// Package modules provides a mechanism for running commonly used services
// as subprocesses and client functionality for accessing those services.
// Such services and functions are collectively called 'commands' and
// are managed by a 'Registry'. The Shell is analagous to the UNIX shell and
// maintains a key, value store of environment variables and config settings
// that are accessible to the commands that it hosts. Simple variable
// expansion is supported.
//
// Four types of 'commands' may be invoked via a Shell.
//
// - functions of type Shell.Main as subprocesses via fork/exec
// - functions of type Shell.Main as functions within the current process
// - arbitrary non-Vanadium commands available on the underlying
//   operating system such as '/bin/cp', 'bash' etc.
// - arbtirary Vanadium commands available on the underlying
//   operating system such as precompiled Vanadium services.
//
// The first two types require that the function to be executed is compiled
// into the binary executing the calls to the Shell. These functions
// are registered with a single, per-process, registry.
// The signature of the function that implements the command is the
// same for both types of command and is defined by the Main function type.
// In particular stdin, stdout and stderr are provided as parameters, as is
// a map representation of the shell's environment.
//
// The second two types allow for arbitrary binaries to be executed. The
// distinction between a Vanadium and non-Vanadium command is that the
// Vanadium command implements the protocol used by v.io/x/ref/lib/exec
// package to synchronise between the parent and child processes and to share
// information such as the ConfigKey key,value store supported by the Shell,
// a shared secret etc.
//
// The registry provides the following functions:
// - RegisterChild: generally called from an init function to register a
//   shell.Main to be executed in a subprocess by fork/exec'ing the calling
//   process.
// - Dispatch: which must be called in the child process to lookup the
//   requested function in the registry and to invoke it. This will typically
//   be called from a TestMain. modules.IsModulesChildProcess can be used
//   to determine if the calling process is a child started via this package.
// - DispatchAndExit: essentially the same as Dispatch but intended to be
//   called as the first thing in a main function.
// - RegisterFunction: for an in-process function that will be invoked
//   via function call.
//
// The v23 tool can automate generation of TestMain and calls to RegisterChild,
// and RegisterFunction. Adding the comment below to a test file will
// generate the appropriate code.
//
// //go:generate v23 test generate .
//
// Use 'v23 test generate --help' to get a complete explanation.
//
// In all cases commands are started by invoking the StartWithOpts method
// on the Shell with the name of the command to run. An instance of the Handle
// interface is returned which can be used to interact with the function
// or subprocess, and in particular to read/write data from/to it using io
// channels that follow the stdin, stdout, stderr convention. The StartOpts
// struct is used to control the detailed behaviour of each such invocation.
// Various helper functions are provided both for creating appropriate
// instances of StartOpts and for common uses of StartWithOpts.
//
// Each successful call to StartWithOpts returns a handle representing
// the running command. This handle can be used to gain access to that
// command's stdin, stdout, stderr and to request or synchronize with
// its termination via the Shutdown method. The Shutdown method can
// optionally be used to read any remaining output from the commands stdout
// and stderr.
// The Shell maintains a record of all such handles and will call Shutdown
// on them in LIFO order when the Shell's Cleanup method is called.
//
// A simple protocol must be followed by all modules.Main commands,
// in particular, they should wait for their stdin stream to be closed
// before exiting. The caller can then coordinate with any command by writing
// to that stdin stream and reading responses from the stdout stream, and it
// can close stdin when it's ready for the command to exit using the
// CloseStdin method on the command's handle. Any binary or script that
// follows this protocol can be used as well.
//
// By default, every Shell created by NewShell starts a security agent
// to manage principals for child processes. These default credentials
// can be overridden by passing a nil context to NewShell then specifying
// VeyronCredentials in the environment provided as a parameter to the
// StartWithOpts method. It is also possible to specify custom credentials
// via StartOpts.
//
// Interacting with Commands:
//
// Handle.Stdout(), Stdin(), Stderr():
// StartWithOpts returns a Handle which can be used to interact with the
// running command. In particular, its Stdin() and Stdout() methods give
// access to the running process' corresponding stdin and stdout and hence
// can be used to communicate with it. Stderr is handled differently and is
// configured so that the child's stderr is written to a log file rather
// than a pipe. This is in order to maximise the liklihood of capturing
// stderr output from a crashed child process.
//
// Handle.Shutdown(stdout, stderr io.Writer):
// The Shutdown method is used to gracefully shutdown a command and to
// synchronise with its termination. In particular, Shutdown can be used
// to read any unread output from the command's stdout and stderr. Note that
// since Stderr is buffered to a file, Shutdown is able to return the entire
// contents of that file. This is useful for debugging misbehaving/crashing
// child processes.
//
// Shell.Cleanup(stdout, stderr io.Writer):
// The Shell keeps track of all Handles that it has issued and in particular
// if Shutdown (or Forget) have not been called, it will call Shutdown for
// each such Handle in LIFO order. This ensures that all commands will be
// Shutdown even if the developer does not explicitly take care to do so
// for every invocation.
//
// Pipes:
// StartWithOpts allows the caller to pass an io.Reader to the command
// (StartOpts.Stdin) for it to read from, rather than creating a new pipe
// internally. This makes it possible to connect the output of one
// command to the input of another directly.
//
// Command Line Arguments:
// The arguments passed in calls to Start are appended to any system required
// ones (e.g. for propagating test timeouts, verbosity etc) and the child
// process will call the command with the result of flag.Args(). In this way
// the caller can provide flags used by libraries in the child process
// as well as those specific to the command and the command will only
// receive the args specific to it. The usual "--" convention can be
// used to override this default behaviour.
//
// Caveats:
//
// Handle.Shutdown assumes that the child command/process will terminate
// when its stdin stream is closed. This assumption is unlikely to be valid
// for 'external' commands (e.g. /bin/cp) and in these cases Kill or some other
// application specific mechanism will need to be used.
package modules

