qemu-nbd: Allow users to adjust handshake limit

Although defaulting the handshake limit to 10 seconds was a nice QoI
change to weed out intentionally slow clients, it can interfere with
integration testing done with manual NBD_OPT commands over 'nbdsh
--opt-mode'.  Expose a command line option to allow the user to alter
the timeout away from the default.  This option is unlikely to be used
in enough scenarios to warrant a short option letter.

The option --handshake-limit intentionally differs from the name of
the constant added in commit fb1c2aaa98 (limit instead of max_secs)
and the QMP name to be added in the next commit; this is because
typing a longer command-line name is undesirable and there is
sufficient --help text to document the units.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-ID: <20250203222722.650694-5-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>

docs/tools/qemu-nbd.rst

@@ -156,6 +156,11 @@ driver options if :option:`--image-opts` is specified.
   Set the NBD volume export description, as a human-readable
   string.
 
+.. option:: --handshake-limit=N
+
+  Set the timeout for a client to successfully complete its handshake
+  to N seconds (default 10), or 0 for no limit.
+
 .. option:: -L, --list
 
   Connect as a client and list all details about the exports exposed by

qemu-nbd.c

@@ -57,19 +57,20 @@
 #define HAVE_NBD_DEVICE 0
 #endif
 
-#define SOCKET_PATH                "/var/lock/qemu-nbd-%s"
-#define QEMU_NBD_OPT_CACHE         256
-#define QEMU_NBD_OPT_AIO           257
-#define QEMU_NBD_OPT_DISCARD       258
-#define QEMU_NBD_OPT_DETECT_ZEROES 259
-#define QEMU_NBD_OPT_OBJECT        260
-#define QEMU_NBD_OPT_TLSCREDS      261
-#define QEMU_NBD_OPT_IMAGE_OPTS    262
-#define QEMU_NBD_OPT_FORK          263
-#define QEMU_NBD_OPT_TLSAUTHZ      264
-#define QEMU_NBD_OPT_PID_FILE      265
-#define QEMU_NBD_OPT_SELINUX_LABEL 266
-#define QEMU_NBD_OPT_TLSHOSTNAME   267
+#define SOCKET_PATH                  "/var/lock/qemu-nbd-%s"
+#define QEMU_NBD_OPT_CACHE           256
+#define QEMU_NBD_OPT_AIO             257
+#define QEMU_NBD_OPT_DISCARD         258
+#define QEMU_NBD_OPT_DETECT_ZEROES   259
+#define QEMU_NBD_OPT_OBJECT          260
+#define QEMU_NBD_OPT_TLSCREDS        261
+#define QEMU_NBD_OPT_IMAGE_OPTS      262
+#define QEMU_NBD_OPT_FORK            263
+#define QEMU_NBD_OPT_TLSAUTHZ        264
+#define QEMU_NBD_OPT_PID_FILE        265
+#define QEMU_NBD_OPT_SELINUX_LABEL   266
+#define QEMU_NBD_OPT_TLSHOSTNAME     267
+#define QEMU_NBD_OPT_HANDSHAKE_LIMIT 268
 
 #define MBR_SIZE 512
 
@@ -80,6 +81,7 @@ static int nb_fds;
 static QIONetListener *server;
 static QCryptoTLSCreds *tlscreds;
 static const char *tlsauthz;
+static int handshake_limit = NBD_DEFAULT_HANDSHAKE_MAX_SECS;
 
 static void usage(const char *name)
 {
@@ -101,6 +103,7 @@ static void usage(const char *name)
 "  -v, --verbose             display extra debugging information\n"
 "  -x, --export-name=NAME    expose export by name (default is empty string)\n"
 "  -D, --description=TEXT    export a human-readable description\n"
+"      --handshake-limit=N   limit client's handshake to N seconds (default 10)\n"
 "\n"
 "Exposing part of the image:\n"
 "  -o, --offset=OFFSET       offset into the image\n"
@@ -390,8 +393,7 @@ static void nbd_accept(QIONetListener *listener, QIOChannelSocket *cioc,
 
     nb_fds++;
     nbd_update_server_watch();
-    /* TODO - expose handshake timeout as command line option */
-    nbd_client_new(cioc, NBD_DEFAULT_HANDSHAKE_MAX_SECS,
+    nbd_client_new(cioc, handshake_limit,
                    tlscreds, tlsauthz, nbd_client_closed, NULL);
 }
 
@@ -569,6 +571,8 @@ int main(int argc, char **argv)
         { "object", required_argument, NULL, QEMU_NBD_OPT_OBJECT },
         { "export-name", required_argument, NULL, 'x' },
         { "description", required_argument, NULL, 'D' },
+        { "handshake-limit", required_argument, NULL,
+          QEMU_NBD_OPT_HANDSHAKE_LIMIT },
         { "tls-creds", required_argument, NULL, QEMU_NBD_OPT_TLSCREDS },
         { "tls-hostname", required_argument, NULL, QEMU_NBD_OPT_TLSHOSTNAME },
         { "tls-authz", required_argument, NULL, QEMU_NBD_OPT_TLSAUTHZ },
@@ -815,6 +819,13 @@ int main(int argc, char **argv)
         case QEMU_NBD_OPT_SELINUX_LABEL:
             selinux_label = optarg;
             break;
+        case QEMU_NBD_OPT_HANDSHAKE_LIMIT:
+            if (qemu_strtoi(optarg, NULL, 0, &handshake_limit) < 0 ||
+                handshake_limit < 0) {
+                error_report("Invalid handshake limit '%s'", optarg);
+                exit(EXIT_FAILURE);
+            }
+            break;
         }
     }