exportfs -r without -B fails because it skips the initial protocol.

For example, executing this one on host A
% aux/listen1 -tv tcp!$addr!64470 /bin/exportfs -r /dev
and executing this on host B
% import -A tcp!$addr!64470 
will typically either fail to read the initial string or result in a bad tree.

I believe that exportfs should only skip the initial protocol with -B, -S, etc.
The fix entails reading the path requested by import and discarding it.
Notes:
Fri May 7 12:20:27 EDT 2010 geoff
    I believe exportfs is working as documented.


Reference: /n/sources/patch/sorry/exportfs-r-problem
Date: Wed Apr 21 03:59:16 CES 2010
Signed-off-by: jrw972@gmail.com
Reviewed-by: geoff

--- /sys/src/cmd/exportfs/exportfs.c	Wed Apr 21 03:59:12 2010
+++ /sys/src/cmd/exportfs/exportfs.c	Wed Apr 21 03:59:10 2010
@@ -200,7 +200,7 @@
 
 	if(na){
 		if(srv == nil)
-			sysfatal("-B requires -s");
+			sysfatal("-B requires -r");
 
 		local = "me";
 		remote = na;
@@ -253,7 +253,7 @@
 	if(srvfd != -1) {
 		/* do nothing */
 	}
-	else if(srv) {
+	else if(na) {
 		chdir(srv);
 		DEBUG(DFD, "invoked as server for %s", srv);
 		strncpy(buf, srv, sizeof buf);
@@ -269,6 +269,10 @@
 			exits(buf);
 		}
 		buf[n] = 0;
+		if(srv) {
+			/* Ignore requested tree. */
+			strncpy(buf, srv, sizeof buf);
+		}
 		if(chdir(buf) < 0) {
 			errstr(ebuf, sizeof ebuf);
 			fprint(0, "chdir(%d:\"%s\"): %s", n, buf, ebuf);
@@ -282,7 +286,7 @@
 
 	DEBUG(DFD, "exportfs: %s\n", buf);
 
-	if(srv == nil && srvfd == -1 && write(0, "OK", 2) != 2)
+	if(na == nil && srvfd == -1 && write(0, "OK", 2) != 2)
 		fatal("open ack write");
 
 	if (readn(netfd, &initial, sizeof(ulong)) < sizeof(ulong))