qemu: Handle huge number of queues correctly

Currently, kernel supports up to 8 queues for a multiqueue tap device.
However, if user tries to enter a huge number (e.g. one million) the tap
allocation fails, as expected. But what is not expected is the log full
of warnings:

    warning : virFileClose:83 : Tried to close invalid fd 0

The problem is, upon error we iterate over an array of FDs (handlers to
queues) and VIR_FORCE_CLOSE() over each item. However, the array is
pre-filled with zeros. Hence, we repeatedly close stdin. Ouch.
But there's more. The queues allocation is done in virNetDevTapCreate()
which cleans up the FDs in case of error. Then, its caller, the
virNetDevTapCreateInBridgePort() iterates over the FD array and tries to
close them too. And so does qemuNetworkIfaceConnect() and
qemuBuildInterfaceCommandLine().

src/qemu/qemu_command.c

@@ -406,7 +406,7 @@ qemuNetworkIfaceConnect(virDomainDefPtr def,
 cleanup:
     if (ret < 0) {
         size_t i;
-        for (i = 0; i < *tapfdSize; i++)
+        for (i = 0; i < *tapfdSize && tapfd[i] >= 0; i++)
             VIR_FORCE_CLOSE(tapfd[i]);
         if (template_ifname)
             VIR_FREE(net->ifname);
@@ -7338,6 +7338,8 @@ qemuBuildInterfaceCommandLine(virCommandPtr cmd,
             VIR_ALLOC_N(tapfdName, tapfdSize) < 0)
             goto cleanup;
 
+        memset(tapfd, -1, tapfdSize * sizeof(tapfd[0]));
+
         if (qemuNetworkIfaceConnect(def, conn, driver, net,
                                     qemuCaps, tapfd, &tapfdSize) < 0)
             goto cleanup;
@@ -7365,6 +7367,8 @@ qemuBuildInterfaceCommandLine(virCommandPtr cmd,
             VIR_ALLOC_N(vhostfdName, vhostfdSize))
             goto cleanup;
 
+        memset(vhostfd, -1, vhostfdSize * sizeof(vhostfd[0]));
+
         if (qemuOpenVhostNet(def, net, qemuCaps, vhostfd, &vhostfdSize) < 0)
             goto cleanup;
     }
@@ -7424,13 +7428,13 @@ qemuBuildInterfaceCommandLine(virCommandPtr cmd,
 cleanup:
     if (ret < 0)
         virDomainConfNWFilterTeardown(net);
-    for (i = 0; tapfd && i < tapfdSize; i++) {
+    for (i = 0; tapfd && i < tapfdSize && tapfd[i] >= 0; i++) {
         if (ret < 0)
             VIR_FORCE_CLOSE(tapfd[i]);
         if (tapfdName)
             VIR_FREE(tapfdName[i]);
     }
-    for (i = 0; vhostfd && i < vhostfdSize; i++) {
+    for (i = 0; vhostfd && i < vhostfdSize && vhostfd[i] >= 0; i++) {
         if (ret < 0)
             VIR_FORCE_CLOSE(vhostfd[i]);
         if (vhostfdName)

src/util/virnetdevtap.c

@@ -445,6 +445,7 @@ int virNetDevTapCreateInBridgePort(const char *brname,
 {
     virMacAddr tapmac;
     char macaddrstr[VIR_MAC_STRING_BUFLEN];
+    size_t i;
 
     if (virNetDevTapCreate(ifname, tapfd, tapfdSize, flags) < 0)
         return -1;
@@ -498,8 +499,8 @@ int virNetDevTapCreateInBridgePort(const char *brname,
     return 0;
 
 error:
-    while (tapfdSize)
-        VIR_FORCE_CLOSE(tapfd[--tapfdSize]);
+    for (i = 0; i < tapfdSize && tapfd[i] >= 0; i++)
+        VIR_FORCE_CLOSE(tapfd[i]);
 
     return -1;
 }