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[libvirt] [PATCHv2] macvtap: log an error if on failure to connect to netlink socket

(v2 change: add log messages for 2 other previously silent error conditions in nlComm)
A bug in libnl (see https://bugzilla.redhat.com/show_bug.cgi?id=677724
and https://bugzilla.redhat.com/show_bug.cgi?id=677725) makes it very
easy to create a failure to connect to the netlink socket when trying
to open a macvtap network device ("type='direct'" in domain interface
XML). When that error occurred (during a call to libnl's nl_connect()
from libvirt's nlComm(), there was no log message, leading virsh (for
example) to report "unknown error".

There were two other cases in nlComm where an error in a libnl
function might return with failure but no error reported. In all three
cases, this patch logs a message which will hopefully be more useful.

Note that more detailed information about the failure might be
available from libnl's nl_geterror() function, but it calls
strerror(), which is not threadsafe, so we can't use it.
 src/util/macvtap.c |   13 ++++++++++---
 1 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/src/util/macvtap.c b/src/util/macvtap.c
index a71db86..00ee4ee 100644
--- a/src/util/macvtap.c
+++ b/src/util/macvtap.c
@@ -123,10 +123,15 @@ int nlComm(struct nl_msg *nl_msg,
     struct nl_handle *nlhandle = nl_handle_alloc();
     struct nlmsghdr *nlmsg = nlmsg_hdr(nl_msg);
-    if (!nlhandle)
+    if (!nlhandle) {
+        virReportSystemError(errno,
+                             "%s", _("cannot allocate nlhandle for netlink"));
         return -1;
+    }
     if (nl_connect(nlhandle, NETLINK_ROUTE) < 0) {
+        virReportSystemError(errno,
+                             "%s", _("cannot connect to netlink socket"));
         rc = -1;
         goto err_exit;
@@ -161,9 +166,11 @@ int nlComm(struct nl_msg *nl_msg,
     *respbuflen = nl_recv(nlhandle, &nladdr, respbuf, NULL);
-    if (*respbuflen <= 0)
+    if (*respbuflen <= 0) {
+        virReportSystemError(errno,
+                             "%s", _("nl_recv failed"));
         rc = -1;
+    }
     if (rc == -1) {

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