libcontainer: configs: ensure can build on darwin

configs package can no longer be built on non-Linux OS, such as Darwin.

When running `GOOS=darwin go build` on the packge, we had the following
errors:
```
./configs/mount.go:34:16: undefined: unix.MountAttr
./configs/mount.go:47:22: undefined: unix.MS_BIND
```

Let's ensure that the linux specific bits are handled in mount_linux.go,
and introduce a _unsupported file, similar to how cgroups file is
handled within the package. This'll facilitate utilization of the pkg
for other projects that care about Darwin.

Signed-off-by: Eric Ernst <eric_ernst@apple.com>

libcontainer/configs/mount.go

@@ -1,42 +1,7 @@
 package configs
 
-import "golang.org/x/sys/unix"
-
 const (
 	// EXT_COPYUP is a directive to copy up the contents of a directory when
 	// a tmpfs is mounted over it.
 	EXT_COPYUP = 1 << iota //nolint:golint // ignore "don't use ALL_CAPS" warning
 )
-
-type Mount struct {
-	// Source path for the mount.
-	Source string `json:"source"`
-
-	// Destination path for the mount inside the container.
-	Destination string `json:"destination"`
-
-	// Device the mount is for.
-	Device string `json:"device"`
-
-	// Mount flags.
-	Flags int `json:"flags"`
-
-	// Propagation Flags
-	PropagationFlags []int `json:"propagation_flags"`
-
-	// Mount data applied to the mount.
-	Data string `json:"data"`
-
-	// Relabel source if set, "z" indicates shared, "Z" indicates unshared.
-	Relabel string `json:"relabel"`
-
-	// RecAttr represents mount properties to be applied recursively (AT_RECURSIVE), see mount_setattr(2).
-	RecAttr *unix.MountAttr `json:"rec_attr"`
-
-	// Extensions are additional flags that are specific to runc.
-	Extensions int `json:"extensions"`
-}
-
-func (m *Mount) IsBind() bool {
-	return m.Flags&unix.MS_BIND != 0
-}

libcontainer/configs/mount_linux.go

@@ -0,0 +1,36 @@
+package configs
+
+import "golang.org/x/sys/unix"
+
+type Mount struct {
+	// Source path for the mount.
+	Source string `json:"source"`
+
+	// Destination path for the mount inside the container.
+	Destination string `json:"destination"`
+
+	// Device the mount is for.
+	Device string `json:"device"`
+
+	// Mount flags.
+	Flags int `json:"flags"`
+
+	// Propagation Flags
+	PropagationFlags []int `json:"propagation_flags"`
+
+	// Mount data applied to the mount.
+	Data string `json:"data"`
+
+	// Relabel source if set, "z" indicates shared, "Z" indicates unshared.
+	Relabel string `json:"relabel"`
+
+	// RecAttr represents mount properties to be applied recursively (AT_RECURSIVE), see mount_setattr(2).
+	RecAttr *unix.MountAttr `json:"rec_attr"`
+
+	// Extensions are additional flags that are specific to runc.
+	Extensions int `json:"extensions"`
+}
+
+func (m *Mount) IsBind() bool {
+	return m.Flags&unix.MS_BIND != 0
+}

libcontainer/configs/mount_unsupported.go

@@ -0,0 +1,10 @@
+//go:build !linux
+// +build !linux
+
+package configs
+
+type Mount struct{}
+
+func (m *Mount) IsBind() bool {
+	return false
+}