TaskWindow: SWIs don't return on TaskWindow exit

Consider this loop, in a module command handler so running in (para-virtualised) supervisor mode:

char buff[256];
int c;
int in = -1;
while (true) {

  // Copy everything from pseudo-terminal master to RISC OS output stream.
  while(true) {
    c = ix_read(ptmx, buff, sizeof(buff));
    if (c <= 0) break;
    ix_MESSAGE(2,"[");
    error = _swix(OS_WriteN, _INR(0, 1), buff, c);
    ix_MESSAGE(2,"]");
    if (error) break;
  }

  // Exit if error or slave end closed
  if (error || c != -ix_EAGAIN) break;

  // Copy character from RISC OS output stream to pseudo-terminal
  // using "in" as a 1 byte buffer.
  if (in < 0) {
    int flag;
    ix_MESSAGE(2,"<");
    error = _swix(OS_Byte, _INR(0, 2) | _OUTR(1, 2), OsByte_INKEY, 1, 0, &in, &flag);
    ix_MESSAGE(2,">");
    if (error) break;
    switch(flag) {
      case 27:
        in = 3; // Escape becomes CTRL C
        ix_MESSAGE(2,"Escape in IXRun\n");
        _kernel_osbyte(OsByte_AcknowledgeEscape, 0, 0);
        break;
      case 255:
        in = -1;
        break;
    }
  }
  if (in >= 0) {
    c = ix_write(ptmx, &in, 1);
    if (c == 1) in = -1;
  }
}
ix_close(ptmx);
int wstatus;
ix_waitpid(pid, &wstatus, 0);

If I run it in a TaskWindow, and close the TaskWindow whilst it running either OS_Byte or OS_WriteN does not return, the thread just disappears without trace, ix_close(ptmx) isn’t executed and one or more Linux processes continue to run.

I made a portable test case, expected behaviour is that a TaskWindow should fail to close.

DIM C% 256
P%=C%
[
SWI "XOS_EnterOS"
SWI "XOS_ReadC"
B C%
]
CALL C%

It appears from the TaskWindow source code, that ErrorBlock_TaskWindow_Dying should be returned, but what should happen after the supervisor stack is unwound?

And yes, this is the same function as last weeks bug. :-)

That’s interesting. To restate the issue, if you run the BASIC program in a task window, and then select Quit from the task manager, the task doesn’t quit.

If we go

  SWI XOS_EnterOS
.a
  SWI XOS_ReadC
  SWI XReport_Regs
  B   a

we see that the SWI does return the error as expected. To make it return each time you have to press a key again. (MsgMon is also useful for looking at this kind of thing.)

The task will not exit whilst it remains in supervisor mode; you’d have to do SWI XOS_LeaveOS.

In terms of your C code, can you give a similar portable assembler example that shows the problem with OS_WriteN or OS_Byte INKEY since there’s no issue with OS_ReadC? I had a go with INKEY and it seemed to return.

I think I’ve tracked it down, Message_Quit, which is sent by the TaskManager works but TaskWindow_Morite, which is sent by Zap and Edit when closing the window doesn’t.

It erroneously immediately calling OS_Exit when a TaskWindow_Morite message was received. I’ve submitted a simple fix:

diff --git a/s/Taskman b/s/Taskman
--- a/s/Taskman
+++ b/s/Taskman
@@ -4573,11 +4573,7 @@ Null
 50
         LDR     r2, =Morite
         TEQ     r0, r2            ; Is it die?
-        BNE     %f60              ; Branch if not
-        ASSERT  Morite <> 0
-        STR     r0, Moribund      ; Remember if so
-        B       abex_exit
-60
+        BEQ     %b20              ; Branch if so
         LDR     r2, =Suspend      ; Is it suspend?
         TEQ     r0, r2
         STREQ   r0, Suspended     ; Now suspended
--