Fix 'docker cp' mount table explosion, take four
* Supersedes #38993
* Supersedes #43583
* Supersedes #44171

**- What I did**
* Fixed #38995
* Fixed #43390
* Made the issue reproducible in a DinD environment so it can be covered by a regression test

**- How I did it**
* I got the issue to reproduce in a DinD container by changing the mount propagation mode to shared, which matches the behaviour of SystemD PID 1 outside of a container.
* I arranged to have the daemon bind-mount the container volumes and perform the archiving operation inside an unshared mount namespace so the mount events would not propagate into the container's namespace.

**- How to verify it**
* Shiny new integration test

**- Description for the changelog**
<!--
Write a short (one line) summary that describes the changes in this
pull request for inclusion in the changelog:
-->
* Fixed `docker cp` failing with "no space left on device" when the daemon root is mounted into the container as a volume


**- A picture of a cute animal (not mandatory but encouraged)**


pkg/chrootarchive/archive_unix.go
//go:build !windows
// +build !windows

package chrootarchive // import "github.com/docker/docker/pkg/chrootarchive"

import (
	"bytes"
	"encoding/json"
	"flag"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"runtime"
	"strings"

	"github.com/docker/docker/pkg/archive"
	"github.com/docker/docker/pkg/reexec"
	"github.com/pkg/errors"
)

// untar is the entry-point for docker-untar on re-exec. This is not used on
// Windows as it does not support chroot, hence no point sandboxing through
// chroot and rexec.
func untar() {
	runtime.LockOSThread()
	flag.Parse()

	var options archive.TarOptions

	// read the options from the pipe "ExtraFiles"
	if err := json.NewDecoder(os.NewFile(3, "options")).Decode(&options); err != nil {
		fatal(err)
	}

	dst := flag.Arg(0)
	var root string
	if len(flag.Args()) > 1 {
		root = flag.Arg(1)
	}

	if root == "" {
		root = dst
	}

	if err := chroot(root); err != nil {
		fatal(err)
	}

	if err := archive.Unpack(os.Stdin, dst, &options); err != nil {
		fatal(err)
	}
	// fully consume stdin in case it is zero padded
	if _, err := flush(os.Stdin); err != nil {
		fatal(err)
	}

	os.Exit(0)
}

func invokeUnpack(decompressedArchive io.Reader, dest string, options *archive.TarOptions, root string) error {
	if root == "" {
		return errors.New("must specify a root to chroot to")
	}

	// We can't pass a potentially large exclude list directly via cmd line
	// because we easily overrun the kernel's max argument/environment size
	// when the full image list is passed (e.g. when this is used by
	// `docker load`). We will marshall the options via a pipe to the
	// child
	r, w, err := os.Pipe()
	if err != nil {
		return fmt.Errorf("Untar pipe failure: %v", err)
	}

	if root != "" {
		relDest, err := filepath.Rel(root, dest)
		if err != nil {
			return err
		}
		if relDest == "." {
			relDest = "/"
		}
		if relDest[0] != '/' {
			relDest = "/" + relDest
		}
		dest = relDest
	}

	cmd := reexec.Command("docker-untar", dest, root)
	cmd.Stdin = decompressedArchive

	cmd.ExtraFiles = append(cmd.ExtraFiles, r)
	output := bytes.NewBuffer(nil)
	cmd.Stdout = output
	cmd.Stderr = output

	// reexec.Command() sets cmd.SysProcAttr.Pdeathsig on Linux, which
	// causes the started process to be signaled when the creating OS thread
	// dies. Ensure that the reexec is not prematurely signaled. See
	// https://go.dev/issue/27505 for more information.
	runtime.LockOSThread()
	defer runtime.UnlockOSThread()
	if err := cmd.Start(); err != nil {
		w.Close()
		return fmt.Errorf("Untar error on re-exec cmd: %v", err)
	}

	// write the options to the pipe for the untar exec to read
	if err := json.NewEncoder(w).Encode(options); err != nil {
		w.Close()
		return fmt.Errorf("Untar json encode to pipe failed: %v", err)
	}
	w.Close()

	if err := cmd.Wait(); err != nil {
		// when `xz -d -c -q | docker-untar ...` failed on docker-untar side,
		// we need to exhaust `xz`'s output, otherwise the `xz` side will be
		// pending on write pipe forever
		io.Copy(io.Discard, decompressedArchive)

		return fmt.Errorf("Error processing tar file(%v): %s", err, output)
	}
	return nil
}

func tar() {
	runtime.LockOSThread()
	flag.Parse()

	src := flag.Arg(0)
	var root string
	if len(flag.Args()) > 1 {
		root = flag.Arg(1)
	}

