Allow 39 atached EBS devices.

AWS has soft support limit for 40 attached EBS devices. Assuming there is just
one root device, use the rest for persistent volumes.

The devices will have name /dev/xvdba - /dev/xvdcm, leaving /dev/sda - /dev/sdz
to the system.

Also, add better error handling and propagate error
"Too many EBS volumes attached to node XYZ" to a pod.

pkg/cloudprovider/providers/aws/aws.go

@@ -80,6 +80,11 @@ const MaxReadThenCreateRetries = 30
 // need hardcoded defaults.
 const DefaultVolumeType = "gp2"
 
+// Amazon recommends having no more that 40 volumes attached to an instance,
+// and at least one of those is for the system root volume.
+// See http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/volume_limits.html#linux-specific-volume-limits
+const DefaultMaxEBSVolumes = 39
+
 // Used to call aws_credentials.Init() just once
 var once sync.Once
 
@@ -902,10 +907,17 @@ type mountDevice string
 // TODO: Also return number of mounts allowed?
 func (self *awsInstanceType) getEBSMountDevices() []mountDevice {
 	// See: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/block-device-mapping-concepts.html
+	// We will generate "ba", "bb", "bc"..."bz", "ca", ..., up to DefaultMaxEBSVolumes
 	devices := []mountDevice{}
-	for c := 'f'; c <= 'p'; c++ {
-		devices = append(devices, mountDevice(fmt.Sprintf("%c", c)))
+	count := 0
+	for first := 'b'; count < DefaultMaxEBSVolumes; first++ {
+		for second := 'a'; count < DefaultMaxEBSVolumes && second <= 'z'; second++ {
+			device := mountDevice(fmt.Sprintf("%c%c", first, second))
+			devices = append(devices, device)
+			count++
+		}
 	}
+
 	return devices
 }
 
@@ -1014,7 +1026,7 @@ func (self *awsInstance) getMountDevice(volumeID string, assign bool) (assigned
 		if strings.HasPrefix(name, "/dev/xvd") {
 			name = name[8:]
 		}
-		if len(name) != 1 {
+		if len(name) < 1 || len(name) > 2 {
 			glog.Warningf("Unexpected EBS DeviceName: %q", aws.StringValue(blockDevice.DeviceName))
 		}
 		deviceMappings[mountDevice(name)] = aws.StringValue(blockDevice.Ebs.VolumeId)
@@ -1051,7 +1063,7 @@ func (self *awsInstance) getMountDevice(volumeID string, assign bool) (assigned
 
 	if chosen == "" {
 		glog.Warningf("Could not assign a mount device (all in use?).  mappings=%v, valid=%v", deviceMappings, valid)
-		return "", false, nil
+		return "", false, fmt.Errorf("Too many EBS volumes attached to node %s.", self.nodeName)
 	}
 
 	self.attaching[chosen] = volumeID