Allow overriding the default /proc folder in blkioController

A host may share its /proc FS folder with a privileged container, mounted in an
alternative folder (e.g. /host/proc), as some users prefer to run their
monitoring software inside a container instead of as a host process.

This patch allows overriding the default /proc folder to allow the blkio
Controller working inside containerized monitoring software.

The /proc root can be overriden via an optional function (ProcRoot) to
avoid introducing a new NewBlkio constructor or introducing breaking changes
in the current API.

Then, the NewBlkio constructor can be invoked as usual, or in the following
form, to override the /proc path:

ctrl := NewBlkio("/sys/fs/cgroup", ProcRoot("/host/proc"))

Signed-off-by: Mario Macias <mariomac@gmail.com>

blkio.go

@@ -30,14 +30,29 @@ import (
 	specs "github.com/opencontainers/runtime-spec/specs-go"
 )
 
-func NewBlkio(root string) *blkioController {
-	return &blkioController{
-		root: filepath.Join(root, string(Blkio)),
+// NewBlkio returns a Blkio controller given the root folder of cgroups.
+// It may optionally accept other configuration options, such as ProcRoot(path)
+func NewBlkio(root string, options ...func(controller *blkioController)) *blkioController {
+	ctrl := &blkioController{
+		root:     filepath.Join(root, string(Blkio)),
+		procRoot: "/proc",
+	}
+	for _, opt := range options {
+		opt(ctrl)
+	}
+	return ctrl
+}
+
+// ProcRoot overrides the default location of the "/proc" filesystem
+func ProcRoot(path string) func(controller *blkioController) {
+	return func(c *blkioController) {
+		c.procRoot = path
 	}
 }
 
 type blkioController struct {
-	root string
+	root     string
+	procRoot string
 }
 
 func (b *blkioController) Name() Name {
@@ -123,7 +138,7 @@ func (b *blkioController) Stat(path string, stats *v1.Metrics) error {
 			},
 		)
 	}
-	f, err := os.Open("/proc/diskstats")
+	f, err := os.Open(filepath.Join(b.procRoot, "diskstats"))
 	if err != nil {
 		return err
 	}

blkio_test.go

@@ -49,3 +49,31 @@ func TestGetDevices(t *testing.T) {
 		t.Fatalf("expected device name %q but received %q", expected, name)
 	}
 }
+
+func TestNewBlkio(t *testing.T) {
+	const root = "/test/folder"
+	const expected = "/test/folder/blkio"
+	const expectedProc = "/proc"
+
+	ctrl := NewBlkio(root)
+	if ctrl.root != expected {
+		t.Fatalf("expected cgroups root %q but received %q", expected, ctrl.root)
+	}
+	if ctrl.procRoot != expectedProc {
+		t.Fatalf("expected proc FS root %q but received %q", expectedProc, ctrl.procRoot)
+	}
+}
+
+func TestNewBlkio_Proc(t *testing.T) {
+	const root = "/test/folder"
+	const expected = "/test/folder/blkio"
+	const expectedProc = "/test/proc"
+
+	ctrl := NewBlkio(root, ProcRoot(expectedProc))
+	if ctrl.root != expected {
+		t.Fatalf("expected cgroups root %q but received %q", expected, ctrl.root)
+	}
+	if ctrl.procRoot != expectedProc {
+		t.Fatalf("expected proc FS root %q but received %q", expectedProc, ctrl.procRoot)
+	}
+}