exec.go

/*
Copyright 2014 Google Inc. All rights reserved.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package exec

import (
	osexec "os/exec"
	"syscall"
)

// Interface is an interface that presents a subset of the os/exec API.  Use this
// when you want to inject fakeable/mockable exec behavior.
type Interface interface {
	// Command returns a Cmd instance which can be used to run a single command.
	// This follows the pattern of package os/exec.
	Command(cmd string, args ...string) Cmd
}

// Cmd is an interface that presents an API that is very similar to Cmd from os/exec.
// As more functionality is needed, this can grow.  Since Cmd is a struct, we will have
// to replace fields with get/set method pairs.
type Cmd interface {
	// CombinedOutput runs the command and returns its combined standard output
	// and standard error.  This follows the pattern of package os/exec.
	CombinedOutput() ([]byte, error)
}

// ExitError is an interface that presents an API similar to os.ProcessState, which is
// what ExitError from os/exec is.  This is designed to make testing a bit easier and
// probably loses some of the cross-platform properties of the underlying library.
type ExitError interface {
	String() string
	Error() string
	Exited() bool
	ExitStatus() int
}

// Implements Interface in terms of really exec()ing.
type executor struct{}

// New returns a new Interface which will os/exec to run commands.
func New() Interface {
	return &executor{}
}

// Command is part of the Interface interface.
func (executor *executor) Command(cmd string, args ...string) Cmd {
	return (*cmdWrapper)(osexec.Command(cmd, args...))
}

// Wraps exec.Cmd so we can capture errors.
type cmdWrapper osexec.Cmd

// CombinedOutput is part of the Cmd interface.
func (cmd *cmdWrapper) CombinedOutput() ([]byte, error) {
	out, err := (*osexec.Cmd)(cmd).CombinedOutput()
	if err != nil {
		ee, ok := err.(*osexec.ExitError)
		if !ok {
			return out, err
		}
		// Force a compile fail if exitErrorWrapper can't convert to ExitError.
		var x ExitError = &exitErrorWrapper{ee}
		return out, x
	}
	return out, nil
}

// exitErrorWrapper is an implementation of ExitError in terms of os/exec ExitError.
// Note: standard exec.ExitError is type *os.ProcessState, which already implements Exited().
type exitErrorWrapper struct {
	*osexec.ExitError
}

// ExitStatus is part of the ExitError interface.
func (eew exitErrorWrapper) ExitStatus() int {
	ws, ok := eew.Sys().(syscall.WaitStatus)
	if !ok {
		panic("can't call ExitStatus() on a non-WaitStatus exitErrorWrapper")
	}
	return ws.ExitStatus()
}