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>

2 files changed, 55 insertions, 0 deletions

diff --git a/libpod/define/errors.go b/libpod/define/errors.go
index 568f8e88d..d37bc397e 100644
--- a/libpod/define/errors.go
+++ b/libpod/define/errors.go
@@ -35,6 +35,10 @@ var (
 	// aliases.
 	ErrNoAliases = errors.New("no aliases for container")
 
+	// ErrMissingPlugin indicates that the requested operation requires a
+	// plugin that is not present on the system or in the configuration.
+	ErrMissingPlugin = errors.New("required plugin missing")
+
 	// ErrCtrExists indicates a container with the same name or ID already
 	// exists
 	ErrCtrExists = errors.New("container already exists")
diff --git a/libpod/define/volume_inspect.go b/libpod/define/volume_inspect.go
new file mode 100644
index 000000000..20602ea16
--- /dev/null
+++ b/libpod/define/volume_inspect.go
@@ -0,0 +1,51 @@
+package define
+
+import (
+	"time"
+)
+
+// InspectVolumeData is the output of Inspect() on a volume. It is matched to
+// the format of 'docker volume inspect'.
+type InspectVolumeData struct {
+	// Name is the name of the volume.
+	Name string `json:"Name"`
+	// Driver is the driver used to create the volume.
+	// If set to "local" or "", the Local driver (Podman built-in code) is
+	// used to service the volume; otherwise, a volume plugin with the given
+	// name is used to mount and manage the volume.
+	Driver string `json:"Driver"`
+	// Mountpoint is the path on the host where the volume is mounted.
+	Mountpoint string `json:"Mountpoint"`
+	// CreatedAt is the date and time the volume was created at. This is not
+	// stored for older Libpod volumes; if so, it will be omitted.
+	CreatedAt time.Time `json:"CreatedAt,omitempty"`
+	// Status is used to return information on the volume's current state,
+	// if the volume was created using a volume plugin (uses a Driver that
+	// is not the local driver).
+	// Status is provided to us by an external program, so no guarantees are
+	// made about its format or contents. Further, it is an optional field,
+	// so it may not be set even in cases where a volume plugin is in use.
+	Status map[string]interface{} `json:"Status,omitempty"`
+	// Labels includes the volume's configured labels, key:value pairs that
+	// can be passed during volume creation to provide information for third
+	// party tools.
+	Labels map[string]string `json:"Labels"`
+	// Scope is unused and provided solely for Docker compatibility. It is
+	// unconditionally set to "local".
+	Scope string `json:"Scope"`
+	// Options is a set of options that were used when creating the volume.
+	// For the Local driver, these are mount options that will be used to
+	// determine how a local filesystem is mounted; they are handled as
+	// parameters to Mount in a manner described in the volume create
+	// manpage.
+	// For non-local drivers, these are passed as-is to the volume plugin.
+	Options map[string]string `json:"Options"`
+	// UID is the UID that the volume was created with.
+	UID int `json:"UID,omitempty"`
+	// GID is the GID that the volume was created with.
+	GID int `json:"GID,omitempty"`
+	// Anonymous indicates that the volume was created as an anonymous
+	// volume for a specific container, and will be be removed when any
+	// container using it is removed.
+	Anonymous bool `json:"Anonymous,omitempty"`
+}