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devproc: make sure /proc/n/wait waits for the right process children

theres a race when we wait for a process children and that
process exits before we sleep().

--- a/sys/src/9/port/devproc.c

+++ b/sys/src/9/port/devproc.c

@@ -680,12 +680,23 @@

 	return tproduced > tconsumed;

 }

+static int

+prochaswaitq(void *x)

+{

+	Chan *c;

+	Proc *p;

+

+	c = (Chan *)x;

+	p = proctab(SLOT(c->qid));

+	return p->pid != PID(c->qid) || p->waitq != 0;

+}

+

 static long

 procread(Chan *c, void *va, long n, vlong off)

 {

 	/* NSEG*32 was too small for worst cases */

 	char *a, flag[10], *sps, *srv, statbuf[NSEG*64];

-	int i, j, m, navail, ne, pid, rsize;

+	int i, j, m, navail, ne, rsize;

 	long l;

 	uchar *rptr;

 	ulong offset;

@@ -924,18 +935,19 @@

 		}

 		lock(&p->exl);

-		pid = p->pid;

-		while(p->waitq == 0) {

+		while(p->waitq == 0 && p->pid == PID(c->qid)) {

 			if(up == p && p->nchild == 0) {

 				unlock(&p->exl);

 				error(Enochild);

 			}

 			unlock(&p->exl);

-			sleep(&p->waitr, haswaitq, p);

-			if(p->pid != pid)

-				error(Eprocdied);

+			sleep(&p->waitr, prochaswaitq, c);

 			lock(&p->exl);

 		}

+		if(p->pid != PID(c->qid)){

+			unlock(&p->exl);

+			error(Eprocdied);

+		}

 		wq = p->waitq;

 		p->waitq = wq->next;

 		p->nwait--;

@@ -1229,11 +1241,12 @@

 		error(Einuse);

 	if(procstopped(p) || p->state == Broken)

 		return;

-

+	pid = p->pid;

+	if(pid == 0)

+		error(Eprocdied);

 	if(ctl != 0)

 		p->procctl = ctl;

 	p->pdbg = up;

-	pid = p->pid;

 	qunlock(&p->debug);

 	up->psstate = "Stopwait";

 	if(waserror()) {

--- a/sys/src/9/port/portfns.h

+++ b/sys/src/9/port/portfns.h

@@ -124,7 +124,6 @@

 ulong		getrealloctag(void*);

 void		gotolabel(Label*);

 char*		getconfenv(void);

-int		haswaitq(void*);

 long		hostdomainwrite(char*, int);

 long		hostownerwrite(char*, int);

 void		hzsched(void);

--- a/sys/src/9/port/proc.c

+++ b/sys/src/9/port/proc.c

@@ -1205,7 +1205,7 @@

 	panic("pexit");

 }

-int

+static int

 haswaitq(void *x)

 {

 	Proc *p;