nbdkit/0006-server-Send-the-last-error-to-the-NBD-client.patch

From 46484ca8e6a35c45fe96b6c972ceba8984d401e8 Mon Sep 17 00:00:00 2001
From: "Richard W.M. Jones" <rjones@redhat.com>
Date: Tue, 23 Jul 2024 15:45:04 +0100
Subject: [PATCH] server: Send the last error to the NBD client

This sends the last error saved in the connection handle back to the
NBD client.  This is informational and best effort.

qemu reports the error already, for example:

  $ nbdkit --log=null \
           eval open=' echo EPERM Go Away >&2; exit 1 ' get_size=' echo 100 ' \
           --run 'qemu-img info "$uri"'
  qemu-img: Could not open 'nbd+unix://?socket=/tmp/nbdkitIDl6iy/socket': Requested export not available
  server reported: /tmp/nbdkitRDAfXH/open: Go Away

This goes back to at least qemu 2.12.0 (RHEL 7) and possibly earlier,
so we can just assume that qemu does this for the test.

libnbd requires a patch to display this information.
---
 server/protocol-handshake-newstyle.c | 43 ++++++++++++++++------
 tests/Makefile.am                    |  2 +
 tests/test-last-error.sh             | 55 ++++++++++++++++++++++++++++
 3 files changed, 88 insertions(+), 12 deletions(-)
 create mode 100755 tests/test-last-error.sh

diff --git a/server/protocol-handshake-newstyle.c b/server/protocol-handshake-newstyle.c
index 6b3bc76f..c18d32e5 100644
--- a/server/protocol-handshake-newstyle.c
+++ b/server/protocol-handshake-newstyle.c
@@ -57,28 +57,47 @@ send_newstyle_option_reply (uint32_t option, uint32_t reply)
 {
   GET_CONN;
   struct nbd_fixed_new_option_reply fixed_new_option_reply;
+  const char *last_error = NULL;
+  uint32_t replylen = 0;
+
+  if (NBD_REP_IS_ERR (reply)) {
+    last_error = threadlocal_get_last_error ();
+    /* Note that calling nbdkit_error will invalidate last_error, so
+     * be careful below.
+     */
+    if (last_error) {
+      size_t len = strlen (last_error);
+      if (len <= NBD_MAX_STRING)
+        replylen = len;
+    }
+  }
 
   fixed_new_option_reply.magic = htobe64 (NBD_REP_MAGIC);
   fixed_new_option_reply.option = htobe32 (option);
   fixed_new_option_reply.reply = htobe32 (reply);
-  fixed_new_option_reply.replylen = htobe32 (0);
+  fixed_new_option_reply.replylen = htobe32 (replylen);
 
   debug ("replying to %s with %s", name_of_nbd_opt (option),
          name_of_nbd_rep (reply));
   if (conn->send (&fixed_new_option_reply,
-                  sizeof fixed_new_option_reply, 0) == -1) {
-    /* The protocol document says that the client is allowed to simply
-     * drop the connection after sending NBD_OPT_ABORT, or may read
-     * the reply.
-     */
-    if (option == NBD_OPT_ABORT)
-      debug ("write: %s: %m", name_of_nbd_opt (option));
-    else
-      nbdkit_error ("write: %s: %m", name_of_nbd_opt (option));
-    return -1;
-  }
+                  sizeof fixed_new_option_reply,
+                  replylen > 0 ? SEND_MORE : 0) == -1)
+    goto err;
+  if (replylen > 0 && conn->send (last_error, replylen, 0) == -1)
+    goto err;
 
   return 0;
+
+err:
+  /* The protocol document says that the client is allowed to simply
+   * drop the connection after sending NBD_OPT_ABORT, or may read
+   * the reply.
+   */
+  if (option == NBD_OPT_ABORT)
+    debug ("write: %s: %m", name_of_nbd_opt (option));
+  else
+    nbdkit_error ("write: %s: %m", name_of_nbd_opt (option));
+  return -1;
 }
 
 /* Reply to NBD_OPT_LIST with the plugin's list of export names.
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 8c7d6b8c..89c5fa9d 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -292,6 +292,7 @@ TESTS += \
 	test-read-password-interactive.sh \
 	test-nbd-client.sh \
 	test-nbd-client-tls.sh \
+	test-last-error.sh \
 	$(NULL)
 if !IS_WINDOWS
 TESTS += \
@@ -324,6 +325,7 @@ EXTRA_DIST += \
 	test-help-plugin.sh \
 	test-ipv4-lo.sh \
 	test-ipv6-lo.sh \
+	test-last-error.sh \
 	test-long-name.sh \
 	test-nbd-client-tls.sh \
 	test-nbd-client.sh \
diff --git a/tests/test-last-error.sh b/tests/test-last-error.sh
new file mode 100755
index 00000000..fc720606
--- /dev/null
+++ b/tests/test-last-error.sh
@@ -0,0 +1,55 @@
+#!/usr/bin/env bash
+# nbdkit
+# Copyright Red Hat
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met:
+#
+# * Redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer.
+#
+# * Redistributions in binary form must reproduce the above copyright
+# notice, this list of conditions and the following disclaimer in the
+# documentation and/or other materials provided with the distribution.
+#
+# * Neither the name of Red Hat nor the names of its contributors may be
+# used to endorse or promote products derived from this software without
+# specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND
+# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+# THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+# PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
+# CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
+# USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+# ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+# OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+# OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+# SUCH DAMAGE.
+
+source ./functions.sh
+set -e
+set -x
+
+# Test informational error messages sent to the NBD client.
+# qemu-img supports this since at least 2.12.0.
+
+requires_run
+requires_plugin eval
+requires qemu-img --version
+
+out=last-error.out
+rm -f $out
+cleanup_fn rm -f $out
+
+export out
+
+nbdkit eval \
+       open=' echo EPERM Go Away >&2; exit 1 ' get_size=' echo 0 ' \
+       --run ' qemu-img info "$uri" > $out 2>&1 ||: '
+cat $out
+
+grep "Go Away" $out
-- 
2.43.0