[PATCH for-6.2 1/2] nbd/server: Don't complain on certain client disconnects

[Eric Blake <eblake@redhat.com>]

When a client disconnects abruptly, but did not have any pending
requests (for example, when using nbdsh without calling h.shutdown),
we used to output the following message:

$ qemu-nbd -f raw file
$ nbdsh -u 'nbd://localhost:10809' -c 'h.trim(1,0)'
qemu-nbd: Disconnect client, due to: Failed to read request: Unexpected end-of-file before all bytes were read

Then in commit f148ae7, we refactored nbd_receive_request() to use
nbd_read_eof(); when this returns 0, we regressed into tracing
uninitialized memory (if tracing is enabled) and reporting a
less-specific:

qemu-nbd: Disconnect client, due to: Request handling failed in intermediate state

Note that with Unix sockets, we have yet another error message,
unchanged by the 6.0 regression:

$ qemu-nbd -k /tmp/sock -f raw file
$ nbdsh -u 'nbd+unix:///?socket=/tmp/sock' -c 'h.trim(1,0)'
qemu-nbd: Disconnect client, due to: Failed to send reply: Unable to write to socket: Broken pipe

But in all cases, the error message goes away if the client performs a
soft shutdown by using NBD_CMD_DISC, rather than a hard shutdown by
abrupt disconnect:

$ nbdsh -u 'nbd://localhost:10809' -c 'h.trim(1,0)' -c 'h.shutdown()'

This patch fixes things to avoid uninitialized memory, and in general
avoids warning about a client that does a hard shutdown when not in
the middle of a packet.  A client that aborts mid-request, or which
does not read the full server's reply, can still result in warnings,
but those are indeed much more unusual situations.

CC: qemu-stable@nongnu.org
Fixes: f148ae7d36 (nbd/server: Quiesce coroutines on context switch)
Signed-off-by: Eric Blake <eblake@redhat.com>
---
 nbd/server.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/nbd/server.c b/nbd/server.c
index d9164ee6d0da..85877f630533 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -1418,6 +1418,9 @@ static int nbd_receive_request(NBDClient *client, NBDRequest *request,
     if (ret < 0) {
         return ret;
     }
+    if (ret == 0) {
+        return -EIO;
+    }

     /* Request
        [ 0 ..  3]   magic   (NBD_REQUEST_MAGIC)
@@ -2285,7 +2288,7 @@ static int nbd_co_receive_request(NBDRequestData *req, NBDRequest *request,
     assert(client->recv_coroutine == qemu_coroutine_self());
     ret = nbd_receive_request(client, request, errp);
     if (ret < 0) {
-        return  ret;
+        return ret;
     }

     trace_nbd_co_receive_request_decode_type(request->handle, request->type,
@@ -2662,7 +2665,7 @@ static coroutine_fn void nbd_trip(void *opaque)
     }

     if (ret < 0) {
-        /* It wans't -EIO, so, according to nbd_co_receive_request()
+        /* It wasn't -EIO, so, according to nbd_co_receive_request()
          * semantics, we should return the error to the client. */
         Error *export_err = local_err;

-- 
2.33.1