libthread: add threadmaybackground

Programs that want to background themselves now need
to define threadmaybackground returning 1.
This avoids a confusing (to people and debuggers)
extra parent process for all the threaded programs
that will never want to background themselves.

src/libthread/bg.c

@@ -0,0 +1,7 @@
+#include "threadimpl.h"
+
+int
+threadmaybackground(void)
+{
+	return 0;
+}

src/libthread/daemonize.c

@@ -8,7 +8,7 @@
 #undef wait
 
 static int sigpid;
-static int threadpassfd;
+static int threadpassfd = -1;
 static int gotsigchld;
 
 static void
@@ -163,9 +163,9 @@ _threadsetupdaemonize(void)
 void
 _threaddaemonize(void)
 {
-	if(threadpassfd >= 0){
-		write(threadpassfd, "0", 1);
-		close(threadpassfd);
-		threadpassfd = -1;
-	}
+	if(threadpassfd < 0)
+		sysfatal("threads in main proc exited w/o threadmaybackground");
+	write(threadpassfd, "0", 1);
+	close(threadpassfd);
+	threadpassfd = -1;
 }

src/libthread/mkfile

@@ -4,6 +4,7 @@ SYSOFILES=`{sh ./sysofiles.sh}
 LIB=libthread.a
 OFILES=\
 	$SYSOFILES\
+	bg.$O\
 	channel.$O\
 	daemonize.$O\
 	exec.$O\

src/libthread/thread.c

@@ -844,7 +844,7 @@ main(int argc, char **argv)
 	// Easier to just run in pthread-per-thread mode.
 	pthreadperthread = 1;
 #endif
-	if(strstr(opts, "nodaemon") == nil && getenv("NOLIBTHREADDAEMONIZE") == nil)
+	if(threadmaybackground() && strstr(opts, "nodaemon") == nil && getenv("NOLIBTHREADDAEMONIZE") == nil)
 		_threadsetupdaemonize();
 
 	threadargc = argc;