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pipe.go
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pipe.go
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// Copyright 2022 Google LLC
//
// 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 pipe extends os.exec, making it easier to create pipes to subcommands.
package pipe
import (
"context"
"fmt"
"io"
"os"
"os/exec"
)
// Cmd is drop-in replacement for exec.Cmd, extended with the Pipe method.
type Cmd struct {
*exec.Cmd
closeAfterStart []io.Closer // closed after Start()
closeAfterWait []io.Closer // closed after Wait()
}
// CommandContext returns a new Cmd. See exec.CommandContext for details.
func CommandContext(ctx context.Context, name string, arg ...string) *Cmd {
return &Cmd{Cmd: exec.CommandContext(ctx, name, arg...)}
}
// RPipe returns the reading side of a pipe that will be connected to the
// subprocess when the command starts.
//
// The subprocess can write to the pipe using the file descriptor returned by
// RPipe. Cmd.ExtraFiles should not be modified directly if RPipe is called.
//
// Wait will close the pipe after seeing the command exit, so most callers need
// not close the pipe themselves. It is thus incorrect to call Wait before all
// reads from the pipe have completed. For the same reason, it is incorrect to
// use Run when using Pipe. See the exec.Cmd.StdoutPipe example [1] for
// idiomatic usage.
//
// [1]: https://pkg.go.dev/os/exec#example-Cmd.StdoutPipe
func (c *Cmd) RPipe() (uintptr, io.ReadCloser, error) {
if c.Process != nil {
return 0, nil, fmt.Errorf("capture: RPipe after process started")
}
r, w, err := os.Pipe()
if err != nil {
return 0, nil, err
}
fd := c.registerPipe(r, w)
return fd, r, nil
}
// WPipe returns the writer side of a pipe that will be connected to the
// subprocess when the command starts.
//
// The subprocess can read from the pipe using the file descriptor returned by
// WPipe. Cmd.ExtraFiles should not be modified directly if WPipe is called.
//
// The pipe will be closed automatically after Wait sees the command exit. A
// caller need only call Close to force the pipe to close sooner. For example,
// if the command being run will not exit until standard input is closed, the
// caller must close the pipe.
func (c *Cmd) WPipe() (uintptr, io.WriteCloser, error) {
if c.Process != nil {
return 0, nil, fmt.Errorf("capture: WPipe after process started")
}
// TODO: StdinPipe [1] makes sure w is only closed once. Understand why
// they do that and do the same if needed.
//
// [1]: https://cs.opensource.google/go/go/+/refs/tags/go1.18.4:src/os/exec/exec.go;l=593
r, w, err := os.Pipe()
if err != nil {
return 0, nil, err
}
fd := c.registerPipe(w, r)
return fd, w, nil
}
func (c *Cmd) registerPipe(local, remote *os.File) uintptr {
c.closeAfterStart = append(c.closeAfterStart, remote)
c.closeAfterWait = append(c.closeAfterWait, local)
return addInheritedFile(c.Cmd, remote)
}
// Start is identical to exec.Command.Start.
func (c *Cmd) Start() error {
if err := c.Cmd.Start(); err != nil {
return err
}
closeAll(&c.closeAfterStart)
return nil
}
// Wait is identical to exec.Command.Wait.
func (c *Cmd) Wait() error {
if err := c.Cmd.Wait(); err != nil {
return err
}
closeAll(&c.closeAfterWait)
return nil
}
// Cleanup cleans up any unused resources.
func (c *Cmd) Cleanup() {
closeAll(&c.closeAfterStart)
closeAll(&c.closeAfterWait)
}
func closeAll(files *[]io.Closer) {
for _, f := range *files {
f.Close()
}
*files = nil
}