cmd/docker: fix completion for --context

registerCompletionFuncForGlobalFlags was called from newDockerCommand,
at which time no context-store is initialized yet, so it would return
a nil value, probably resulting in `store.Names` to panic, but these
errors are not shown when running the completion. As a result, the flag
completion would fall back to completing from filenames.

This patch changes the function to dynamically get the context-store;
this fixes the problem mentioned above, because at the time the completion
function is _invoked_, the CLI is fully initialized, and does have a
context-store available.

A (non-exported) interface is defined to allow the function to accept
alternative implementations (not requiring a full command.DockerCLI).

Before this patch:

    docker context create one
    docker context create two

    docker --context <TAB>
    .DS_Store                   .idea/                      Makefile
    .dockerignore               .mailmap                    build/
    ...

With this patch:

    docker context create one
    docker context create two

    docker --context <TAB>
    default  one      two

Signed-off-by: Sebastiaan van Stijn <[email protected]>

cmd/docker/completions.go

@@ -6,17 +6,15 @@ import (
 	"github.com/spf13/cobra"
 )
 
-func registerCompletionFuncForGlobalFlags(contextStore store.Store, cmd *cobra.Command) error {
-	err := cmd.RegisterFlagCompletionFunc(
-		"context",
-		func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
-			names, err := store.Names(contextStore)
-			if err != nil {
-				return nil, cobra.ShellCompDirectiveError
-			}
-			return names, cobra.ShellCompDirectiveNoFileComp
-		},
-	)
+type contextStoreProvider interface {
+	ContextStore() store.Store
+}
+
+func registerCompletionFuncForGlobalFlags(dockerCLI contextStoreProvider, cmd *cobra.Command) error {
+	err := cmd.RegisterFlagCompletionFunc("context", func(*cobra.Command, []string, string) ([]string, cobra.ShellCompDirective) {
+		names, _ := store.Names(dockerCLI.ContextStore())
+		return names, cobra.ShellCompDirectiveNoFileComp
+	})
 	if err != nil {
 		return err
 	}

cmd/docker/docker.go

@@ -100,7 +100,7 @@ func newDockerCommand(dockerCli *command.DockerCli) *cli.TopLevelCommand {
 	cmd.SetErr(dockerCli.Err())
 
 	opts, helpCmd = cli.SetupRootCommand(cmd)
-	_ = registerCompletionFuncForGlobalFlags(dockerCli.ContextStore(), cmd)
+	_ = registerCompletionFuncForGlobalFlags(dockerCli, cmd)
 	cmd.Flags().BoolP("version", "v", false, "Print version information and quit")
 	setFlagErrorFunc(dockerCli, cmd)