIND_GLOBAL("ErrorInner",
     else
         prompt := "brk> ";
     fi;
+    shellOut := GAP_ERROR_STREAM;
+    shellIn := "*errin*";
+    if IsHPCGAP then
+        if HaveMultiThreadedUI then
+            shellOut := "*defout*";
+            shellIn := "*defin*";
+        fi;
+    fi;
     if not justQuit then
-        res := SHELL(context,mayReturnVoid,mayReturnObj,3,false,prompt,false,"*errin*",GAP_ERROR_STREAM,false);
+        res := SHELL(context,mayReturnVoid,mayReturnObj,3,false,prompt,false,shellIn,shellOut,false);
     else
         res := fail;
     fi;
@@ -149,8 +263,8 @@ BIND_GLOBAL("ErrorInner",
         fi;
         if ErrorLevel = 0 then LEAVE_ALL_NAMESPACES(); fi;
         if not justQuit then
-           # dont try and do anything else after this before the longjump       
-            SetUserHasQuit(1);  
+            # dont try and do anything else after this before the longjump
+            SetUserHasQuit(1);
         fi;
         JUMP_TO_CATCH(3);
     fi;