lxc: Check the dereferencedAlias really exists on the remote before e…

…xporting it

When doing a `lxc image export ..`, dereferencing the provided image name either returns
a fingerprint or a human readable alias. Both forms can be fine. However, since this `dereferenceAlias`
operation is not really checking if the image associated with this alias exists on the remote,
we were creating a local target based on the same string that a user entered. Most of the time, this
`targetMeta` string was resolved to a fingerprint (e.g, `/var/lib/snapd/hostfs/home/gab/d2a43fade9d0055e454a091b2e9bd819f7eb715a3d4d46667644a5784d40172`)
and this was fine because there is only the fingerprint file created and no intermediate directories.

Anyway, I think it is safer to systematically check for the image existence prior to creating
any file resources. This way we avoid changing the filesystem write logic that could annoy exsting users and break their
export image pipeline:

```
target := "."
targetMeta := fingerprint
if len(args) > 1 {
	target = args[1]
	if shared.IsDir(shared.HostPathFollow(args[1])) {
		targetMeta = filepath.Join(args[1], targetMeta)
	} else {
		targetMeta = args[1]
	}
}
targetMeta = shared.HostPathFollow(targetMeta)
```

Signed-off-by: Gabriel Mougard <gabriel.mougard@canonical.com>

lxc/image.go

@@ -13,7 +13,7 @@ import (
 	"github.com/spf13/cobra"
 	"gopkg.in/yaml.v2"
 
-	"github.com/canonical/lxd/client"
+	lxd "github.com/canonical/lxd/client"
 	"github.com/canonical/lxd/shared"
 	"github.com/canonical/lxd/shared/api"
 	cli "github.com/canonical/lxd/shared/cmd"
@@ -524,6 +524,13 @@ func (c *cmdImageExport) Run(cmd *cobra.Command, args []string) error {
 
 	fingerprint := c.image.dereferenceAlias(remoteServer, imageType, name)
 
+	// Check if the returned fingerprint (that could also be an alias)
+	// exists on the remote before creating any files on the host system
+	_, _, err = remoteServer.GetImage(fingerprint)
+	if err != nil {
+		return err
+	}
+
 	// Default target is current directory
 	target := "."
 	targetMeta := fingerprint