shell completion: add completion for transports names

Make sure skopeo copy/inspect/delete show the transport names when shell
completion is used to not regress compared to the old bash completion
script.

In theory I would highly recommend to set completion functions for
every flag and command. This can be ensured with a test like this:
https://github.com/containers/podman/blob/main/cmd/podman/shell_completion_test.go
But this is a lot of work to get right and I am neither a skopeo user or
maintainer so I am missing a lot of context for most options. I think
this would be better handled by a person who knows skopeo better.

Normally options should either use AutocompleteNone() or
AutocompleteDefault() from c/common/pkg/completion.
Even better would be to add custom completion functions for arguments
that only accept fixed values, see AutocompleteSupportedTransports() in
this commit.

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

cmd/skopeo/completions.go

@@ -0,0 +1,16 @@
+package main
+
+import (
+	"github.com/containers/image/v5/transports"
+	"github.com/spf13/cobra"
+)
+
+// autocompleteSupportedTransports list all supported transports with the colon suffix.
+func autocompleteSupportedTransports(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
+	tps := transports.ListNames()
+	suggestions := make([]string, 0, len(tps))
+	for _, tp := range tps {
+		suggestions = append(suggestions, tp+":")
+	}
+	return suggestions, cobra.ShellCompDirectiveNoFileComp
+}

cmd/skopeo/copy.go

@@ -64,8 +64,9 @@ Supported transports:
 
 See skopeo(1) section "IMAGE NAMES" for the expected format
 `, strings.Join(transports.ListNames(), ", ")),
-		RunE:    commandAction(opts.run),
-		Example: `skopeo copy docker://quay.io/skopeo/stable:latest docker://registry.example.com/skopeo:latest`,
+		RunE:              commandAction(opts.run),
+		Example:           `skopeo copy docker://quay.io/skopeo/stable:latest docker://registry.example.com/skopeo:latest`,
+		ValidArgsFunction: autocompleteSupportedTransports,
 	}
 	adjustUsage(cmd)
 	flags := cmd.Flags()

cmd/skopeo/delete.go

@@ -35,8 +35,9 @@ Supported transports:
 %s
 See skopeo(1) section "IMAGE NAMES" for the expected format
 `, strings.Join(transports.ListNames(), ", ")),
-		RunE:    commandAction(opts.run),
-		Example: `skopeo delete docker://registry.example.com/example/pause:latest`,
+		RunE:              commandAction(opts.run),
+		Example:           `skopeo delete docker://registry.example.com/example/pause:latest`,
+		ValidArgsFunction: autocompleteSupportedTransports,
 	}
 	adjustUsage(cmd)
 	flags := cmd.Flags()

cmd/skopeo/inspect.go

@@ -55,6 +55,7 @@ See skopeo(1) section "IMAGE NAMES" for the expected format
 		Example: `skopeo inspect docker://registry.fedoraproject.org/fedora
   skopeo inspect --config docker://docker.io/alpine
   skopeo inspect  --format "Name: {{.Name}} Digest: {{.Digest}}" docker://registry.access.redhat.com/ubi8`,
+		ValidArgsFunction: autocompleteSupportedTransports,
 	}
 	adjustUsage(cmd)
 	flags := cmd.Flags()