	if root == "" {
		root = src
	}

	if err := realChroot(root); err != nil {
		fatal(err)
	}

	var options archive.TarOptions
	if err := json.NewDecoder(os.Stdin).Decode(&options); err != nil {
		fatal(err)
	}

	rdr, err := archive.TarWithOptions(src, &options)
	if err != nil {
		fatal(err)
	}
	defer rdr.Close()

	if _, err := io.Copy(os.Stdout, rdr); err != nil {
		fatal(err)
	}

	os.Exit(0)
}

func invokePack(srcPath string, options *archive.TarOptions, root string) (io.ReadCloser, error) {
	if root == "" {
		return nil, errors.New("root path must not be empty")
	}

	relSrc, err := filepath.Rel(root, srcPath)
	if err != nil {
		return nil, err
	}
	if relSrc == "." {
		relSrc = "/"
	}
	if relSrc[0] != '/' {
		relSrc = "/" + relSrc
	}

	// make sure we didn't trim a trailing slash with the call to `Rel`
	if strings.HasSuffix(srcPath, "/") && !strings.HasSuffix(relSrc, "/") {
		relSrc += "/"
	}

	cmd := reexec.Command("docker-tar", relSrc, root)

	errBuff := bytes.NewBuffer(nil)
	cmd.Stderr = errBuff

	tarR, tarW := io.Pipe()
	cmd.Stdout = tarW

	stdin, err := cmd.StdinPipe()
	if err != nil {
		return nil, errors.Wrap(err, "error getting options pipe for tar process")
	}

	started := make(chan error)
	go func() {
		// reexec.Command() sets cmd.SysProcAttr.Pdeathsig on Linux,
		// which causes the started process to be signaled when the
		// creating OS thread dies. Ensure that the subprocess is not
		// prematurely signaled. See https://go.dev/issue/27505 for more
		// information.
		runtime.LockOSThread()
		defer runtime.UnlockOSThread()
		if err := cmd.Start(); err != nil {
			started <- err
			return
		}
		close(started)
		err := cmd.Wait()
		err = errors.Wrapf(err, "error processing tar file: %s", errBuff)
		tarW.CloseWithError(err)
	}()
	if err := <-started; err != nil {
		return nil, errors.Wrap(err, "tar error on re-exec cmd")
	}

	if err := json.NewEncoder(stdin).Encode(options); err != nil {
		stdin.Close()
		return nil, errors.Wrap(err, "tar json encode to pipe failed")
	}
	stdin.Close()

	return tarR, nil
}

//go:build !windows
// +build !windows

package chrootarchive // import "github.com/docker/docker/pkg/chrootarchive"

import (
	"io"
	"path/filepath"
	"strings"

	"github.com/docker/docker/pkg/archive"
	"github.com/pkg/errors"
)

func invokeUnpack(decompressedArchive io.Reader, dest string, options *archive.TarOptions, root string) error {
	relDest, err := resolvePathInChroot(root, dest)
	if err != nil {
		return err
	}

	done := make(chan error)
	err = goInChroot(root, func() { done <- archive.Unpack(decompressedArchive, relDest, options) })
	if err != nil {
		return err
	}
	return <-done
}

func invokePack(srcPath string, options *archive.TarOptions, root string) (io.ReadCloser, error) {
	relSrc, err := resolvePathInChroot(root, srcPath)
	if err != nil {
		return nil, err
	}

	// make sure we didn't trim a trailing slash with the call to `resolvePathInChroot`
	if strings.HasSuffix(srcPath, "/") && !strings.HasSuffix(relSrc, "/") {
		relSrc += "/"
	}

	tb, err := archive.NewTarballer(relSrc, options)
	if err != nil {
		return nil, errors.Wrap(err, "error processing tar file")
	}
	err = goInChroot(root, tb.Do)
	if err != nil {
		return nil, errors.Wrap(err, "could not chroot")
	}
	return tb.Reader(), nil
}

// resolvePathInChroot returns the equivalent to path inside a chroot rooted at root.
// The returned path always begins with '/'.
//
//   - resolvePathInChroot("/a/b", "/a/b/c/d") -> "/c/d"
//   - resolvePathInChroot("/a/b", "/a/b")     -> "/"
//
// The implementation is buggy, and some bugs may be load-bearing.
// Here be dragons.
func resolvePathInChroot(root, path string) (string, error) {
	if root == "" {
		return "", errors.New("root path must not be empty")
	}
	rel, err := filepath.Rel(root, path)
	if err != nil {
		return "", err
	}
	if rel == "." {
		rel = "/"
	}
	if rel[0] != '/' {
		rel = "/" + rel
	}
	return rel, nil
}

You put in a lot more effort figuring out what this code does than I did. I just spotted some code literally written twice verbatim and DRYed it out. I'd be glad to document it, especially now that the hard part is already done 😁 