nbd: Don't inf-loop on early EOF

Commit 7d3123e1 converted a single read_sync() into a while loop
that assumed that read_sync() would either make progress or give
an error. But when the server hangs up early, the client sees
EOF (a read_sync() of 0) and never makes progress, which in turn
caused qemu-iotest './check -nbd 83' to go into an infinite loop.

Rework the loop to accomodate reads cut short by EOF.
Reported-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <1478551093-32757-1-git-send-email-eblake@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

nbd/client.c

@@ -90,20 +90,21 @@ static QTAILQ_HEAD(, NBDExport) exports = QTAILQ_HEAD_INITIALIZER(exports);
  * the amount of bytes consumed. */
 static ssize_t drop_sync(QIOChannel *ioc, size_t size)
 {
-    ssize_t ret, dropped = size;
+    ssize_t ret = 0;
     char small[1024];
     char *buffer;
 
     buffer = sizeof(small) < size ? small : g_malloc(MIN(65536, size));
     while (size > 0) {
-        ret = read_sync(ioc, buffer, MIN(65536, size));
-        if (ret < 0) {
+        ssize_t count = read_sync(ioc, buffer, MIN(65536, size));
+
+        if (count <= 0) {
             goto cleanup;
         }
-        assert(ret <= size);
-        size -= ret;
+        assert(count <= size);
+        size -= count;
+        ret += count;
     }
-    ret = dropped;
 
  cleanup:
     if (buffer != small) {