import (
	"fmt"
	"io"
	"io/ioutil"
	"os"
	"sync"
	"syscall"
	"time"

	"v.io/v23"
	"v.io/v23/context"
	"v.io/v23/security"
	"v.io/x/ref/lib/exec"
	"v.io/x/ref/lib/flags/consts"
	"v.io/x/ref/lib/testutil/expect"
	"v.io/x/ref/security/agent"
	"v.io/x/ref/security/agent/keymgr"
)

const (
	shellBlessingExtension = "test-shell"
	childBlessingExtension = "child"

	defaultStartTimeout    = time.Minute
	defaultShutdownTimeout = time.Minute
	defaultExpectTimeout   = time.Minute
)

var defaultStartOpts = StartOpts{
	StartTimeout:    defaultStartTimeout,
	ShutdownTimeout: defaultShutdownTimeout,
	ExpectTimeout:   defaultExpectTimeout,
	ExecProtocol:    true,
}

// Shell represents the context within which commands are run.
type Shell struct {
	mu               sync.Mutex
	env              map[string]string
	handles          map[Handle]struct{}
	lifoHandles      []Handle
	defaultStartOpts StartOpts
	// tmpCredDir is the temporary directory created by this
	// shell. This must be removed when the shell is cleaned up.
	tempCredDir      string
	config           exec.Config
	principal        security.Principal
	agent            *keymgr.Agent
	ctx              *context.T
	sessionVerbosity bool
	cancelCtx        func()
}

// NewShell creates a new instance of Shell.
//
// If ctx is non-nil, the shell will manage Principals for child processes.
//
// If p is non-nil, any child process created has its principal blessed
// by the default blessings of 'p', Else any child process created has its
// principal blessed by the default blessings of ctx's principal.
func NewShell(ctx *context.T, p security.Principal) (*Shell, error) {
	sh := &Shell{
		env:              make(map[string]string),
		handles:          make(map[Handle]struct{}),
		config:           exec.NewConfig(),
		defaultStartOpts: defaultStartOpts,
	}
	if ctx == nil {
		return sh, nil
	}
	var err error
	ctx, sh.cancelCtx = context.WithCancel(ctx)
	if ctx, err = v23.SetNewStreamManager(ctx); err != nil {
		return nil, err
	}
	sh.ctx = ctx

	if sh.tempCredDir, err = ioutil.TempDir("", "shell_credentials"); err != nil {
		return nil, err
	}
	if sh.agent, err = keymgr.NewLocalAgent(ctx, sh.tempCredDir, nil); err != nil {
		return nil, err
	}
	sh.principal = p
	if sh.principal == nil {
		sh.principal = v23.GetPrincipal(ctx)
	}
	return sh, nil
}

