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.

include/thread.h

@@ -15,6 +15,7 @@ void		threadexits(char *);
 void		threadexitsall(char *);
 void		threadsetname(char*, ...);
 void		threadsetstate(char*, ...);
+void		threadneedbackground(void);
 char		*threadgetname(void);
 int			threadyield(void);
 int			threadidle(void);
@@ -60,6 +61,8 @@ void		**threaddata(void);
 void		threadmain(int argc, char *argv[]);
 extern	int	mainstacksize;
 
+int	threadmaybackground(void);
+
 /*
  * channel communication
  */

man/man3/thread.3

@@ -33,6 +33,7 @@ threadintgrp,
 threadkill,
 threadkillgrp,
 threadmain,
+threadmaybackground,
 threadnotify,
 threadid,
 threadpid,
@@ -80,6 +81,7 @@ struct Alt {
 .ft L
 .ta \w'\fLChannel* 'u +4n +4n +4n +4n
 void	threadmain(int argc, char *argv[])
+int	threadmaybackground(void)
 int	mainstacksize
 int	proccreate(void (*fn)(void*), void *arg, uint stacksize)
 int	threadcreate(void (*fn)(void*), void *arg, uint stacksize)
@@ -238,6 +240,14 @@ When the last thread in
 .IR threadmain 's
 proc exits, the program will appear to its parent to have exited.
 The remaining procs will still run together, but as a background program.
+This functionality can only be relied upon if the program defines a function
+.I threadmaybackground
+returning a non-zero result.
+Programs that do not define such a
+.I threadmaybackground
+will crash instead should the last thread in
+.IR threadmain 's
+proc exit leaving behind other running procs.
 .PP
 The threads in a proc are coroutines, scheduled nonpreemptively
 in a round-robin fashion.

src/cmd/9pfuse/main.c

@@ -98,6 +98,12 @@ usage(void)
 void fusereader(void*);
 void watchfd(void*);
 
+int
+threadmaybackground(void)
+{
+	return 1;
+}
+
 void
 threadmain(int argc, char **argv)
 {

src/cmd/9pserve.c

@@ -137,6 +137,12 @@ usage(void)
 	threadexitsall("usage");
 }
 
+int
+threadmaybackground(void)
+{
+	return 1;
+}
+
 uchar vbuf[128];
 extern int _threaddebuglevel;
 void

src/cmd/auth/factotum/main.c

@@ -20,6 +20,12 @@ usage(void)
 	threadexitsall("usage");
 }
 
+int
+threadmaybackground(void)
+{
+	return 1;
+}
+
 void
 threadmain(int argc, char *argv[])
 {

src/cmd/auth/ssh-agent.c

@@ -90,6 +90,12 @@ usage(void)
 	threadexitsall("usage");
 }
 
+int
+threadmaybackground(void)
+{
+	return 1;
+}
+
 void
 threadmain(int argc, char **argv)
 {

src/cmd/fossil/fossil.c

@@ -59,6 +59,12 @@ readCmdPart(char *file, char ***pcmd, int *pncmd)
 	*pncmd = ncmd;
 }
 
+int
+threadmaybackground(void)
+{
+	return 1;
+}
+
 void
 threadmain(int argc, char* argv[])
 {

src/cmd/import.c

@@ -51,6 +51,12 @@ fatal(char *fmt, ...)
 	threadexitsall("fatal");
 }
 
+int
+threadmaybackground(void)
+{
+	return 1;
+}
+
 void
 threadmain(int argc, char *argv[])
 {

src/cmd/ndb/dns.c

@@ -121,6 +121,12 @@ checkaddress(void)
 		fprint(2, "warning: announce mismatch %s %s\n", udpaddr, tcpaddr);
 }
 
+int
+threadmaybackground(void)
+{
+	return 1;
+}
+
 void
 threadmain(int argc, char *argv[])
 {

src/cmd/plumb/plumber.c

@@ -26,6 +26,12 @@ makeports(Ruleset *rules[])
 		addport(rules[i]->port);
 }
 
+int
+threadmaybackground(void)
+{
+	return 1;
+}
+
 void
 threadmain(int argc, char *argv[])
 {

src/cmd/smugfs/main.c

@@ -51,6 +51,12 @@ smuglogin(void)
 	printerrors = 0;
 }
 
+int
+threadmaybackground(void)
+{
+	return 1;
+}
+
 void
 threadmain(int argc, char **argv)
 {

src/cmd/upas/fs/fs.c

@@ -155,6 +155,12 @@ notifyf(void *a, char *s)
 	noted(NDFLT);
 }
 
+int
+threadmaybackground(void)
+{
+	return 1;
+}
+
 void
 threadmain(int argc, char *argv[])
 {

src/cmd/upas/nfs/main.c

@@ -26,6 +26,12 @@ usage(void)
 	threadexitsall("usage");
 }
 
+int
+threadmaybackground(void)
+{
+	return 1;
+}
+
 void
 threadmain(int argc, char **argv)
 {

src/cmd/venti/srv/venti.c

@@ -23,6 +23,12 @@ usage(void)
 	threadexitsall("usage");
 }
 
+int
+threadmaybackground(void)
+{
+	return 1;
+}
+
 void
 threadmain(int argc, char *argv[])
 {

src/lib9p/ramfs.c

@@ -125,6 +125,12 @@ usage(void)
 	threadexitsall("usage");
 }
 
+int
+threadmaybackground(void)
+{
+	return 1;
+}
+
 void
 threadmain(int argc, char **argv)
 {

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){
+	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;