Add option to run pre/post commands for any container (#73)

* Add option to run pre commands on arbitrary container

* Correctly handle quoted args in commands

* Provide defaults for test version arg

* Allow filtering of target containers

* Add documentation on exec commands

* Use mysqldump in exec test

* Add mysqldump section to recipes

* Also run commands test in swarm mode

* Use name instead of id

* Add syntax highlighting

* Add missing license headers

cmd/backup/exec.go

@@ -0,0 +1,122 @@
+// Copyright 2022 - Offen Authors <hioffen@posteo.de>
+// SPDX-License-Identifier: MPL-2.0
+
+package main
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"strings"
+	"sync"
+
+	"github.com/cosiner/argv"
+	"github.com/docker/docker/api/types"
+	"github.com/docker/docker/api/types/filters"
+	"github.com/docker/docker/pkg/stdcopy"
+)
+
+func (s *script) exec(containerRef string, command string) ([]byte, []byte, error) {
+	args, _ := argv.Argv(command, nil, nil)
+	execID, err := s.cli.ContainerExecCreate(context.Background(), containerRef, types.ExecConfig{
+		Cmd:          args[0],
+		AttachStdin:  true,
+		AttachStderr: true,
+	})
+	if err != nil {
+		return nil, nil, fmt.Errorf("exec: error creating container exec: %w", err)
+	}
+
+	resp, err := s.cli.ContainerExecAttach(context.Background(), execID.ID, types.ExecStartCheck{})
+	if err != nil {
+		return nil, nil, fmt.Errorf("exec: error attaching container exec: %w", err)
+	}
+	defer resp.Close()
+
+	var outBuf, errBuf bytes.Buffer
+	outputDone := make(chan error)
+
+	go func() {
+		_, err := stdcopy.StdCopy(&outBuf, &errBuf, resp.Reader)
+		outputDone <- err
+	}()
+
+	select {
+	case err := <-outputDone:
+		if err != nil {
+			return nil, nil, fmt.Errorf("exec: error demultiplexing output: %w", err)
+		}
+		break
+	}
+
+	stdout, err := ioutil.ReadAll(&outBuf)
+	if err != nil {
+		return nil, nil, fmt.Errorf("exec: error reading stdout: %w", err)
+	}
+	stderr, err := ioutil.ReadAll(&errBuf)
+	if err != nil {
+		return nil, nil, fmt.Errorf("exec: error reading stderr: %w", err)
+	}
+
+	res, err := s.cli.ContainerExecInspect(context.Background(), execID.ID)
+	if err != nil {
+		return nil, nil, fmt.Errorf("exec: error inspecting container exec: %w", err)
+	}
+
+	if res.ExitCode > 0 {
+		return stdout, stderr, fmt.Errorf("exec: running command exited %d", res.ExitCode)
+	}
+
+	return stdout, stderr, nil
+}
+
+func (s *script) runLabeledCommands(label string) error {
+	f := []filters.KeyValuePair{
+		{Key: "label", Value: label},
+	}
+	if s.c.ExecLabel != "" {
+		f = append(f, filters.KeyValuePair{
+			Key:   "label",
+			Value: fmt.Sprintf("docker-volume-backup.exec-label=%s", s.c.ExecLabel),
+		})
+	}
+	containersWithCommand, err := s.cli.ContainerList(context.Background(), types.ContainerListOptions{
+		Quiet:   true,
+		Filters: filters.NewArgs(f...),
+	})
+	if err != nil {
+		return fmt.Errorf("runLabeledCommands: error querying for containers", err)
+	}
+
+	if len(containersWithCommand) == 0 {
+		return nil
+	}
+
+	wg := sync.WaitGroup{}
+	wg.Add(len(containersWithCommand))
+
+	var cmdErrors []error
+	for _, container := range containersWithCommand {
+		go func(c types.Container) {
+			cmd, _ := c.Labels[label]
+			s.logger.Infof("Running %s command %s for container %s", label, cmd, strings.TrimPrefix(c.Names[0], "/"))
+			stdout, stderr, err := s.exec(c.ID, cmd)
+			if err != nil {
+				cmdErrors = append(cmdErrors, err)
+			}
+			if s.c.ExecForwardOutput {
+				os.Stderr.Write(stderr)
+				os.Stdout.Write(stdout)
+			}
+			wg.Done()
+		}(container)
+	}
+
+	wg.Wait()
+	if len(cmdErrors) != 0 {
+		return join(cmdErrors...)
+	}
+	return nil
+}