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, 6 insertions, 4 deletions

diff --git a/docs/source/markdown/podman-volume-create.1.md b/docs/source/markdown/podman-volume-create.1.md
index 118f024df..a06411000 100644
--- a/docs/source/markdown/podman-volume-create.1.md
+++ b/docs/source/markdown/podman-volume-create.1.md
@@ -17,7 +17,7 @@ driver options can be set using the **--opt** flag.
 
 #### **--driver**=*driver*
 
-Specify the volume driver name (default local).
+Specify the volume driver name (default **local**). Setting this to a value other than **local** Podman will attempt to create the volume using a volume plugin with the given name. Such plugins must be defined in the **volume_plugins** section of the **containers.conf**(5) configuration file.
 
 #### **--help**
 
@@ -30,13 +30,14 @@ Set metadata for a volume (e.g., --label mykey=value).
 #### **--opt**=*option*, **-o**
 
 Set driver specific options.
-For the default driver, `local`, this allows a volume to be configured to mount a filesystem on the host.
+For the default driver, **local**, this allows a volume to be configured to mount a filesystem on the host.
 For the `local` driver the following options are supported: `type`, `device`, and `o`.
 The `type` option sets the type of the filesystem to be mounted, and is equivalent to the `-t` flag to **mount(8)**.
 The `device` option sets the device to be mounted, and is equivalent to the `device` argument to **mount(8)**.
 The `o` option sets options for the mount, and is equivalent to the `-o` flag to **mount(8)** with two exceptions.
 The `o` option supports `uid` and `gid` options to set the UID and GID of the created volume that are not normally supported by **mount(8)**.
-Using volume options with the `local` driver requires root privileges.
+Using volume options with the **local** driver requires root privileges.
+When not using the **local** driver, the given options will be passed directly to the volume plugin. In this case, supported options will be dictated by the plugin in question, not Podman.
 
 ## EXAMPLES
 
@@ -53,7 +54,8 @@ $ podman volume create --label foo=bar myvol
 ```
 
 ## SEE ALSO
-podman-volume(1), mount(8)
+**podman-volume**(1), **mount**(8), **containers.conf**(5)
 
 ## HISTORY
+January 2020, updated with information on volume plugins by Matthew Heon <mheon@redhat.com>
 November 2018, Originally compiled by Urvashi Mohnani <umohnani@redhat.com>