Fix Lua scriptlet runaway child detection

Commit a810765 moved the detection
to parent using waitpid(), but this causes it to complain on any
process in the same process group. In the usual rpm context we don't
have any, but we can't very well tell API users not to have any
children. And then failed to detect a runaway Lua child in one of
our own tests. Uff.

rpmlog() in the child has the issues mentioned in the originating
commit, but that's a problem that needs to be solved elsewhere.
Revert back to issuing a warning when we actually *are* in the child
process, so there are no false positive possible. Use EXIT_FAILURE
like in the original version, dunno why I'd changed that.

Update the rpmlua test to expect the deserved warning, and use
stdio for printing its "normal" output, the catch there is that we
need to flush the io in the child.

Reported at https://bugzilla.redhat.com/show_bug.cgi?id=2254463

rpmio/rpmlua.c

@@ -239,17 +239,14 @@ static int luaopt(int c, const char *oarg, int oint, void *data)
 static int rpm_pcall(lua_State *L, int nargs, int nresults, int errfunc)
 {
     pid_t pid = getpid();
-    int status;
 
     int rc = lua_pcall(L, nargs, nresults, errfunc);
 
     /* Terminate unhandled fork from Lua script */
-    if (pid != getpid())
-	_exit(1);
-
-    if (waitpid(0, &status, WNOHANG) == 0) {
-	rpmlog(RPMLOG_WARNING,
-		_("runaway fork() in Lua script\n"));
+    if (pid != getpid()) {
+	fprintf(stderr, _("warning: runaway fork() in Lua script\n"));
+	fflush(stderr);
+	_exit(EXIT_FAILURE);
     }
 
     return rc;

tests/rpmmacro.at

@@ -1322,13 +1322,13 @@ nil	No such file or directory	2.0
 ],
 [])
 
-# This uses io.stderr:write() because the grue from the previous test 
-# appears to have eaten stdout of the forked child, or something.
 RPMTEST_CHECK([
-runroot_other rpmlua -e 'pid = posix.fork(); if pid == 0 then a,b,c=rpm.redirect2null(-1); io.stderr:write(string.format("%s\t%s\t%s\n", a,b,c)) else posix.wait(pid) end'
+runroot_other rpmlua -e 'pid = posix.fork(); if pid == 0 then a,b,c=rpm.redirect2null(-1); print(string.format("%s\t%s\t%s\n", a,b,c));io.flush() else posix.wait(pid) end'
 ],
 [0],
-[],
 [nil	Bad file descriptor	9.0
-])
+],
+[warning: runaway fork() in Lua script
+]
+)
 RPMTEST_CLEANUP