Fix unhelpful error message for oci-archive scheme (#705)

* Improve error message

Signed-off-by: Dan Luhring <dan+github@luhrings.com>

* Return error from stereoscope immediately

Signed-off-by: Dan Luhring <dan+github@luhrings.com>

* Bump version of stereoscope

Signed-off-by: Dan Luhring <dan+github@luhrings.com>

* Conditionally retry image retrieval

Signed-off-by: Dan Luhring <dan+github@luhrings.com>

* Update error message for source construction failure

Signed-off-by: Dan Luhring <dan+github@luhrings.com>

* Update stereoscope

Signed-off-by: Dan Luhring <dan+github@luhrings.com>

* Retry image pull without predetermined image source

Signed-off-by: Dan Luhring <dan+github@luhrings.com>

* Add comment to image pull source determination

Signed-off-by: Dan Luhring <dan+github@luhrings.com>

cmd/packages.go

@@ -262,7 +262,7 @@ func packagesExecWorker(userInput string) <-chan error {
 
 		src, cleanup, err := source.New(userInput, appConfig.Registry.ToOptions(), appConfig.Exclusions)
 		if err != nil {
-			errs <- fmt.Errorf("failed to determine image source: %w", err)
+			errs <- fmt.Errorf("failed to construct source from user input %q: %w", userInput, err)
 			return
 		}
 		if cleanup != nil {

go.mod

@@ -13,7 +13,7 @@ require (
 	github.com/anchore/go-testutils v0.0.0-20200925183923-d5f45b0d3c04
 	github.com/anchore/go-version v1.2.2-0.20200701162849-18adb9c92b9b
 	github.com/anchore/packageurl-go v0.0.0-20210922164639-b3fa992ebd29
-	github.com/anchore/stereoscope v0.0.0-20211203160213-5a5e323a5c89
+	github.com/anchore/stereoscope v0.0.0-20211222141827-6e663afeef5d
 	// we are hinting brotli to latest due to warning when installing archiver v3:
 	// go: warning: github.com/andybalholm/brotli@v1.0.1: retracted by module author: occasional panics and data corruption
 	github.com/andybalholm/brotli v1.0.4 // indirect

go.sum

@@ -111,8 +111,8 @@ github.com/anchore/go-version v1.2.2-0.20200701162849-18adb9c92b9b h1:e1bmaoJfZV
 github.com/anchore/go-version v1.2.2-0.20200701162849-18adb9c92b9b/go.mod h1:Bkc+JYWjMCF8OyZ340IMSIi2Ebf3uwByOk6ho4wne1E=
 github.com/anchore/packageurl-go v0.0.0-20210922164639-b3fa992ebd29 h1:K9LfnxwhqvihqU0+MF325FNy7fsKV9EGaUxdfR4gnWk=
 github.com/anchore/packageurl-go v0.0.0-20210922164639-b3fa992ebd29/go.mod h1:Oc1UkGaJwY6ND6vtAqPSlYrptKRJngHwkwB6W7l1uP0=
-github.com/anchore/stereoscope v0.0.0-20211203160213-5a5e323a5c89 h1:pcMFN7wjIQIxdZm8NA0R3JiRRFn5Eyx9mhagHOVS4Bw=
+github.com/anchore/stereoscope v0.0.0-20211222141827-6e663afeef5d h1:ggdC5SRb8rAXmBkwxah8gfSxwpoAVcpp12tQ+XbDi20=
-github.com/anchore/stereoscope v0.0.0-20211203160213-5a5e323a5c89/go.mod h1:FNm1rtauEjkC3elA3jr3bJkU+/4QiovApwJdPnHQ9x0=
+github.com/anchore/stereoscope v0.0.0-20211222141827-6e663afeef5d/go.mod h1:FNm1rtauEjkC3elA3jr3bJkU+/4QiovApwJdPnHQ9x0=
 github.com/andreyvit/diff v0.0.0-20170406064948-c7f18ee00883/go.mod h1:rCTlJbsFo29Kk6CurOXKm700vrz8f0KW0JNfpkRJY/8=
 github.com/andybalholm/brotli v1.0.1/go.mod h1:loMXtMfwqflxFJPmdbJO0a3KNoPuLBgiu3qAvBg8x/Y=
 github.com/andybalholm/brotli v1.0.4 h1:V7DdXeJtZscaqfNuAdSRuRFzuiKlHSC/Zh3zl9qY3JY=

