Add helper functions for Block volume Capacity detection

Similar to how NewMetricsStatFS() works, the new NewMetricsBlock()
provides the GetMetrics() interface for Block volumes.

Additional metrics for Block volumes are difficult to gather. There is
no guarantee that there is a filesystem on the volume, which makes most
of the volume metrics useless.

Advanced storage might be able to detect the actual consumption (when
thin-provisioned) vs the capacity. However, this is out of the scope for
a standard helper function and requires intimate knowledge of the used
storage system.

pkg/volume/metrics_block.go

@@ -0,0 +1,87 @@
+/*
+Copyright 2021 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package volume
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"runtime"
+
+	"k8s.io/apimachinery/pkg/api/resource"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+)
+
+var _ MetricsProvider = &metricsBlock{}
+
+// metricsBlock represents a MetricsProvider that detects the size of the
+// BlockMode Volume.
+type metricsBlock struct {
+	// the device node where the volume is attached to.
+	device string
+}
+
+// NewMetricsStatfs creates a new metricsBlock with the device node of the
+// Volume.
+func NewMetricsBlock(device string) MetricsProvider {
+	return &metricsBlock{device}
+}
+
+// See MetricsProvider.GetMetrics
+// GetMetrics detects the size of the BlockMode volume for the device node
+// where the Volume is attached.
+//
+// Note that only the capacity of the device can be detected with standard
+// tools. Storage systems may have more information that they can provide by
+// going through specialized APIs.
+func (mb *metricsBlock) GetMetrics() (*Metrics, error) {
+	// TODO: Windows does not yet support VolumeMode=Block
+	if runtime.GOOS == "windows" {
+		return nil, NewNotImplementedError("Windows does not support Block volumes")
+	}
+
+	metrics := &Metrics{Time: metav1.Now()}
+	if mb.device == "" {
+		return metrics, NewNoPathDefinedError()
+	}
+
+	err := mb.getBlockInfo(metrics)
+	if err != nil {
+		return metrics, err
+	}
+
+	return metrics, nil
+}
+
+// getBlockInfo fetches metrics.Capacity by opening the device and seeking to
+// the end.
+func (mb *metricsBlock) getBlockInfo(metrics *Metrics) error {
+	dev, err := os.Open(mb.device)
+	if err != nil {
+		return fmt.Errorf("unable to open device %q: %w", mb.device, err)
+	}
+	defer dev.Close()
+
+	end, err := dev.Seek(0, io.SeekEnd)
+	if err != nil {
+		return fmt.Errorf("failed to detect size of %q: %w", mb.device, err)
+	}
+
+	metrics.Capacity = resource.NewQuantity(end, resource.BinarySI)
+
+	return nil
+}

pkg/volume/metrics_block_test.go

@@ -0,0 +1,98 @@
+/*
+Copyright 2021 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package volume_test
+
+import (
+	"io/fs"
+	"os"
+	"runtime"
+	"testing"
+
+	. "k8s.io/kubernetes/pkg/volume"
+	volumetest "k8s.io/kubernetes/pkg/volume/testing"
+)
+
+func TestGetMetricsBlockInvalid(t *testing.T) {
+	metrics := NewMetricsBlock("")
+	actual, err := metrics.GetMetrics()
+	expected := &Metrics{}
+	if !volumetest.MetricsEqualIgnoreTimestamp(actual, expected) {
+		t.Errorf("Expected empty Metrics from uninitialized MetricsBlock, actual %v", *actual)
+	}
+	if err == nil {
+		t.Errorf("Expected error when calling GetMetrics on uninitialized MetricsBlock, actual nil")
+	}
+
+	metrics = NewMetricsBlock("/nonexistent/device/node")
+	actual, err = metrics.GetMetrics()
+	if !volumetest.MetricsEqualIgnoreTimestamp(actual, expected) {
+		t.Errorf("Expected empty Metrics from incorrectly initialized MetricsBlock, actual %v", *actual)
+	}
+	if err == nil {
+		t.Errorf("Expected error when calling GetMetrics on incorrectly initialized MetricsBlock, actual nil")
+	}
+}
+
+func TestGetMetricsBlock(t *testing.T) {
+	// FIXME: this test is Linux specific
+	if runtime.GOOS == "windows" {
+		t.Skip("Block device detection is Linux specific, no Windows support")
+	}
+
+	// find a block device
+	// get all available block devices
+	//  - ls /sys/block
+	devices, err := os.ReadDir("/dev")
+	if err != nil {
+		t.Skipf("Could not read devices from /dev: %v", err)
+	} else if len(devices) == 0 {
+		t.Skip("No devices found")
+	}
+
+	// for each device, check if it is available in /dev
+	devNode := ""
+	var stat fs.FileInfo
+	for _, device := range devices {
+		// if the device exists, use it, return
+		devNode = "/dev/" + device.Name()
+		stat, err = os.Stat(devNode)
+		if err == nil {
+			if stat.Mode().Type() == fs.ModeDevice {
+				break
+			}
+		}
+		// set to an empty string, so we can do validation of the last
+		// device too
+		devNode = ""
+	}
+
+	// if no devices are found, or none exists in /dev, skip this part
+	if devNode == "" {
+		t.Skip("Could not find a block device under /dev")
+	}
+
+	// when we get here, devNode points to an existing block device
+	metrics := NewMetricsBlock(devNode)
+	actual, err := metrics.GetMetrics()
+	if err != nil {
+		t.Errorf("Unexpected error when calling GetMetrics: %v", err)
+	}
+
+	if a := actual.Capacity.Value(); a <= 0 {
+		t.Errorf("Expected Capacity %d to be greater than 0.", a)
+	}
+}

pkg/volume/metrics_errors.go

@@ -35,6 +35,14 @@ func NewNotSupportedError() *MetricsError {
 	}
 }
 
+// NewNotImplementedError creates a new MetricsError with code NotSupported.
+func NewNotImplementedError(reason string) *MetricsError {
+	return &MetricsError{
+		Code: ErrCodeNotSupported,
+		Msg:  fmt.Sprintf("metrics support is not implemented: %s", reason),
+	}
+}
+
 // NewNotSupportedErrorWithDriverName creates a new MetricsError with code NotSupported.
 // driver name is added to the error message.
 func NewNotSupportedErrorWithDriverName(name string) *MetricsError {