// NewExpectShell creates a new instance of Shell as per NewShell, but with
// the default StartOpts set to include the specified expect.Testing parameter.
func NewExpectShell(ctx *context.T, p security.Principal, t expect.Testing, verbosity bool) (*Shell, error) {
	sh, err := NewShell(ctx, p)
	if err != nil {
		return nil, err
	}
	sh.sessionVerbosity = verbosity
	sh.SetDefaultStartOpts(DefaultStartOpts().WithSessions(t, time.Minute))
	return sh, nil
}

// DefaultStartOpts returns the current StartOpts stored with the Shell.
func (sh *Shell) DefaultStartOpts() StartOpts {
	return sh.defaultStartOpts
}

// SetDefaultStartOpts sets the default StartOpts stored with the Shell.
func (sh *Shell) SetDefaultStartOpts(opts StartOpts) {
	sh.defaultStartOpts = opts
}

// CustomCredentials encapsulates a Principal which can be shared with
// one or more processes run by a Shell.
type CustomCredentials struct {
	p     security.Principal
	agent *keymgr.Agent
	id    []byte
}

// Principal returns the Principal.
func (c *CustomCredentials) Principal() security.Principal {
	return c.p
}

// File returns a socket which can be used to connect to the agent
// managing this principal. Typically you would pass this to a child
// process.
func (c *CustomCredentials) File() (*os.File, error) {
	return c.agent.NewConnection(c.id)
}

func dup(conn *os.File) (int, error) {
	syscall.ForkLock.RLock()
	fd, err := syscall.Dup(int(conn.Fd()))
	if err != nil {
		syscall.ForkLock.RUnlock()
		return -1, err
	}
	syscall.CloseOnExec(fd)
	syscall.ForkLock.RUnlock()
	return fd, nil
}

// NewCustomCredentials creates a new Principal for StartWithOpts.
// Returns nil if the shell is not managing principals.
func (sh *Shell) NewCustomCredentials() (cred *CustomCredentials, err error) {
	// Create child principal.
	if sh.ctx == nil {
		return nil, nil
	}
	id, conn, err := sh.agent.NewPrincipal(sh.ctx, true)
	if err != nil {
		return nil, err
	}
	fd, err := dup(conn)
	conn.Close()
	if err != nil {
		return nil, err
	}
	p, err := agent.NewAgentPrincipal(sh.ctx, fd, v23.GetClient(sh.ctx))
	if err != nil {
		syscall.Close(fd)
		return nil, err
	}
	return &CustomCredentials{p, sh.agent, id}, nil
}

// NewChildCredentials creates a new principal, served via the
// security agent, that have a blessing from this shell's principal.
// All processes started by this shell will have access to such a
// principal.
// Returns nil when the shell is not managing principals.
func (sh *Shell) NewChildCredentials() (c *CustomCredentials, err error) {
	creds, err := sh.NewCustomCredentials()
	if creds == nil {
		return nil, err
	}
	p := creds.p

	// Bless the child principal with blessings derived from the default blessings
	// of shell's principal.
	blessings := sh.principal.BlessingStore().Default()
	blessingForChild, err := sh.principal.Bless(p.PublicKey(), blessings, childBlessingExtension, security.UnconstrainedUse())
	if err != nil {
		return nil, err
	}
	if err := p.BlessingStore().SetDefault(blessingForChild); err != nil {
		return nil, err
	}
	if _, err := p.BlessingStore().Set(blessingForChild, security.AllPrincipals); err != nil {
		return nil, err
	}
	if err := p.AddToRoots(blessingForChild); err != nil {
		return nil, err
	}

	return creds, nil
}

type Main func(stdin io.Reader, stdout, stderr io.Writer, env map[string]string, args ...string) error

// String returns a string representation of the Shell, which is a
// list of the commands currently available in the shell.
func (sh *Shell) String() string {
	return registry.help("")
}

// Help returns the help message for the specified command.
func (sh *Shell) Help(command string) string {
	return registry.help(command)
}

// Start is shorthand for StartWithOpts(sh.DefaultStartOpts(), ...)
func (sh *Shell) Start(name string, env []string, args ...string) (Handle, error) {
	return sh.StartWithOpts(sh.DefaultStartOpts(), env, name, args...)
}

// StartOpts represents the options that can be passed to the
// StartWithOpts method.
type StartOpts struct {
	// Error is set when creating/intializing instances of StartOpts
	// via one of the factory methods and returned when StartWithOpts
	// is called. This allows usage of of the form:
	//
	// err := sh.StartWithOpts(sh.DefaultStartOpts()...)
	//
	// as opposed to:
	//
	// opts, err := sh.DefaultStartOpts(....)
	// if err != nil {
	//     panic(...)
	// }
	// sh.StartWithOpts(opts, ....)
	Error error

