make mkfs cope with growing or shrinking files in the source tree
by copying exactly the number of bytes it claimed in the archive header
it wrote (as tar does), padding or truncating as necessary.

Reference: /n/sources/patch/applied/keep-mkfs-sync
Date: Wed Feb  9 08:32:46 CET 2005

--- /sys/src/cmd/disk/mkfs.c	Wed Feb  9 08:31:10 2005
+++ /sys/src/cmd/disk/mkfs.c	Wed Feb  9 08:31:07 2005
@@ -345,8 +345,8 @@
 copy(Dir *d)
 {
 	char cptmp[LEN], *p;
-	long tot;
-	int f, t, n, needwrite;
+	int f, t, n, needwrite, nowarnyet = 1;
+	vlong tot, len;
 	Dir nd;
 
 	f = open(oldfile, OREAD);
@@ -372,14 +372,24 @@
 	}
 
 	needwrite = 0;
-	for(tot = 0;; tot += n){
-		n = read(f, buf, buflen);
-		if(n < 0){
-			warn("can't read %q: %r", oldfile);
-			break;
+	for(tot = 0; tot < d->length; tot += n){
+		len = d->length - tot;
+		/* don't read beyond d->length */
+		if (len > buflen)
+			len = buflen;
+		n = read(f, buf, len);
+		if(n <= 0) {
+			if(n < 0 && nowarnyet) {
+				warn("can't read %q: %r", oldfile);
+				nowarnyet = 0;
+			}
+			/*
+			 * don't quit: pad to d->length (in pieces) to agree
+			 * with the length in the header, already emitted.
+			 */
+			memset(buf, 0, len);
+			n = len;
 		}
-		if(n == 0)
-			break;
 		if(fskind == Archive){
 			if(Bwrite(&bout, buf, n) != n)
 				error("write error: %r");
@@ -399,10 +409,12 @@
 			error("can't write zero at end of %q: %r", newfile);
 	}
 	if(tot != d->length){
-		warn("wrong number bytes written to %q (was %d should be %d)\n",
+		/* this should no longer happen */
+		warn("wrong number of bytes written to %q (was %lld should be %lld)\n",
 			newfile, tot, d->length);
 		if(fskind == Archive){
 			warn("seeking to proper position\n");
+			/* does no good if stdout is a pipe */
 			Bseek(&bout, d->length - tot, 1);
 		}
 	}