[PULL 06/15] virtiofsd: Simplify skip byte logic

["Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>]

From: Vivek Goyal <vgoyal@redhat.com>

We need to skip bytes in two cases.

a. Before we start reading into in_sg, we need to skip iov_len bytes
   in the beginning which typically will have fuse_out_header.

b. If preadv() does a short read, then we need to retry preadv() with
   remainig bytes and skip the bytes preadv() read in short read.

For case a, there is no reason that skipping logic be inside the while
loop. Move it outside. And only retain logic "b" inside while loop.

Also get rid of variable "skip_size". Looks like we can do without it.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Connor Kuehl <ckuehl@redhat.com>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Message-Id: <20210518213538.693422-6-vgoyal@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 tools/virtiofsd/fuse_virtio.c | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/tools/virtiofsd/fuse_virtio.c b/tools/virtiofsd/fuse_virtio.c
index ed5146d7a6..49c7dd788a 100644
--- a/tools/virtiofsd/fuse_virtio.c
+++ b/tools/virtiofsd/fuse_virtio.c
@@ -392,17 +392,11 @@ int virtio_send_data_iov(struct fuse_session *se, struct fuse_chan *ch,
     unsigned int in_sg_cpy_count = in_num;
 
     /* skip over parts of in_sg that contained the header iov */
-    size_t skip_size = iov_len;
+    iov_discard_front(&in_sg_ptr, &in_sg_cpy_count, iov_len);
 
     do {
-        if (skip_size != 0) {
-            iov_discard_front(&in_sg_ptr, &in_sg_cpy_count, skip_size);
-        }
-
-        fuse_log(FUSE_LOG_DEBUG,
-                 "%s: after skip skip_size=%zd in_sg_cpy_count=%d "
-                 "len remaining=%zd\n", __func__, skip_size, in_sg_cpy_count,
-                 len);
+        fuse_log(FUSE_LOG_DEBUG, "%s: in_sg_cpy_count=%d len remaining=%zd\n",
+                 __func__, in_sg_cpy_count, len);
 
         ret = preadv(buf->buf[0].fd, in_sg_ptr, in_sg_cpy_count,
                      buf->buf[0].pos);
@@ -421,7 +415,7 @@ int virtio_send_data_iov(struct fuse_session *se, struct fuse_chan *ch,
         if (ret < len && ret) {
             fuse_log(FUSE_LOG_DEBUG, "%s: ret < len\n", __func__);
             /* Skip over this much next time around */
-            skip_size = ret;
+            iov_discard_front(&in_sg_ptr, &in_sg_cpy_count, ret);
             buf->buf[0].pos += ret;
             len -= ret;
 
-- 
2.31.1