	// Stdin, if non-nil, will be used as the stdin for the child process.
	// If this option is set, then the Stdin() method on the returned Handle
	// will return nil. The client of this API maintains ownership of stdin
	// and must close it, i.e. the shell will not do so.
	Stdin io.Reader
	// Credentials, if non-nil, will be used as the credentials for the
	// child process. If the creds are nil or the shell is not managing
	// principals, the credentials are ignored.
	Credentials *CustomCredentials
	// ExecProtocol indicates whether the child process is expected to
	// implement the v.io/x.ref/lib/exec parent/child protocol.
	// It should be set to false when running non-vanadium commands
	// (e.g. /bin/cp).
	ExecProtocol bool
	// External indicates if the command is an external process rather than
	// a Shell.Main function.
	External bool
	// StartTimeout specifies the amount of time to wait for the
	// child process to signal its correct intialization for Vanadium
	// processes that implement the exec parent/child protocol. It has no
	// effect if External is set to true.
	StartTimeout time.Duration
	// ShutdownTimeout specifics the amount of time to wait for the child
	// process to exit when the Shutdown method is called on that
	// child's handle.
	ShutdownTimeout time.Duration
	// ExpectTesting is used when creating an instance of expect.Session
	// to embed in Handle.
	ExpectTesting expect.Testing
	// ExpectTimeout is the timeout to use with expect.Session.
	ExpectTimeout time.Duration
}

// DefaultStartOpts returns an instance of Startops with the current default
// values. The defaults have values for timeouts, no credentials
// (StartWithOpts will then create credentials each time it is called),
// and with ExecProtocol set to true.
// This is expected to be the common use case.
func DefaultStartOpts() StartOpts {
	return defaultStartOpts
}

// WithCustomCredentials returns an instance of StartOpts with the specified
// credentials. All other options are set to the current defaults.
func (opts StartOpts) WithCustomCredentials(creds *CustomCredentials) StartOpts {
	opts.Credentials = creds
	return opts
}

// WithSessions returns a copy of opts with the specified expect.Testing and
// associated timeout.
func (opts StartOpts) WithSessions(t expect.Testing, timeout time.Duration) StartOpts {
	opts.ExpectTesting = t
	opts.ExpectTimeout = timeout
	return opts
}

// WithStdin returns a copy of opts with the specified Stdin io.Reader.
func (opts StartOpts) WithStdin(stdin io.Reader) StartOpts {
	opts.Stdin = stdin
	return opts
}

// NoExecCommand returns a copy of opts with the External option
// enabled and ExecProtocol disabled.
func (opts StartOpts) NoExecCommand() StartOpts {
	opts.External = true
	opts.ExecProtocol = false
	return opts
}

// ExternalCommand returns a copy of StartOpts with the
// External option enabled.
func (opts StartOpts) ExternalCommand() StartOpts {
	opts.External = true
	return opts
}

// StartWithOpts starts the specified command according to the supplied
// StartOpts and returns a Handle which can be used for interacting with
// that command.
//
// The environment variables for the command are set by merging variables
// from the OS environment, those in this Shell and those provided as a
// parameter to it. In general, it prefers values from its parameter over
// those from the Shell, over those from the OS. However, the VeyronCredentials
// and agent FdEnvVar variables will never use the value from the Shell or OS.
//
// If the shell is managing principals, the command is configured to
// connect to the shell's agent. Custom credentials may be specified
// via StartOpts. If the shell is not managing principals, set
// the VeyronCredentials environment variable in the 'env' parameter.
//
// The Shell tracks all of the Handles that it creates so that it can shut
// them down when asked to. The returned Handle may be non-nil even when an
// error is returned, in which case it may be used to retrieve any output
// from the failed command.
//
// StartWithOpts will return a valid handle for errors that occur during the
// child processes startup process. It is thus possible to call Shutdown
// to obtain the error output. Handle will be nil if the error is due to
// some other reason, such as failure to create pipes/files before starting
// the child process. A common use will therefore be:
//
//    h, err := sh.Start(env, "/bin/echo", "hello")
//    if err != nil {
//        if h != nil {
//            h.Shutdown(nil,os.Stderr)
//        }
//        t.Fatal(err)
//    }
func (sh *Shell) StartWithOpts(opts StartOpts, env []string, name string, args ...string) (Handle, error) {
	if opts.Error != nil {
		return nil, opts.Error
	}
	var desc *commandDesc
	if opts.External {
		desc = registry.getExternalCommand(name)
	} else if desc = registry.getCommand(name); desc == nil {
		return nil, fmt.Errorf("%s: not registered", name)
	}
	if opts.Credentials == nil {
		var err error
		opts.Credentials, err = sh.NewChildCredentials()
		if err != nil {
			return nil, err
		}
	}
	cmd := desc.factory()
	expanded := append([]string{name}, sh.expand(args...)...)
	cenv, err := sh.setupCommandEnv(env)
	if err != nil {
		return nil, err
	}

