Bug-fix: don't assume mutexes are "recursive" (standard pthread mutexes aren't)

which caused lock-ups in RingBufWrite().

src/sbbs3/ringbuf.c

@@ -126,21 +126,24 @@ void RINGBUFCALL RingBufDispose( RingBuf* rb)
 	memset(rb,0,sizeof(RingBuf));
 }
 
+/* Non-mutex-protected (standard pthread mutexes are non-recursive) */
+static DWORD ringbuf_full(RingBuf* rb)
+{
+	if(rb->pHead >= rb->pTail)
+		return(rb->pHead - rb->pTail);
+
+	return(rb->size - (rb->pTail - (rb->pHead + 1)));
+}
+
 DWORD RINGBUFCALL RingBufFull( RingBuf* rb )
 {
-	DWORD	head,tail,retval;
+	DWORD	retval;
 
 #ifdef RINGBUF_MUTEX
 	pthread_mutex_lock(&rb->mutex);
 #endif
 
-	head = (DWORD) rb->pHead;
-	tail = (DWORD) rb->pTail;
-
-	if(head >= tail)
-		retval = head - tail;
-	else
-		retval = rb->size - (tail - (head + 1));
+	retval = ringbuf_full(rb);
 
 #ifdef RINGBUF_EVENT
 	if(rb->empty_event!=NULL) {
@@ -210,7 +213,7 @@ DWORD RINGBUFCALL RingBufWrite( RingBuf* rb, BYTE* src,  DWORD cnt )
 
 #ifdef RINGBUF_SEM
 	sem_post(&rb->sem);
-	if(rb->highwater_mark!=0 && RingBufFull(rb)>=rb->highwater_mark)
+	if(rb->highwater_mark!=0 && ringbuf_full(rb)>=rb->highwater_mark)
 		sem_post(&rb->highwater_sem);
 #endif
 #ifdef RINGBUF_EVENT