Initial implementation of volume plugins

This implements support for mounting and unmounting volumes
backed by volume plugins. Support for actually retrieving
plugins requires a pull request to land in containers.conf and
then that to be vendored, and as such is not yet ready. Given
this, this code is only compile tested. However, the code for
everything past retrieving the plugin has been written - there is
support for creating, removing, mounting, and unmounting volumes,
which should allow full functionality once the c/common PR is
merged.

A major change is the signature of the MountPoint function for
volumes, which now, by necessity, returns an error. Named volumes
managed by a plugin do not have a mountpoint we control; instead,
it is managed entirely by the plugin. As such, we need to cache
the path in the DB, and calls to retrieve it now need to access
the DB (and may fail as such).

Notably absent is support for SELinux relabelling and chowning
these volumes. Given that we don't manage the mountpoint for
these volumes, I am extremely reluctant to try and modify it - we
could easily break the plugin trying to chown or relabel it.

Also, we had no less than *5* separate implementations of
inspecting a volume floating around in pkg/infra/abi and
pkg/api/handlers/libpod. And none of them used volume.Inspect(),
the only correct way of inspecting volumes. Remove them all and
consolidate to using the correct way. Compat API is likely still
doing things the wrong way, but that is an issue for another day.

Fixes #4304

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

1 files changed, 46 insertions, 11 deletions

diff --git a/libpod/volume.go b/libpod/volume.go
index ed08d375f..4c137cb8e 100644
--- a/libpod/volume.go
+++ b/libpod/volume.go
@@ -7,6 +7,7 @@ import (
 
 	"github.com/containers/podman/v2/libpod/define"
 	"github.com/containers/podman/v2/libpod/lock"
+	"github.com/containers/podman/v2/libpod/plugin"
 )
 
 // Volume is a libpod named volume.
@@ -18,6 +19,7 @@ type Volume struct {
 	state  *VolumeState
 
 	valid   bool
+	plugin  *plugin.VolumePlugin
 	runtime *Runtime
 	lock    lock.Locker
 }
@@ -31,7 +33,7 @@ type VolumeConfig struct {
 	// Labels for the volume.
 	Labels map[string]string `json:"labels"`
 	// The volume driver. Empty string or local does not activate a volume
-	// driver, all other volumes will.
+	// driver, all other values will.
 	Driver string `json:"volumeDriver"`
 	// The location the volume is mounted at.
 	MountPoint string `json:"mountPoint"`
@@ -53,6 +55,10 @@ type VolumeConfig struct {
 // Volumes are not guaranteed to have a state. Only volumes using the Local
 // driver that have mount options set will create a state.
 type VolumeState struct {
+	// Mountpoint is the location where the volume was mounted.
+	// This is only used for volumes using a volume plugin, which will mount
+	// at non-standard locations.
+	MountPoint string `json:"mountPoint,omitempty"`
 	// MountCount is the number of times this volume has been requested to
 	// be mounted.
 	// It is incremented on mount() and decremented on unmount().
@@ -115,8 +121,20 @@ func (v *Volume) Labels() map[string]string {
 }
 
 // MountPoint returns the volume's mountpoint on the host
-func (v *Volume) MountPoint() string {
-	return v.config.MountPoint
+func (v *Volume) MountPoint() (string, error) {
+	// For the sake of performance, avoid locking unless we have to.
+	if v.UsesVolumeDriver() {
+		v.lock.Lock()
+		defer v.lock.Unlock()
+
+		if err := v.update(); err != nil {
+			return "", err
+		}
+
+		return v.state.MountPoint, nil
+	}
+
+	return v.config.MountPoint, nil
 }
 
 // Options return the volume's options
@@ -139,14 +157,19 @@ func (v *Volume) UID() (int, error) {
 	v.lock.Lock()
 	defer v.lock.Unlock()
 
-	if !v.valid {
-		return -1, define.ErrVolumeRemoved
+	if err := v.update(); err != nil {
+		return -1, err
 	}
 
+	return v.uid(), nil
+}
+
+// Internal, unlocked accessor for UID.
+func (v *Volume) uid() int {
 	if v.state.UIDChowned > 0 {
-		return v.state.UIDChowned, nil
+		return v.state.UIDChowned
 	}
-	return v.config.UID, nil
+	return v.config.UID
 }
 
 // GID returns the GID the volume will be created as.
@@ -154,14 +177,19 @@ func (v *Volume) GID() (int, error) {
 	v.lock.Lock()
 	defer v.lock.Unlock()
 
-	if !v.valid {
-		return -1, define.ErrVolumeRemoved
+	if err := v.update(); err != nil {
+		return -1, err
 	}
 
+	return v.gid(), nil
+}
+
+// Internal, unlocked accessor for GID.
+func (v *Volume) gid() int {
 	if v.state.GIDChowned > 0 {
-		return v.state.GIDChowned, nil
+		return v.state.GIDChowned
 	}
-	return v.config.GID, nil
+	return v.config.GID
 }
 
 // CreatedTime returns the time the volume was created at. It was not tracked
@@ -198,3 +226,10 @@ func (v *Volume) IsDangling() (bool, error) {
 	}
 	return len(ctrs) == 0, nil
 }
+
+// UsesVolumeDriver determines whether the volume uses a volume driver. Volume
+// drivers are pluggable backends for volumes that will manage the storage and
+// mounting.
+func (v *Volume) UsesVolumeDriver() bool {
+	return !(v.config.Driver == define.VolumeDriverLocal || v.config.Driver == "")
+}