	var p *os.File
	if opts.Credentials != nil {
		p, err = opts.Credentials.File()
		if err != nil {
			return nil, err
		}
	}

	h, err := cmd.start(sh, p, &opts, cenv, expanded...)
	if err != nil {
		return h, err
	}
	sh.mu.Lock()
	sh.handles[h] = struct{}{}
	sh.lifoHandles = append(sh.lifoHandles, h)
	sh.mu.Unlock()
	return h, nil
}

// CommandEnvelope returns the command line and environment that would be
// used for running the subprocess or function if it were started with the
// specifed arguments.
func (sh *Shell) CommandEnvelope(name string, env []string, args ...string) ([]string, []string) {
	cmd := registry.getCommand(name)
	if cmd == nil {
		return []string{}, []string{}
	}
	menv, err := sh.setupCommandEnv(env)
	if err != nil {
		return []string{}, []string{}
	}
	return cmd.factory().envelope(sh, menv, args...)
}

// Forget tells the Shell to stop tracking the supplied Handle. This is
// generally used when the application wants to control the order that
// commands are shutdown in.
func (sh *Shell) Forget(h Handle) {
	sh.mu.Lock()
	delete(sh.handles, h)
	sh.mu.Unlock()
}

func (sh *Shell) expand(args ...string) []string {
	exp := []string{}
	for _, a := range args {
		if len(a) > 0 && a[0] == '$' {
			if v, present := sh.env[a[1:]]; present {
				exp = append(exp, v)
				continue
			}
		}
		exp = append(exp, a)
	}
	return exp
}

// GetVar returns the variable associated with the specified key
// and an indication of whether it is defined or not.
func (sh *Shell) GetVar(key string) (string, bool) {
	sh.mu.Lock()
	defer sh.mu.Unlock()
	v, present := sh.env[key]
	return v, present
}

// SetVar sets the value to be associated with key.
func (sh *Shell) SetVar(key, value string) {
	sh.mu.Lock()
	defer sh.mu.Unlock()
	// TODO(cnicolaou): expand value
	sh.env[key] = value
}

// ClearVar removes the speficied variable from the Shell's environment
func (sh *Shell) ClearVar(key string) {
	sh.mu.Lock()
	defer sh.mu.Unlock()
	delete(sh.env, key)
}

// GetConfigKey returns the value associated with the specified key in
// the Shell's config and an indication of whether it is defined or
// not.
func (sh *Shell) GetConfigKey(key string) (string, bool) {
	v, err := sh.config.Get(key)
	return v, err == nil
}

// SetConfigKey sets the value of the specified key in the Shell's
// config.
func (sh *Shell) SetConfigKey(key, value string) {
	sh.config.Set(key, value)
}

// ClearConfigKey removes the speficied key from the Shell's config.
func (sh *Shell) ClearConfigKey(key string) {
	sh.config.Clear(key)
}

// Env returns the entire set of environment variables associated with this
// Shell as a string slice.
func (sh *Shell) Env() []string {
	vars := []string{}
	sh.mu.Lock()
	defer sh.mu.Unlock()
	for k, v := range sh.env {
		vars = append(vars, k+"="+v)
	}
	return vars
}

