Add cmdio.Command interface

Due to interface assertions not working on embedded interface types,
code that inspected the capabilities of an underlying command
represented as an io.ReadWriter failed, requiring manual passthrough of
all implemented methods.

This was not a sustainable way of handling methods with differing
sets of command implementations.

cmdio.Command represents the broadest possible interface of any
command. For implementations that choose not to implement optional
interfaces, NopCommand is provided as an embeddable type to
automatically fulfill the interface with dummy implementations, which
can be selectively overridden as desired.

ctr/cmd.go

@@ -2,14 +2,12 @@ package ctr
 
 import (
 	"context"
-	"fmt"
-	"io"
 
 	"lesiw.io/cmdio"
 )
 
 type cmd struct {
-	io.ReadWriter
+	cmdio.Command
 	cdr *cdr
 	ctx context.Context
 	env map[string]string
@@ -18,7 +16,7 @@ type cmd struct {
 
 func newCmd(
 	cdr *cdr, ctx context.Context, env map[string]string, args ...string,
-) io.ReadWriter {
+) cmdio.Command {
 	c := &cmd{
 		ctx: ctx,
 		env: env,
@@ -31,26 +29,12 @@ func newCmd(
 
 func (c *cmd) Attach() error {
 	c.setCmd(true)
-	if a, ok := c.ReadWriter.(cmdio.Attacher); ok {
+	if a, ok := c.Command.(cmdio.Attacher); ok {
 		return a.Attach()
 	}
 	return nil
 }
 
-func (c *cmd) Close() error {
-	if cl, ok := c.ReadWriter.(io.Closer); ok {
-		return cl.Close()
-	}
-	return nil
-}
-
-func (c *cmd) String() string {
-	if s, ok := c.ReadWriter.(fmt.Stringer); ok {
-		return s.String()
-	}
-	return fmt.Sprintf("<%T>", c)
-}
-
 func (c *cmd) setCmd(attach bool) {
 	cmd := []string{"container", "exec", "-i"}
 	if attach {
@@ -65,5 +49,5 @@ func (c *cmd) setCmd(attach bool) {
 	}
 	cmd = append(cmd, c.cdr.ctrid)
 	cmd = append(cmd, c.arg...)
-	c.ReadWriter = c.cdr.rnr.Commander.Command(c.ctx, nil, cmd...)
+	c.Command = c.cdr.rnr.Commander.Command(c.ctx, nil, cmd...)
 }

ctr/runner.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"crypto/sha1"
 	"fmt"
-	"io"
 	"io/fs"
 	"os"
 	"path/filepath"
@@ -29,7 +28,7 @@ type cdr struct {
 
 func (c *cdr) Command(
 	ctx context.Context, env map[string]string, args ...string,
-) io.ReadWriter {
+) cmdio.Command {
 	return newCmd(c, ctx, env, args...)
 }
 

io.go

@@ -18,6 +18,7 @@ package cmdio
 
 import (
 	"context"
+	"fmt"
 	"io"
 	"os"
 
@@ -28,21 +29,13 @@ import (
 //
 // The Command function accepts a [context.Context], a map of environment
 // variables, and a variable number of arguments representing the command
-// itself.
-//
-// The command must not begin execution until the first time it is read from or
-// written to. It must return [io.EOF] once execution has completed and all
-// output has been consumed.
-//
-// In general, the Write method will correspond to standard in and the Read
-// method will correspond to standard out. Commands may implement [Logger] to
-// represent standard error.
+// itself. It returns a [Command].
 type Commander interface {
 	Command(
 		ctx context.Context,
 		env map[string]string,
 		arg ...string,
-	) (cmd io.ReadWriter)
+	) (cmd Command)
 }
 
 // An Enver has environment variables.
@@ -77,6 +70,36 @@ type Attacher interface {
 	Attach() error
 }
 
+// A [Command] is the broadest possible command interface.
+//
+// Commands must not begin execution until the first time they are read from or
+// written to. They must return [io.EOF] once execution has completed and all
+// output has been consumed.
+//
+// In general, the Write method will correspond to standard in, the Read
+// method will correspond to standard out, and an [io.Writer] may be passed
+// to Log for handling standard error.
+type Command interface {
+	io.ReadWriteCloser
+	fmt.Stringer
+	Attacher
+	Coder
+	Logger
+}
+
+// A [NopCommand] is an empty [Command] implementation. It is useful for
+// embedding in command implementations that may not choose to implement
+// optional interfaces.
+type NopCommand struct{}
+
+func (NopCommand) Read([]byte) (int, error)  { return 0, nil }
+func (NopCommand) Write([]byte) (int, error) { return 0, nil }
+func (NopCommand) Close() error              { return nil }
+func (NopCommand) String() string            { return "<nop>" }
+func (NopCommand) Attach() error             { return nil }
+func (NopCommand) Code() int                 { return 0 }
+func (NopCommand) Log(io.Writer)             {}
+
 // Trace is an [io.Writer] to which command tracing information is written.
 // To disable tracing, set this variable to [io.Discard].
 var Trace io.Writer = prefix.NewWriter("+ ", stderr)

sub/runner.go

@@ -2,7 +2,6 @@ package sub
 
 import (
 	"context"
-	"io"
 
 	"lesiw.io/cmdio"
 	"lesiw.io/cmdio/sys"
@@ -15,7 +14,7 @@ type cdr struct {
 
 func (c *cdr) Command(
 	ctx context.Context, env map[string]string, args ...string,
-) io.ReadWriter {
+) cmdio.Command {
 	return c.rnr.Commander.Command(ctx, env, append(c.cmd, args...)...)
 }
 

sys/cmd.go

@@ -11,9 +11,13 @@ import (
 	"slices"
 	"strings"
 	"sync"
+
+	"lesiw.io/cmdio"
 )
 
 type cmd struct {
+	cmdio.NopCommand
+
 	ctx  context.Context
 	cmd  *exec.Cmd
 	env  map[string]string
@@ -38,7 +42,9 @@ func (c *cmd) Attach() error {
 	return nil
 }
 
-func newCmd(ctx context.Context, env map[string]string, args ...string) *cmd {
+func newCmd(
+	ctx context.Context, env map[string]string, args ...string,
+) cmdio.Command {
 	c := new(cmd)
 	c.ctx = ctx
 	c.cmd = exec.CommandContext(ctx, args[0], args[1:]...)

sys/runner.go

@@ -2,7 +2,6 @@ package sys
 
 import (
 	"context"
-	"io"
 
 	"lesiw.io/cmdio"
 )
@@ -11,7 +10,7 @@ type cdr struct{}
 
 func (cdr) Command(
 	ctx context.Context, env map[string]string, args ...string,
-) io.ReadWriter {
+) cmdio.Command {
 	return newCmd(ctx, env, args...)
 }