iov: reorganize iov_send_recv, part 2

Do not touch the "bytes" argument anymore.  Instead, remember the
original length of the last iovec if we touch it, and restore it
afterwards.

This requires undoing the changes in opposite order.  The previous
algorithm didn't care.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Orit Wassermann <owasserm@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

util/iov.c

@@ -145,7 +145,9 @@ ssize_t iov_send_recv(int sockfd, struct iovec *iov, unsigned iov_cnt,
                       bool do_send)
 {
     ssize_t ret;
+    size_t orig_len, tail;
     unsigned si, ei;            /* start and end indexes */
+
     if (bytes == 0) {
         /* Catch the do-nothing case early, as otherwise we will pass an
          * empty iovec to sendmsg/recvmsg(), and not all implementations
@@ -174,31 +176,29 @@ ssize_t iov_send_recv(int sockfd, struct iovec *iov, unsigned iov_cnt,
     }
     /* Find the end position skipping `bytes' bytes: */
     /* first, skip all full-sized elements */
-    for (ei = 0; ei < iov_cnt && iov[ei].iov_len <= bytes; ++ei) {
-        bytes -= iov[ei].iov_len;
+    tail = bytes;
+    for (ei = 0; ei < iov_cnt && iov[ei].iov_len <= tail; ++ei) {
+        tail -= iov[ei].iov_len;
     }
-    if (bytes) {
-        /* second, fixup the last element, and remember
-         * the length we've cut from the end of it in `bytes' */
-        size_t tail;
+    if (tail) {
+        /* second, fixup the last element, and remember the original
+         * length */
         assert(ei < iov_cnt);
-        assert(iov[ei].iov_len > bytes);
-        tail = iov[ei].iov_len - bytes;
-        iov[ei].iov_len = bytes;
-        bytes = tail;  /* bytes is now equal to the tail size */
-        ++ei;
+        assert(iov[ei].iov_len > tail);
+        orig_len = iov[ei].iov_len;
+        iov[ei++].iov_len = tail;
     }
 
     ret = do_send_recv(sockfd, iov, ei, do_send);
 
     /* Undo the changes above */
+    if (tail) {
+        iov[ei-1].iov_len = orig_len;
+    }
     if (offset) {
         iov[0].iov_base -= offset;
         iov[0].iov_len += offset;
     }
-    if (bytes) {
-        iov[ei-1].iov_len += bytes;
-    }
 
     return ret;
 }