util: get clusterID for the passed in mon string

as part of migration support, the clusterID has to be fetched
from passed in mon. Because the intree RBD storage class only
got monitor and not `clusterID` parameter support. However, in
CSI, SC has the `clusterID` parameter support but not mon. Due
to that we have to fetch the clusterID from config file for the
passed in mon and use it in our operations. This adds a helper
function to retrieve clusterID from passed in mon string.

Updates https://github.com/ceph/ceph-csi/issues/2509

Signed-off-by: Humble Chirammal <hchiramm@redhat.com>

internal/util/csiconfig.go

@@ -161,3 +161,44 @@ func GetClusterID(options map[string]string) (string, error) {
 
 	return clusterID, nil
 }
+
+// GetClusterIDFromMon will be called with a mon string to fetch
+// clusterID based on the passed in mon string. If passed in 'mon'
+// string has been found in the config the clusterID is returned,
+// else error.
+func GetClusterIDFromMon(mon string) (string, error) {
+	clusterID, err := readClusterInfoWithMon(CsiConfigFile, mon)
+	if err != nil {
+		return "", err
+	}
+
+	return clusterID, nil
+}
+
+func readClusterInfoWithMon(pathToConfig, mon string) (string, error) {
+	var config []ClusterInfo
+
+	// #nosec
+	content, err := ioutil.ReadFile(pathToConfig)
+	if err != nil {
+		err = fmt.Errorf("error fetching configuration file %q: %w", pathToConfig, err)
+
+		return "", err
+	}
+
+	err = json.Unmarshal(content, &config)
+	if err != nil {
+		return "", fmt.Errorf("unmarshal failed (%w), raw buffer response: %s",
+			err, string(content))
+	}
+
+	for _, cluster := range config {
+		for _, m := range cluster.Monitors {
+			if m == mon {
+				return cluster.ClusterID, nil
+			}
+		}
+	}
+
+	return "", fmt.Errorf("missing configuration of cluster ID for mon %q", mon)
+}