add RFNOWAIT. this was probably a bad idea.

src/lib9/mkfile

@@ -118,6 +118,7 @@ LIB9OFILES=\
 	mallocz.$O\
 	nan.$O\
 	needsrcquote.$O\
+	needstack.$O\
 	netmkaddr.$O\
 	notify.$O\
 	nrand.$O\

src/lib9/rfork.c

@@ -1,4 +1,6 @@
 #include <u.h>
+#include <sys/wait.h>
+#include <signal.h>
 #include <libc.h>
 #include "9proc.h"
 #undef rfork
@@ -6,16 +8,71 @@
 int
 p9rfork(int flags)
 {
-	int pid;
+	int pid, status;
+	int p[2];
+	int n;
+	char buf[128], *q;
 
+	_p9uproc(0);
 	if((flags&(RFPROC|RFFDG|RFMEM)) == (RFPROC|RFFDG)){
 		/* check other flags before we commit */
 		flags &= ~(RFPROC|RFFDG);
-		if(flags & ~(RFNOTEG|RFNAMEG)){
+		n = (flags & ~(RFNOTEG|RFNAMEG|RFNOWAIT));
-			werrstr("unknown flags %08ux in rfork", flags);
+		if(n){
+			werrstr("unknown flags %08ux in rfork", n);
 			return -1;
 		}
+		if(flags&RFNOWAIT){
+			if(pipe(p) < 0)
+				return -1;
+		}
 		pid = fork();
+		if(pid == -1)
+			return -1;
+		if(flags&RFNOWAIT){
+			flags &= ~RFNOWAIT;
+			if(pid){
+				/*
+				 * Parent - wait for child to fork wait-free child.
+				 * Then read pid from pipe.  Assume pipe buffer can absorb the write.
+				 */
+				close(p[1]);
+				wait4(pid, &status, 0, 0);
+				n = readn(p[0], buf, sizeof buf-1);
+				close(p[0]);
+				if(!WIFEXITED(status) || WEXITSTATUS(status)!=0 || n <= 0){
+					werrstr("pipe dance failed in rfork");
+					return -1;
+				}
+				buf[n] = 0;
+				n = strtol(buf, &q, 0);
+				if(*q != 0){
+					werrstr("%s", q);
+					return -1;
+				}
+				pid = n;
+			}else{
+				/*
+				 * Child - fork a new child whose wait message can't 
+				 * get back to the parent because we're going to exit!
+				 */
+				signal(SIGCHLD, SIG_IGN);
+				close(p[0]);
+				pid = fork();
+				if(pid){
+					/* Child parent - send status over pipe and exit. */
+					if(pid > 0)
+						fprint(p[1], "%d", pid);
+					else
+						fprint(p[1], " %r");
+					close(p[1]);
+					_exit(0);
+				}else{
+					/* Child child - close pipe. */
+					close(p[1]);
+				}
+			}
+		}
 		_p9uproc(0);
 		if(pid != 0)
 			return pid;
@@ -32,6 +89,10 @@ p9rfork(int flags)
 		setpgid(0, getpid());
 		flags &= ~RFNOTEG;
 	}
+	if(flags&RFNOWAIT){
+		werrstr("cannot use RFNOWAIT without RFPROC");
+		return -1;
+	}
 	if(flags){
 		werrstr("unknown flags %08ux in rfork", flags);
 		return -1;