cmd/create: Don't block user interaction while fetching the image size

It takes 'skopeo inspect' a few seconds to fetch the image size from the
remote registry, and while that happens the user can't interact with the
image download prompt:
  $ toolbox create
  Image required to create toolbox container.
  <wait for a few seconds>
  Download registry.fedoraproject.org/fedora-toolbox:39 (359.8MB)? [y/N]:

This feels awkward because it's not clear to the user what's going on
during those few seconds.  Moreover, while knowing the image size can be
convenient at times, for example when disk space and network bandwidth
are limited, it's not always important.

It will be better if 'skopeo inspect' ran in the background, while
waiting for the user to respond to the image download prompt, and once
the image size has been fetched, the image download prompt can be
updated to include it.

So, initially:
  $ toolbox create
  Image required to create toolbox container.
  Download registry.fedoraproject.org/fedora-toolbox:39 (...)? [y/N]:

... and then once the size is available:
  $ toolbox create
  Image required to create toolbox container.
  Download registry.fedoraproject.org/fedora-toolbox:39 (359.8MB)? [y/N]:

If skopeo(1) is missing or too old, then the prompt can continue without
the size, as it did before:
  $ toolbox create
  Image required to create toolbox container.
  Download registry.fedoraproject.org/fedora-toolbox:39 [y/N]:

Updating the image download prompt with the results of 'skopeo inspect'
is vulnerable to races.  At the same time as the terminal's cursor is
moved to the beginning of the current line to overwrite the earlier
prompt with the new one, the user can keep typing, which will keep
moving the cursor forward.  This competition over the cursor can lead to
awkward outcomes. For example, the prompt can overwrite the characters
typed in by the user, leaving characters in the terminal's input buffer
waiting for the user to hit ENTER, even though they are not visible on
the screen.  Another example is that hitting BACKSPACE can end up
deleting parts of the prompt, instead of stopping at the edge.

This is solved by putting the terminal device into non-canonical mode
input and disabling the echoing of input characters, while the prompt is
being updated.  This prevents input from moving the terminal's cursor
forward, and from accumulating in the terminal's input buffer even if
it might not be visible.  Any input during this interim period is
discarded and replaced by '...', and a fresh new prompt is shown in the
following line.

In practice, this race shouldn't be too common.  It can only happen if
the user is typing right when the prompt is being updated, which is
unlikely because it's only supposed to be a short 'yes' or 'no' input.

The use of the context.Cause and context.WithCancelCause functions [1]
requires Go >= 1.20.  Bumping the Go version in src/go.mod then requires
a 'go mod tidy'.  Otherwise, it leads to:
  $ meson compile -C builddir --verbose
  ...
  /home/rishi/devel/containers/git/toolbox/src/go-build-wrapper
    /home/rishi/devel/containers/git/toolbox/src
    /home/rishi/devel/containers/git/toolbox/builddir src/toolbox
    0.0.99.4 cc /lib64/ld-linux-x86-64.so.2 false
  go: updates to go.mod needed; to update it:
          go mod tidy
  ninja: build stopped: subcommand failed.

[1] https://pkg.go.dev/context

#752
#1263

Author: debarshiray

.github/workflows/ubuntu-tests.yaml

@@ -44,7 +44,7 @@ jobs:
             fish \
             gcc \
             go-md2man \
-            golang \
+            golang-1.20 \
             meson \
             ninja-build \
             openssl \
@@ -54,6 +54,10 @@ jobs:
             systemd \
             udisks2
 
+      - name: Set up PATH for Go 1.20
+        run: |
+          echo "PATH=/usr/lib/go-1.20/bin:$PATH" >> "$GITHUB_ENV"
+
       - name: Checkout Bats
         uses: actions/checkout@v3
         with:

src/cmd/create.go

@@ -37,6 +37,16 @@ import (
 	"github.com/spf13/cobra"
 )
 
+type promptForDownloadError struct {
+	ImageSize string
+}
+
+type promptOptions struct {
+	UsePlaceholder bool
+}
+
+type promptOption func(*promptOptions)
+
 const (
 	alpha    = `abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ`
 	num      = `0123456789`
@@ -573,8 +583,7 @@ func getFullyQualifiedImageFromRepoTags(image string) (string, error) {
 	return imageFull, nil
 }
 
