[MM-56853] Add mmctl shell completion for commands (#26214)

server/cmd/mmctl/commands/completion.go

@@ -4,8 +4,12 @@
 package commands
 
 import (
+	"context"
 	"os"
+	"strings"
 
+	"github.com/mattermost/mattermost/server/public/model"
+	"github.com/mattermost/mattermost/server/v8/cmd/mmctl/client"
 	"github.com/spf13/cobra"
 )
 
@@ -202,3 +206,69 @@ __mmctl_bash_source <(__mmctl_convert_bash_to_zsh)
 
 	return nil
 }
+
+func noCompletion(_ *cobra.Command, _ []string, _ string) ([]string, cobra.ShellCompDirective) {
+	return nil, cobra.ShellCompDirectiveNoFileComp
+}
+
+type validateArgsFn func(ctx context.Context, c client.Client, cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective)
+
+func validateArgsWithClient(fn validateArgsFn) func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) { //nolint:unused // Remove with https://github.com/mattermost/mattermost/pull/25633
+	return func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
+		ctx, cancel := context.WithTimeout(context.Background(), shellCompleteTimeout)
+		defer cancel()
+
+		c, _, _, err := getClient(ctx)
+		if err != nil {
+			return nil, cobra.ShellCompDirectiveError
+		}
+
+		return fn(ctx, c, cmd, args, toComplete)
+	}
+}
+
+type fetcher[T any] func(ctx context.Context, c client.Client, page int, perPage int) ([]T, *model.Response, error) // fetcher calls the Mattermost API to fetch a list of entities T.
+type matcher[T any] func(t T) []string                                                                              // matcher returns list of field that are T uses for shell completion.
+
+func fetchAndComplete[T any](f fetcher[T], m matcher[T]) validateArgsFn {
+	return func(ctx context.Context, c client.Client, cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
+		res := []string{}
+
+		if toComplete == "" {
+			return res, cobra.ShellCompDirectiveNoFileComp
+		}
+
+		var page int
+		for {
+			entities, _, err := f(ctx, c, page, perPage)
+			if err != nil {
+				// Return what we got so far
+				return res, cobra.ShellCompDirectiveNoFileComp
+			}
+
+			for _, e := range entities {
+				for _, field := range m(e) {
+					if strings.HasPrefix(field, toComplete) {
+						res = append(res, field)
+
+						// Only complete one field per entity.
+						break
+					}
+				}
+			}
+
+			if len(res) > shellCompletionMaxItems {
+				res = res[:shellCompletionMaxItems]
+				break
+			}
+
+			if len(entities) < perPage {
+				break
+			}
+
+			page++
+		}
+
+		return res, cobra.ShellCompDirectiveNoFileComp
+	}
+}