halscope: fix "loadrt scope_rt failed" when reopening the scope

[grandixximo]

Reopening halscope fails after the first time: the window never appears again and the terminal shows loadrt scope_rt failed. From AXIS (Machine > Hal Scope) it just looks like nothing happens until LinuxCNC is restarted.

Regression from d88d10d. scope.c checks whether the realtime half is already loaded before running loadrt scope_rt, and that check was converted from a funct lookup to a component lookup:

- if (!halpr_find_funct_by_name("scope.sample")) {
+ int rv = hal_comp_by_name("scope.sample", NULL);
+ if (-ENOENT == rv) {

There is no component named scope.sample. The component is scope_rt, scope.sample is the funct it exports, so the lookup always returns -ENOENT and halscope always runs halcmd loadrt scope_rt.

Closing halscope does not unload scope_rt, it stays resident and attached to the thread. On the next launch the loadrt therefore fails and halscope calls exit(1) before creating its window.

scope_horiz.c was converted correctly in the same commit, using hal_list_funct(). This does the same in scope.c, restoring the original semantics.

Tested against a live HAL under Xvfb, closing the window with WM_DELETE_WINDOW between launches:

	launch 1	launch 2	launch 3
before	opens	loadrt scope_rt failed, exits	exits
after	opens	opens	opens

scope.sample stays attached to the servo thread across all three, and the halscope component is removed from HAL on each close.

[BsAtHome]

BTW, good catch.

[grandixximo]

You build three times my docs, I open two times your scope...

Wouldn't it be great if the third time you try to open the scope it had yet another issue 😆

[BsAtHome]

I'd like to reach Bingo on the fourth time and an automatic payout from the machine :-)