-func getImageSizeFromRegistry(imageFull string) (string, error) {
-	ctx := context.Background()
+func getImageSizeFromRegistry(ctx context.Context, imageFull string) (string, error) {
 	image, err := skopeo.Inspect(ctx, imageFull)
 	if err != nil {
 		return "", err
@@ -598,6 +607,23 @@ func getImageSizeFromRegistry(imageFull string) (string, error) {
 	return imageSizeHuman, nil
 }
 
+func getImageSizeFromRegistryAsync(ctx context.Context, imageFull string) (<-chan string, <-chan error) {
+	retValCh := make(chan string)
+	errCh := make(chan error)
+
+	go func() {
+		imageSize, err := getImageSizeFromRegistry(ctx, imageFull)
+		if err != nil {
+			errCh <- err
+			return
+		}
+
+		retValCh <- imageSize
+	}()
+
+	return retValCh, errCh
+}
+
 func getServiceSocket(serviceName string, unitName string) (string, error) {
 	logrus.Debugf("Resolving path to the %s socket", serviceName)
 
@@ -710,18 +736,7 @@ func pullImage(image, release, authFile string) (bool, error) {
 	}
 
 	if promptForDownload {
-		fmt.Println("Image required to create toolbox container.")
-
-		var prompt string
-
-		if imageSize, err := getImageSizeFromRegistry(imageFull); err != nil {
-			logrus.Debugf("Getting image size failed: %s", err)
-			prompt = fmt.Sprintf("Download %s? [y/N]:", imageFull)
-		} else {
-			prompt = fmt.Sprintf("Download %s (%s)? [y/N]:", imageFull, imageSize)
-		}
-
-		shouldPullImage = askForConfirmation(prompt)
+		shouldPullImage = showPromptForDownload(imageFull)
 	}
 
 	if !shouldPullImage {
@@ -751,6 +766,248 @@ func pullImage(image, release, authFile string) (bool, error) {
 	return true, nil
 }
 