// Cleanup calls Shutdown on all of the Handles currently being tracked
// by the Shell and writes to stdout and stderr as per the Shutdown
// method in the Handle interface. Cleanup returns the error from the
// last Shutdown that returned a non-nil error. The order that the
// Shutdown routines are executed is not defined.
func (sh *Shell) Cleanup(stdout, stderr io.Writer) error {
	sh.mu.Lock()
	verbose := sh.sessionVerbosity
	sh.mu.Unlock()

	writeMsg := func(format string, args ...interface{}) {
		if !verbose {
			return
		}
		if stderr != nil {
			fmt.Fprintf(stderr, format, args...)
		}
	}

	if verbose {
		writeMsg("---- Shell Cleanup ----\n")
	}
	sh.mu.Lock()
	handles := make([]Handle, 0, len(sh.lifoHandles))
	for _, h := range sh.lifoHandles {
		if _, present := sh.handles[h]; present {
			handles = append(handles, h)
		}
	}
	sh.handles = make(map[Handle]struct{})
	sh.lifoHandles = nil
	sh.mu.Unlock()
	var err error
	for i := len(handles); i > 0; i-- {
		h := handles[i-1]
		switch v := h.(type) {
		case *functionHandle:
			writeMsg("---- Cleanup calling Shutdown on function %q\n", v.name)
		case *execHandle:
			writeMsg("---- Cleanup calling Shutdown on command %q\n", v.name)
		}
		cerr := h.Shutdown(stdout, stderr)
		if cerr != nil {
			err = cerr
		}
		fn := func() string {
			if cerr == nil {
				return ": done"
			} else {
				return ": error: " + err.Error()
			}
		}
		switch v := h.(type) {
		case *functionHandle:
			writeMsg("---- Shutdown on function %q%s\n", v.name, fn())
		case *execHandle:
			writeMsg("---- Shutdown on command %q%s\n", v.name, fn())
		}
	}

	if sh.cancelCtx != nil {
		// Note(ribrdb, caprita): This will shutdown the agents.  If there
		// were errors shutting down it is possible there could be child
		// processes still running, and stopping the agent may cause
		// additional failures.
		sh.cancelCtx()
	}
	os.RemoveAll(sh.tempCredDir)
	return err
}

func (sh *Shell) setupCommandEnv(env []string) ([]string, error) {
	osmap := envSliceToMap(os.Environ())
	evmap := envSliceToMap(env)

	sh.mu.Lock()
	defer sh.mu.Unlock()
	m1 := mergeMaps(osmap, sh.env)
	// Clear any VeyronCredentials directory in m1 as we never
	// want the child to directly use the directory specified
	// by the shell's VeyronCredentials.
	delete(m1, consts.VeyronCredentials)
	delete(m1, agent.FdVarName)

	m2 := mergeMaps(m1, evmap)
	r := []string{}
	for k, v := range m2 {
		r = append(r, k+"="+v)
	}
	return r, nil
}

// ExpectSession is a subset of v.io/x/ref/lib/testutil/expect.Session's methods
// that are embedded in Handle.
type ExpectSession interface {
	Expect(expected string)
	ExpectEOF() error
	ExpectRE(pattern string, n int) [][]string
	ExpectSetEventuallyRE(expected ...string) [][]string
	ExpectSetRE(expected ...string) [][]string
	ExpectVar(name string) string
	Expectf(format string, args ...interface{})
	ReadAll() (string, error)
	ReadLine() string
	SetVerbosity(bool)
	Failed() bool
	Error() error
}

// Handle represents a running command.
type Handle interface {
	ExpectSession

	// Stdout returns a reader to the running command's stdout stream.
	Stdout() io.Reader

	// Stderr returns a reader to the running command's stderr
	// stream.
	Stderr() io.Reader

	// Stdin returns a writer to the running command's stdin. The
	// convention is for commands to wait for stdin to be closed before
	// they exit, thus the caller should close stdin when it wants the
	// command to exit cleanly.
	Stdin() io.Writer

	// CloseStdin closes stdin in a manner that avoids a data race
	// between any current readers on it.
	CloseStdin()

	// Shutdown closes the Stdin for the command and then reads output
	// from the command's stdout until it encounters EOF, waits for
	// the command to complete and then reads all of its stderr output.
	// The stdout and stderr contents are written to the corresponding
	// io.Writers if they are non-nil, otherwise the content is discarded.
	Shutdown(stdout, stderr io.Writer) error

	// Pid returns the pid of the process running the command
	Pid() int
}

// command is used to abstract the implementations of inprocess and subprocess
// commands.
type command interface {
	envelope(sh *Shell, env []string, args ...string) ([]string, []string)
	start(sh *Shell, agent *os.File, opts *StartOpts, env []string, args ...string) (Handle, error)
}