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libdraw: make ebread() return buffer immidiately if available, cleanup

this reduces number of syscalls and improves performance for vt

--- a/sys/src/libdraw/event.c

+++ b/sys/src/libdraw/event.c

@@ -45,21 +45,10 @@

 ebread(Slave *s)

 {

 	Ebuf *eb;

-	Dir *d;

-	ulong l;

-

-	for(;;){

-		d = dirfstat(epipe[0]);

-		if(d == nil)

-			drawerror(display, "events: eread stat error");

-		l = d->length;

-		free(d);

-		if(s->head && l==0)

-			break;

+ 

+	while((eb = s->head) == 0)

 		extract();

-	}

-	eb = s->head;

-	s->head = s->head->next;

+	s->head = eb->next;

 	if(s->head == 0)

 		s->tail = 0;

 	return eb;

@@ -322,9 +311,10 @@

 	memmove(eb->buf, &ebuf[1], n - 1);

 	eb->next = 0;

 	if(s->head)

-		s->tail = s->tail->next = eb;

+		s->tail->next = eb;

 	else

-		s->head = s->tail = eb;

+		s->head = eb;

+	s->tail = eb;

 }

 static int

-- 

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