+func createPromptForDownload(imageFull, imageSize string, options ...promptOption) string {
+	o := &promptOptions{}
+	for _, option := range options {
+		option(o)
+	}
+
+	if imageSize == "" {
+		if o.UsePlaceholder {
+			imageSize = " ... MB"
+		} else {
+			prompt := fmt.Sprintf("Download %s? [y/N]:", imageFull)
+			return prompt
+		}
+	}
+
+	imageSizeLen := len(imageSize)
+	var padding1 int
+	var padding2 int
+	var padding3 int
+	var padding4 int
+
+	if imageSizeLen < 7 {
+		padding4 = 7 - imageSizeLen
+	}
+
+	if padding4 > 1 {
+		padding3 = padding4 - 1
+		padding4 = 1
+	}
+
+	if padding3 > 1 {
+		padding2 = padding3 - 1
+		padding3 = 1
+	}
+
+	if padding2 > 1 {
+		padding1 = padding2 - 1
+		padding2 = 1
+	}
+
+	prompt := fmt.Sprintf("Download %s (%*s%s%*s)? %*s[y/N]:%*s",
+		imageFull,
+		padding1, "",
+		imageSize,
+		padding2, "",
+		padding3, "",
+		padding4, "")
+
+	return prompt
+}
+
+func showPromptForDownloadFirst(imageFull string) (bool, error) {
+	withPlaceholder := func(o *promptOptions) {
+		o.UsePlaceholder = true
+	}
+
+	prompt := createPromptForDownload(imageFull, "", withPlaceholder)
+
+	parentCtx := context.Background()
+	askCtx, askCancel := context.WithCancelCause(parentCtx)
+	defer askCancel(errors.New("clean-up"))
+
+	askCh, askErrCh := askForConfirmationAsync(askCtx, prompt, nil)
+
+	imageSizeCtx, imageSizeCancel := context.WithCancelCause(parentCtx)
+	defer imageSizeCancel(errors.New("clean-up"))
+
+	imageSizeCh, imageSizeErrCh := getImageSizeFromRegistryAsync(imageSizeCtx, imageFull)
+
+	var imageSize string
+	var shouldPullImage bool
+
+	select {
+	case val := <-askCh:
+		shouldPullImage = val
+		cause := fmt.Errorf("%w: received confirmation without image size", context.Canceled)
+		imageSizeCancel(cause)
+	case err := <-askErrCh:
+		shouldPullImage = false
+		cause := fmt.Errorf("failed to ask for confirmation without image size: %w", err)
+		imageSizeCancel(cause)
+	case val := <-imageSizeCh:
+		imageSize = val
+		cause := fmt.Errorf("%w: received image size", context.Canceled)
+		askCancel(cause)
+	case err := <-imageSizeErrCh:
+		cause := fmt.Errorf("failed to get image size: %w", err)
+		askCancel(cause)
+	}
+
+	if imageSizeCtx.Err() != nil && askCtx.Err() == nil {
+		cause := context.Cause(imageSizeCtx)
+		logrus.Debugf("Show prompt for download: image size canceled: %s", cause)
+		return shouldPullImage, nil
+	}
+
+	var done bool
+
+	if imageSizeCtx.Err() == nil && askCtx.Err() != nil {
+		select {
+		case val := <-askCh:
+			logrus.Debugf("Show prompt for download: received pending confirmation without image size")
+			shouldPullImage = val
+			done = true
+		case err := <-askErrCh:
+			logrus.Debugf("Show prompt for download: failed to ask for confirmation without image size: %s",
+				err)
+		}
+	} else {
+		panic("code should not be reached")
+	}
+
+	cause := context.Cause(askCtx)
+	logrus.Debugf("Show prompt for download: ask canceled: %s", cause)
+
+	if done {
+		return shouldPullImage, nil
+	}
+
+	return false, &promptForDownloadError{imageSize}
+}
+
+func showPromptForDownloadSecond(imageFull string, errFirst *promptForDownloadError) bool {
+	oldState, err := term.GetState(os.Stdin)
+	if err != nil {
+		logrus.Debugf("Show prompt for download: failed to get terminal state: %s", err)
+		return false
+	}
+
+	defer term.SetState(os.Stdin, oldState)
+
+	lockedState := term.NewStateFrom(oldState,
+		term.WithVMIN(1),
+		term.WithVTIME(0),
+		term.WithoutECHO(),
+		term.WithoutICANON())
+
+	if err := term.SetState(os.Stdin, lockedState); err != nil {
+		logrus.Debugf("Show prompt for download: failed to set terminal state: %s", err)
+		return false
+	}
+
+	parentCtx := context.Background()
+	discardCtx, discardCancel := context.WithCancelCause(parentCtx)
+	defer discardCancel(errors.New("clean-up"))
+
+	discardCh, discardErrCh := discardInputAsync(discardCtx)
+
+	var prompt string
+	if errors.Is(errFirst, context.Canceled) {
+		prompt = createPromptForDownload(imageFull, errFirst.ImageSize)
+	} else {
+		prompt = createPromptForDownload(imageFull, "")
+	}
+
+	fmt.Printf("\r")
+
+	askCtx, askCancel := context.WithCancelCause(parentCtx)
+	defer askCancel(errors.New("clean-up"))
+
+	var askForConfirmationPreFnDone bool
+	askForConfirmationPreFn := func() error {
+		defer discardCancel(errors.New("clean-up"))
+		if askForConfirmationPreFnDone {
+			return nil
+		}
+
+		// Save the cursor position.
+		fmt.Printf("\033[s")
+
+		if err := term.SetState(os.Stdin, oldState); err != nil {
+			return fmt.Errorf("failed to restore terminal state: %w", err)
+		}
+
+		cause := errors.New("terminal restored")
+		discardCancel(cause)
+
+		err := <-discardErrCh
+		if !errors.Is(err, context.Canceled) {
+			return fmt.Errorf("failed to discard input: %w", err)
+		}
+
+		logrus.Debugf("Show prompt for download: stopped discarding input: %s", err)
+
+		discardTotal := <-discardCh
+		logrus.Debugf("Show prompt for download: discarded input: %d bytes", discardTotal)
+
+		if discardTotal == 0 {
+			askForConfirmationPreFnDone = true
+			return nil
+		}
+
+		if err := term.SetState(os.Stdin, lockedState); err != nil {
+			return fmt.Errorf("failed to set terminal state: %w", err)
+		}
+
+		discardCtx, discardCancel = context.WithCancelCause(parentCtx)
+		discardCh, discardErrCh = discardInputAsync(discardCtx)
+
+		// Restore the cursor position
+		fmt.Printf("\033[u")
+
+		// Erase to end of line
+		fmt.Printf("\033[K")
+
+		fmt.Printf("...\n")
+		return errContinue
+	}
+
+	askCh, askErrCh := askForConfirmationAsync(askCtx, prompt, askForConfirmationPreFn)
+	var shouldPullImage bool
+
+	select {
+	case val := <-askCh:
+		logrus.Debug("Show prompt for download: received confirmation with image size")
+		shouldPullImage = val
+	case err := <-askErrCh:
+		logrus.Debugf("Show prompt for download: failed to ask for confirmation with image size: %s", err)
+		shouldPullImage = false
+	}
+
+	return shouldPullImage
+}
+
+func showPromptForDownload(imageFull string) bool {
+	fmt.Println("Image required to create toolbox container.")
+
+	shouldPullImage, err := showPromptForDownloadFirst(imageFull)
+	if err == nil {
+		return shouldPullImage
+	}
+
+	var errPromptForDownload *promptForDownloadError
+	if !errors.As(err, &errPromptForDownload) {
+		panicMsg := fmt.Sprintf("unexpected %T: %s", err, err)
+		panic(panicMsg)
+	}
+
+	shouldPullImage = showPromptForDownloadSecond(imageFull, errPromptForDownload)
+	return shouldPullImage
+}
+
 // systemdNeedsEscape checks whether a byte in a potential dbus ObjectPath needs to be escaped
 func systemdNeedsEscape(i int, b byte) bool {
 	// Escape everything that is not a-z-A-Z-0-9
@@ -778,3 +1035,17 @@ func systemdPathBusEscape(path string) string {
 	}
 	return string(n)
 }
+
+func (err *promptForDownloadError) Error() string {
+	innerErr := err.Unwrap()
+	errMsg := innerErr.Error()
+	return errMsg
+}
+
+func (err *promptForDownloadError) Unwrap() error {
+	if err.ImageSize == "" {
+		return errors.New("failed to get image size")
+	}
+
+	return context.Canceled
+}

