nwfilter: save error from DHCP snoop thread to report in main thread

A problem encountered due to a bug in libpcap was reported to the
caller as:

   An error occurred, but the cause is unknown

This was because the error had been logged in the DHCPSnoop
thread. The worker thread handling the API call to start a domain
spins up the DHCPSnoop thread which watches for dhcp packets with
libpcap, then uses virCondSignal() to notify the worker thread (which
has been waiting with virCondWait()). The worker thread knows that
there was an error (because threadStatus != THREAD_STATUS_OK), but the
error info had been stored in thread-specific storage for the other
thread, so the worker thread can only report that there was a failure,
but it doesn't know why.

The solution is to save the error that was logged (with
virErrorPreserveLast() into the object the is used to share info
between the threads, then we can set the error in the worker thread
using virErrorRestore().

In the case of the error I was looking at, this changed the "unknown"
message into:

    internal error: pcap_setfilter: can't remove kernel filter:
    Bad file descriptor
Signed-off-by: NLaine Stump <laine@laine.org>

src/nwfilter/nwfilter_dhcpsnoop.c

@@ -147,6 +147,7 @@ struct _virNWFilterSnoopReq {
     virNWFilterSnoopIPLeasePtr           start;
     virNWFilterSnoopIPLeasePtr           end;
     char                                *threadkey;
+    virErrorPtr                          threadError;
 
     virNWFilterSnoopThreadStatus         threadStatus;
     virCond                              threadStatusCond;
@@ -639,6 +640,7 @@ virNWFilterSnoopReqFree(virNWFilterSnoopReqPtr req)
 
     virMutexDestroy(&req->lock);
     virCondDestroy(&req->threadStatusCond);
+    virFreeError(req->threadError);
 
     VIR_FREE(req);
 }
@@ -1404,10 +1406,12 @@ virNWFilterDHCPSnoopThread(void *req0)
 
     /* let creator know how well we initialized */
     if (error || !threadkey || tmp < 0 || !worker ||
-        ifindex != req->ifindex)
+        ifindex != req->ifindex) {
+        virErrorPreserveLast(&req->threadError);
         req->threadStatus = THREAD_STATUS_FAIL;
-    else
+    } else {
         req->threadStatus = THREAD_STATUS_OK;
+    }
 
     virCondSignal(&req->threadStatusCond);
 
@@ -1713,9 +1717,16 @@ virNWFilterDHCPSnoopReq(virNWFilterTechDriverPtr techdriver,
     }
 
     /* sync with thread */
-    if (virCondWait(&req->threadStatusCond, &req->lock) < 0 ||
-        req->threadStatus != THREAD_STATUS_OK)
+    if (virCondWait(&req->threadStatusCond, &req->lock) < 0) {
+        virReportSystemError(errno, "%s",
+                             _("unable to wait on dhcp snoop thread"));
         goto exit_snoop_cancel;
+    }
+
+    if (req->threadStatus != THREAD_STATUS_OK) {
+        virErrorRestore(&req->threadError);
+        goto exit_snoop_cancel;
+    }
 
     virNWFilterSnoopReqUnlock(req);