syft/source/source.go

@@ -51,7 +51,7 @@ func New(userInput string, registryOptions *image.RegistryOptions, exclusions []
 	case DirectoryScheme:
 		source, cleanupFn, err = generateDirectorySource(fs, location)
 	case ImageScheme:
-		source, cleanupFn, err = generateImageSource(location, userInput, imageSource, registryOptions)
+		source, cleanupFn, err = generateImageSource(userInput, location, imageSource, registryOptions)
 	default:
 		err = fmt.Errorf("unable to process input for scanning: '%s'", userInput)
 	}
@@ -63,17 +63,8 @@ func New(userInput string, registryOptions *image.RegistryOptions, exclusions []
 	return source, cleanupFn, err
 }
 
-func generateImageSource(location, userInput string, imageSource image.Source, registryOptions *image.RegistryOptions) (*Source, func(), error) {
+func generateImageSource(userInput, location string, imageSource image.Source, registryOptions *image.RegistryOptions) (*Source, func(), error) {
-	img, err := stereoscope.GetImageFromSource(location, imageSource, registryOptions)
+	img, cleanup, err := getImageWithRetryStrategy(userInput, location, imageSource, registryOptions)
-	if err != nil {
-		log.Debugf("error parsing location: %s after detecting scheme; pulling image: %s", location, userInput)
-		// we may have been to aggressive reading the source hint
-		// try the input as supplied by the user if our initial parse failed
-		img, err = stereoscope.GetImageFromSource(userInput, imageSource, registryOptions)
-	}
-
-	cleanup := stereoscope.Cleanup
-
 	if err != nil || img == nil {
 		return &Source{}, cleanup, fmt.Errorf("could not fetch image '%s': %w", location, err)
 	}
@@ -86,6 +77,54 @@ func generateImageSource(location, userInput string, imageSource image.Source, r
 	return &s, cleanup, nil
 }
 
+func parseScheme(userInput string) string {
+	parts := strings.SplitN(userInput, ":", 2)
+	if len(parts) < 2 {
+		return ""
+	}
+
+	return parts[0]
+}
+
+func getImageWithRetryStrategy(userInput, location string, imageSource image.Source, registryOptions *image.RegistryOptions) (*image.Image, func(), error) {
+	img, err := stereoscope.GetImageFromSource(location, imageSource, registryOptions)
+	if err == nil {
+		// Success on the first try!
+		return img, stereoscope.Cleanup, nil
+	}
+
+	scheme := parseScheme(userInput)
+	if !(scheme == "docker" || scheme == "registry") {
+		// Image retrieval failed, and we shouldn't retry it. It's most likely that the
+		// user _did_ intend the parsed scheme, but there was a legitimate failure with
+		// using the scheme to load the image. Alert the user to this failure, so they
+		// can fix the problem.
+		return nil, nil, err
+	}
+
+	// Maybe the user wanted "docker" or "registry" to refer to an _image name_
+	// (e.g. "docker:latest"), not a scheme. We'll retry image retrieval with this
+	// alternative interpretation, in an attempt to avoid unnecessary user friction.
+
+	log.Warnf(
+		"scheme %q specified, but it coincides with a common image name; re-examining user input %q"+
+			" without scheme parsing because image retrieval using scheme parsing was unsuccessful: %v",
+		scheme,
+		userInput,
+		err,
+	)
+
+	// We need to determine the image source again, such that this determination
+	// doesn't take scheme parsing into account.
+	imageSource = image.DetermineImagePullSource(userInput)
+	img, err = stereoscope.GetImageFromSource(userInput, imageSource, registryOptions)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	return img, stereoscope.Cleanup, nil
+}
+
 func generateDirectorySource(fs afero.Fs, location string) (*Source, func(), error) {
 	fileMeta, err := fs.Stat(location)
 	if err != nil {