src/go.mod

@@ -1,6 +1,6 @@
 module github.com/containers/toolbox
 
-go 1.14
+go 1.20
 
 require (
 	github.com/HarryMichal/go-version v1.0.1
@@ -15,3 +15,25 @@ require (
 	github.com/stretchr/testify v1.7.0
 	golang.org/x/sys v0.1.0
 )
+
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/fatih/color v1.13.0 // indirect
+	github.com/hashicorp/hcl v1.0.0 // indirect
+	github.com/inconshreveable/mousetrap v1.0.0 // indirect
+	github.com/magiconair/properties v1.8.5 // indirect
+	github.com/mattn/go-colorable v0.1.12 // indirect
+	github.com/mattn/go-isatty v0.0.14 // indirect
+	github.com/mitchellh/mapstructure v1.4.3 // indirect
+	github.com/pelletier/go-toml v1.9.4 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/spf13/afero v1.6.0 // indirect
+	github.com/spf13/cast v1.4.1 // indirect
+	github.com/spf13/jwalterweatherman v1.1.0 // indirect
+	github.com/spf13/pflag v1.0.5 // indirect
+	github.com/subosito/gotenv v1.2.0 // indirect
+	golang.org/x/text v0.3.7 // indirect
+	gopkg.in/ini.v1 v1.66.2 // indirect
+	gopkg.in/yaml.v2 v2.4.0 // indirect
+	gopkg.in/yaml.v3 v3.0.0-20210107192922-496545a6307b // indirect
+)