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9nfs: fix rfork() race with shared pid variable

--- a/sys/src/cmd/9nfs/chat.c

+++ b/sys/src/cmd/9nfs/chat.c

@@ -26,7 +26,7 @@

 void

 chatsrv(char *name)

 {

-	int n, sfd, pfd[2];

+	int n, sfd, pid, pfd[2];

 	char *p, buf[256];

 	if(name && *name)

@@ -44,13 +44,14 @@

 	sfd = create(nbuf, OWRITE, 0600);

 	if(sfd < 0)

 		panic("chatsrv create %s", nbuf);

-	chatpid = rfork(RFPROC|RFMEM);

-	switch(chatpid){

+	pid = rfork(RFPROC|RFMEM);

+	switch(pid){

 	case -1:

 		panic("chatsrv fork");

 	case 0:

 		break;

 	default:

+		chatpid = pid;

 		atexit(killchat);

 		return;

 	}