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snap: fix readseg() to cope with > 2gb segments

--- a/sys/src/cmd/snap/snap.h

+++ b/sys/src/cmd/snap/snap.h

@@ -26,9 +26,9 @@

 struct Seg {

 	char*	name;

 	uvlong	offset;

-	uvlong	 len;

+	uvlong	len;

 	Page**	pg;

-	int	npg;

+	ulong	npg;

 };

 struct Page {

--- a/sys/src/cmd/snap/take.c

+++ b/sys/src/cmd/snap/take.c

@@ -96,47 +96,45 @@

 }

 static Seg*

-readseg(int fd, vlong off, ulong len, char *name)

+readseg(int fd, uvlong off, uvlong len, char *name)

 {

 	char buf[Pagesize];

+	ulong npg;

 	Page **pg;

-	int npg;

 	Seg *s;

-	ulong i;

 	int n;

 	s = emalloc(sizeof(*s));

 	s->name = estrdup(name);

-

 	if(seek(fd, off, 0) < 0) {

 		fprint(2, "seek fails\n");

 		goto Die;

 	}

-

+	s->offset = off;

+	s->len = 0;

 	pg = nil;

 	npg = 0;

-	for(i=0; i<len; ) {

+	while(s->len < len){

 		n = Pagesize;

-		if(n > len-i)

-			n = len-i;

+		if(n > len - s->len)

+			n = len - s->len;

 		if((n = readn(fd, buf, n)) <= 0)

 			break;

-		pg = erealloc(pg, sizeof(*pg)*(npg+1));

+		s->len += n;

+		if((npg & (npg-1)) == 0)

+			pg = erealloc(pg, sizeof(*pg) * (npg==0 | npg*2));

 		pg[npg++] = datapage(buf, n);

-		i += n;

 		if(n != Pagesize)	/* any short read, planned or otherwise */

 			break;

 	}

-

-	if(i==0 && len!=0)

+	if(s->len==0 && len!=0){

+		free(pg);

 		goto Die;

-

-	s->offset = off;

-	s->len = i;

+	}

+	pg = erealloc(pg, sizeof(*pg) * npg);

 	s->pg = pg;

 	s->npg = npg;

 	return s;

-

 Die:

 	free(s->name);

 	free(s);