util: don't re-add the qdisc used for tx filters if it already exists

There will soon be two separate users of tc on virtual networks, and
both will use the "qdisc root handle 1: htb" to add tx filters. One or the
other could get the first chance to add the qdisc, and then if at a
later time the other decides to use it, we need to prevent the 2nd
user from attempting to re-add the qdisc (because that just generates
an error).

We do this by running "tc qdisc show dev $bridge handle 1:" then
checking if the output of that command contains both "qdisc" and " 1:
".[*] If it does then the qdisc has already been added. If not then we
need to add it now.

[*]As of this writing, the output more exactly starts with "qdisc
htb 1: root", but our comparison is made purposefully generous to
increase the chances that it will continue to work properly if tc
modifies the format of its output.

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

src/util/virnetdevbandwidth.c

@@ -805,12 +805,35 @@ int
 virNetDevBandWidthAddTxFilterParentQdisc(const char *ifname,
                                          bool hierarchical_class)
 {
-    g_autoptr(virCommand) cmd = NULL;
+    g_autoptr(virCommand) testCmd = NULL;
+    g_autofree char *testResult = NULL;
 
-    cmd = virCommandNew(TC);
+    /* first check it the qdisc with handle 1: was already added for
-    virCommandAddArgList(cmd, "qdisc", "add", "dev", ifname, "root",
+     * this interface by someone else
+     */
+    testCmd = virCommandNew(TC);
+    virCommandAddArgList(testCmd, "qdisc", "show", "dev", ifname,
+                         "handle", "1:", NULL);
+    virCommandSetOutputBuffer(testCmd, &testResult);
+
+    if (virCommandRun(testCmd, NULL) < 0)
+        return -1;
+
+    /* output will be something like: "qdisc htb 1: root refcnt ..."
+     * if the qdisc was already added. We just search for "qdisc" and
+     * " 1: " anywhere in the output to allow for tc changing its
+     * output format.
+     */
+    if (!(testResult && strstr(testResult, "qdisc") && strstr(testResult, " 1: "))) {
+        /* didn't find qdisc in output, so we need to add one */
+        g_autoptr(virCommand) addCmd = virCommandNew(TC);
+
+        virCommandAddArgList(addCmd, "qdisc", "add", "dev", ifname, "root",
                              "handle", "1:", "htb", "default",
                              hierarchical_class ? "2" : "1", NULL);
 
-    return virCommandRun(cmd, NULL);
+        return virCommandRun(addCmd, NULL);
+    }
+
+    return 0;
 }

tests/virnetdevbandwidthtest.c

@@ -147,6 +147,7 @@ mymain(void)
                 "</bandwidth>",
                 TC " qdisc del dev eth0 root\n"
                 TC " qdisc del dev eth0 ingress\n"
+                TC " qdisc show dev eth0 handle 1:\n"
                 TC " qdisc add dev eth0 root handle 1: htb default 1\n"
                 TC " class add dev eth0 parent 1: classid 1:1 htb rate 1024kbps quantum 87\n"
                 TC " qdisc add dev eth0 parent 1:1 handle 2: sfq perturb 10\n"
@@ -177,6 +178,7 @@ mymain(void)
                 "</bandwidth>",
                 TC " qdisc del dev eth0 root\n"
                 TC " qdisc del dev eth0 ingress\n"
+                TC " qdisc show dev eth0 handle 1:\n"
                 TC " qdisc add dev eth0 root handle 1: htb default 1\n"
                 TC " class add dev eth0 parent 1: classid 1:1 htb rate 1kbps ceil 2kbps burst 4kb quantum 1\n"
                 TC " qdisc add dev eth0 parent 1:1 handle 2: sfq perturb 10\n"
@@ -199,6 +201,7 @@ mymain(void)
                 "</bandwidth>",
                 TC " qdisc del dev eth0 root\n"
                 TC " qdisc del dev eth0 ingress\n"
+                TC " qdisc show dev eth0 handle 1:\n"
                 TC " qdisc add dev eth0 root handle 1: htb default 1\n"
                 TC " class add dev eth0 parent 1: classid 1:1 htb rate 4294967295kbps quantum 366503875\n"
                 TC " qdisc add dev eth0 parent 1:1 handle 2: sfq perturb 10\n"