[PATCH] nbd-server: Silence server warnings on port probes

[Eric Blake]

While testing the use of qemu-nbd in a Pod of a Kubernetes cluster, I
got LOTS of log messages of the forms:

qemu-nbd: option negotiation failed: Failed to read flags: Unexpected 
end-of-file before all data were read
qemu-nbd: option negotiation failed: Failed to read flags: Unable to read from 
socket: Connection reset by peer

While it is nice to warn about clients that aren't following protocol
(in case it helps diagnosing bugs in those clients), a mere port probe
(where the client never write()s any bytes, and where we might even
hit EPIPE in trying to send our greeting to the client) is NOT
abnormal, but merely serves to pollute the log.  And Kubernetes
_really_ likes to do port probes to determine whether a given Pod is
up and running.

Easy ways to demonstrate the above port probes:
$ qemu-nbd -r -f raw path/to/file &
$ nc localhost 10809 </dev/null
$ bash -c 'exec </dev/tcp/localhost/10809'
$ kill $!

Silence the noise by not capturing errors until after our first
successful read() from a client.

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 nbd/server.c | 26 +++++++++++++++++---------
 1 file changed, 17 insertions(+), 9 deletions(-)

diff --git a/nbd/server.c b/nbd/server.c
index c30e687fc8b..f64e47270c0 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -1150,8 +1150,8 @@ nbd_negotiate_meta_queries(NBDClient *client, Error 
**errp)
  * Return:
  * -errno  on error, errp is set
  * 0       on successful negotiation, errp is not set
- * 1       if client sent NBD_OPT_ABORT, i.e. on valid disconnect,
- *         errp is not set
+ * 1       if client sent NBD_OPT_ABORT (i.e. on valid disconnect) or never
+ *         wrote anything (i.e. port probe); errp is not set
  */
 static coroutine_fn int
 nbd_negotiate_options(NBDClient *client, Error **errp)
@@ -1175,8 +1175,13 @@ nbd_negotiate_options(NBDClient *client, Error **errp)
         ...           Rest of request
     */

-    if (nbd_read32(client->ioc, &flags, "flags", errp) < 0) {
-        return -EIO;
+    /*
+     * Intentionally ignore errors on this first read - we do not want
+     * to be noisy about a mere port probe, but only for clients that
+     * start talking the protocol and then quit abruptly.
+     */
+    if (nbd_read32(client->ioc, &flags, "flags", NULL) < 0) {
+        return 1;
     }
     client->mode = NBD_MODE_EXPORT_NAME;
     trace_nbd_negotiate_options_flags(flags);
@@ -1383,8 +1388,8 @@ nbd_negotiate_options(NBDClient *client, Error **errp)
  * Return:
  * -errno  on error, errp is set
  * 0       on successful negotiation, errp is not set
- * 1       if client sent NBD_OPT_ABORT, i.e. on valid disconnect,
- *         errp is not set
+ * 1       if client sent NBD_OPT_ABORT (i.e. on valid disconnect) or never
+ *         wrote anything (i.e. port probe); errp is not set
  */
 static coroutine_fn int nbd_negotiate(NBDClient *client, Error **errp)
 {
@@ -1415,9 +1420,12 @@ static coroutine_fn int nbd_negotiate(NBDClient *client, 
Error **errp)
     stq_be_p(buf + 8, NBD_OPTS_MAGIC);
     stw_be_p(buf + 16, NBD_FLAG_FIXED_NEWSTYLE | NBD_FLAG_NO_ZEROES);

-    if (nbd_write(client->ioc, buf, 18, errp) < 0) {
-        error_prepend(errp, "write failed: ");
-        return -EINVAL;
+    /*
+     * Be silent about failure to write our greeting: there is nothing
+     * wrong with a client testing if our port is alive.
+     */
+    if (nbd_write(client->ioc, buf, 18, NULL) < 0) {
+        return 1;
     }
     ret = nbd_negotiate_options(client, errp);
     if (ret != 0) {
-- 
2.47.0