Add ability for volumes with options to mount/umount

When volume options and the local volume driver are specified,
the volume is intended to be mounted using the 'mount' command.
Supported options will be used to volume the volume before the
first container using it starts, and unmount the volume after the
last container using it dies.

This should work for any local filesystem, though at present I've
only tested with tmpfs and btrfs.

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

1 files changed, 128 insertions, 0 deletions

diff --git a/libpod/volume_internal_linux.go b/libpod/volume_internal_linux.go
new file mode 100644
index 000000000..68ddb3c0d
--- /dev/null
+++ b/libpod/volume_internal_linux.go
@@ -0,0 +1,128 @@
+// +build linux
+
+package libpod
+
+import (
+	"io/ioutil"
+	"os/exec"
+
+	"github.com/pkg/errors"
+	"github.com/sirupsen/logrus"
+	"golang.org/x/sys/unix"
+)
+
+// mount mounts the volume if necessary.
+// A mount is necessary if a volume has any options set.
+// If a mount is necessary, v.state.MountCount will be incremented.
+// If it was 0 when the increment occurred, the volume will be mounted on the
+// host. Otherwise, we assume it is already mounted.
+// Must be done while the volume is locked.
+// Is a no-op on volumes that do not require a mount (as defined by
+// volumeNeedsMount())
+func (v *Volume) mount() error {
+	if !v.needsMount() {
+		return nil
+	}
+
+	// Update the volume from the DB to get an accurate mount counter.
+	if err := v.update(); err != nil {
+		return err
+	}
+
+	// If the count is non-zero, the volume is already mounted.
+	// Nothing to do.
+	if v.state.MountCount > 0 {
+		v.state.MountCount = v.state.MountCount + 1
+		logrus.Debugf("Volume %s mount count now at %d", v.Name(), v.state.MountCount)
+		return v.save()
+	}
+
+	volDevice := v.config.Options["device"]
+	volType := v.config.Options["type"]
+	volOptions := v.config.Options["o"]
+
+	// Some filesystems (tmpfs) don't have a device, but we still need to
+	// give the kernel something.
+	if volDevice == "" && volType != "" {
+		volDevice = volType
+	}
+
+	// We need to use the actual mount command.
+	// Convincing unix.Mount to use the same semantics as the mount command
+	// itself seems prohibitively difficult.
+	// TODO: might want to cache this path in the runtime?
+	mountPath, err := exec.LookPath("mount")
+	if err != nil {
+		return errors.Wrapf(err, "error locating 'mount' binary")
+	}
+	mountArgs := []string{}
+	if volOptions != "" {
+		mountArgs = append(mountArgs, "-o", volOptions)
+	}
+	if volType != "" {
+		mountArgs = append(mountArgs, "-t", volType)
+	}
+	mountArgs = append(mountArgs, volDevice, v.config.MountPoint)
+	mountCmd := exec.Command(mountPath, mountArgs...)
+
+	errPipe, err := mountCmd.StderrPipe()
+	if err != nil {
+		return errors.Wrapf(err, "getting stderr pipe")
+	}
+	if err := mountCmd.Start(); err != nil {
+		out, err2 := ioutil.ReadAll(errPipe)
+		if err2 != nil {
+			return errors.Wrapf(err2, "error reading mount STDERR")
+		}
+		return errors.Wrapf(errors.New(string(out)), "error mounting volume %s", v.Name())
+	}
+
+	logrus.Debugf("Mounted volume %s", v.Name())
+
+	// Increment the mount counter
+	v.state.MountCount = v.state.MountCount + 1
+	logrus.Debugf("Volume %s mount count now at %d", v.Name(), v.state.MountCount)
+	return v.save()
+}
+
+// unmount unmounts the volume if necessary.
+// Unmounting a volume that is not mounted is a no-op.
+// Unmounting a volume that does not require a mount is a no-op.
+// The volume must be locked for this to occur.
+// The mount counter will be decremented if non-zero. If the counter reaches 0,
+// the volume will really be unmounted, as no further containers are using the
+// volume.
+// If force is set, the volume will be unmounted regardless of mount counter.
+func (v *Volume) unmount(force bool) error {
+	if !v.needsMount() {
+		return nil
+	}
+
+	// Update the volume from the DB to get an accurate mount counter.
+	if err := v.update(); err != nil {
+		return err
+	}
+
+	if v.state.MountCount == 0 {
+		logrus.Debugf("Volume %s already unmounted", v.Name())
+		return nil
+	}
+
+	if !force {
+		v.state.MountCount = v.state.MountCount - 1
+	} else {
+		v.state.MountCount = 0
+	}
+
+	logrus.Debugf("Volume %s mount count now at %d", v.Name(), v.state.MountCount)
+
+	if v.state.MountCount == 0 {
+		// Unmount the volume
+		if err := unix.Unmount(v.config.MountPoint, unix.MNT_DETACH); err != nil {
+			return errors.Wrapf(err, "error unmounting volume %s", v.Name())
+		}
+		logrus.Debugf("Unmounted volume %s", v.Name())
+	}
+
+	return v.save()
+}