WIP: make printing output easier (approach 1)

I'm looking if we can make printing output easier, and without linters warning
about all the unhandled errors for `fmt.FPrintXX()`.

One approach could be to use Cobra's print functions, which are convenient,
but the (big!) downside is that we would then have to pass _both_ `cobra.Command`
_and_ `command.Cli` everywhere.

Looking at our code, it looks like the intent is to separate functionality from
cobra (cobra "triggers" the command, but the command itself is agnostic to being
called from cobra).

The other approach is to

- enhance the CLI to have utilities for printing (similar to cobra); `dockerCli.Printf(...)`
- enhance the `streams` package to have utilities, but note that while both `dockerCli.Out()`
  and `dockerCli.Err()` are an `io.Writer`, only `dockerCli.Out()` is a `streams.Out`, so
  we would have to change `dockerCli.Err()` to also be a `streams.Out`. Maybe there
  was a reason for not doing so (`streams.Out` can detect if a terminal is attached,
  maybe that's not possible for `STDERR`, which (unlike `STDOUT` is not buffered? not
  sure if that makes a difference)

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

cli/command/context/list.go

@@ -1,7 +1,7 @@
 package context
 
 import (
-	"fmt"
+	"io"
 	"os"
 	"sort"
 
@@ -29,7 +29,7 @@ func newListCommand(dockerCli command.Cli) *cobra.Command {
 		Short:   "List contexts",
 		Args:    cli.NoArgs,
 		RunE: func(cmd *cobra.Command, args []string) error {
-			return runList(dockerCli, opts)
+			return runList(cmd, dockerCli, opts)
 		},
 		ValidArgsFunction: completion.NoComplete,
 	}
@@ -40,7 +40,7 @@ func newListCommand(dockerCli command.Cli) *cobra.Command {
 	return cmd
 }
 
-func runList(dockerCli command.Cli, opts *listOptions) error {
+func runList(cmd *cobra.Command, dockerCli command.Cli, opts *listOptions) error {
 	if opts.format == "" {
 		opts.format = formatter.TableFormatKey
 	}
@@ -101,19 +101,23 @@ func runList(dockerCli command.Cli, opts *listOptions) error {
 	sort.Slice(contexts, func(i, j int) bool {
 		return sortorder.NaturalLess(contexts[i].Name, contexts[j].Name)
 	})
-	if err := format(dockerCli, opts, contexts); err != nil {
+	if err := format(cmd.OutOrStdout(), opts, contexts); err != nil {
 		return err
 	}
 	if os.Getenv(client.EnvOverrideHost) != "" {
-		_, _ = fmt.Fprintf(dockerCli.Err(), "Warning: %[1]s environment variable overrides the active context. "+
+		cmd.PrintErrf("Warning: %[1]s environment variable overrides the active context. "+
 			"To use a context, either set the global --context flag, or unset %[1]s environment variable.\n", client.EnvOverrideHost)
+
+		// Alternatives:
+		// - dockerCli.PrintErrf(.....)
+		// - dockerCli.Err().Printf(.....)
 	}
 	return nil
 }
 
-func format(dockerCli command.Cli, opts *listOptions, contexts []*formatter.ClientContext) error {
+func format(output io.Writer, opts *listOptions, contexts []*formatter.ClientContext) error {
 	contextCtx := formatter.Context{
-		Output: dockerCli.Out(),
+		Output: output,
 		Format: formatter.NewClientContextFormat(opts.format, opts.quiet),
 	}
 	return formatter.ClientContextWrite(contextCtx, contexts)

cli/command/context/list_test.go

@@ -4,6 +4,7 @@ import (
 	"testing"
 
 	"github.com/docker/cli/cli/command"
+	"github.com/spf13/cobra"
 	"gotest.tools/v3/assert"
 	"gotest.tools/v3/golden"
 )
@@ -26,7 +27,10 @@ func TestList(t *testing.T) {
 	createTestContext(t, cli, "unset")
 	cli.SetCurrentContext("current")
 	cli.OutBuffer().Reset()
-	assert.NilError(t, runList(cli, &listOptions{}))
+	cmd := &cobra.Command{}
+	cmd.SetOut(cli.Out())
+	cmd.SetErr(cli.Err())
+	assert.NilError(t, runList(cmd, cli, &listOptions{}))
 	golden.Assert(t, cli.OutBuffer().String(), "list.golden")
 }
 
@@ -36,7 +40,7 @@ func TestListQuiet(t *testing.T) {
 	createTestContext(t, cli, "other")
 	cli.SetCurrentContext("current")
 	cli.OutBuffer().Reset()
-	assert.NilError(t, runList(cli, &listOptions{quiet: true}))
+	assert.NilError(t, runList(&cobra.Command{}, cli, &listOptions{quiet: true}))
 	golden.Assert(t, cli.OutBuffer().String(), "quiet-list.golden")
 }
 

cmd/docker/docker.go

@@ -62,7 +62,6 @@ func newDockerCommand(dockerCli *command.DockerCli) *cli.TopLevelCommand {
 	setupHelpCommand(dockerCli, cmd, helpCmd)
 	setHelpFunc(dockerCli, cmd)
 
-	cmd.SetOut(dockerCli.Out())
 	commands.AddCommands(cmd, dockerCli)
 
 	cli.DisableFlagsInUseLine(cmd)