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plan9/snd: fix race condition

reported only on amd64 and 386, did not occur on arm64.

--- a/snd_plan9.c

+++ b/snd_plan9.c

@@ -1,9 +1,13 @@

 #include "quakedef.h"

 #include <thread.h>

+typedef struct Mixbuf Mixbuf;

+struct Mixbuf{

+	byte *buf;

+	long sz;

+};

+

 static int afd;

-static QLock alock;

-static uchar *mixbuf;

 static Channel *ach;

 long

@@ -20,15 +24,14 @@

 static void

 auproc(void *p)

 {

-	long sz, n;

+	Mixbuf m;

+	long n;

 	for(;;){

-		if((sz = recvul(p)) < 1)

+		if(recv(p, &m) < 0)

 			break;

-		qlock(&alock);

-		n = write(afd, mixbuf, sz);

-		qunlock(&alock);

-		if(n != sz){

+		n = write(afd, m.buf, m.sz);

+		if(n != m.sz){

 			Con_DPrintf("sndwrite: %r\n");

 			break;

 		}

@@ -44,12 +47,12 @@

 void

 sndwrite(uchar *buf, long sz)

 {

+	Mixbuf m;

+

 	if(ach == nil)

 		return;

-	qlock(&alock);

-	sendul(ach, sz);

-	mixbuf = buf;

-	qunlock(&alock);

+	m = (Mixbuf){buf, sz};

+	send(ach, &m);

 }

 void

@@ -67,7 +70,7 @@

 {

 	if((afd = open("/dev/audio", OWRITE)) < 0)

 		return -1;

-	ach = chancreate(sizeof(ulong), 0);

+	ach = chancreate(sizeof(Mixbuf), 1);

 	proccreate(auproc, ach, 4096);

 	return 0;

 }