Change platform selection logic (#189)

This reverts commit d7551b7.

Defaulting to platform for all providers resulted in syft having unnecessary
when pulling an image that had a particular digest, if that digest didn't match
the architecture of the host running the pull. Revert commit
d7551b7, which introduced that error, but then
add platform defaulting logic back for the OCI Registry Provider, since
defaulting there has been specifically requested. Add integ tests to cover the
new behavior. Also, update integ tests to use the manifest ID for assertsion,
since the RepoDigests array can be empty.

Signed-off-by: Will Murphy <will.murphy@anchore.com>

client.go

@@ -102,13 +102,13 @@ func GetImageFromSource(ctx context.Context, imgStr string, source image.Source,
 func selectImageProvider(imgStr string, source image.Source, cfg config) (image.Provider, error) {
 	var provider image.Provider
 	tempDirGenerator := rootTempDirGenerator.NewGenerator()
-
-	if err := setPlatform(source, &cfg, runtime.GOARCH); err != nil {
-		return nil, err
-	}
+	platformSelectionUnsupported := fmt.Errorf("specified platform=%q however image source=%q does not support selecting platform", cfg.Platform.String(), source.String())
 
 	switch source {
 	case image.DockerTarballSource:
+		if cfg.Platform != nil {
+			return nil, platformSelectionUnsupported
+		}
 		// note: the imgStr is the path on disk to the tar file
 		provider = docker.NewProviderFromTarball(imgStr, tempDirGenerator)
 	case image.DockerDaemonSource:
@@ -130,43 +130,42 @@ func selectImageProvider(imgStr string, source image.Source, cfg config) (image.
 			return nil, err
 		}
 	case image.OciDirectorySource:
+		if cfg.Platform != nil {
+			return nil, platformSelectionUnsupported
+		}
 		provider = oci.NewProviderFromPath(imgStr, tempDirGenerator)
 	case image.OciTarballSource:
+		if cfg.Platform != nil {
+			return nil, platformSelectionUnsupported
+		}
 		provider = oci.NewProviderFromTarball(imgStr, tempDirGenerator)
 	case image.OciRegistrySource:
+		defaultPlatformIfNil(&cfg)
 		provider = oci.NewProviderFromRegistry(imgStr, tempDirGenerator, cfg.Registry, cfg.Platform)
 	case image.SingularitySource:
+		if cfg.Platform != nil {
+			return nil, platformSelectionUnsupported
+		}
 		provider = sif.NewProviderFromPath(imgStr, tempDirGenerator)
 	default:
 		return nil, fmt.Errorf("unable to determine image source")
 	}
 	return provider, nil
 }
 
-func setPlatform(source image.Source, cfg *config, defaultArch string) error {
-	// we should override the platform based on the host architecture if the user did not specify a platform
-	// see https://github.com/anchore/stereoscope/issues/149 for more details
-	defaultPlatform, err := image.NewPlatform(defaultArch)
-	if err != nil {
-		log.WithFields("error", err).Warnf("unable to set default platform to %q", runtime.GOARCH)
-		defaultPlatform = nil
-	}
-
-	switch source {
-	case image.DockerTarballSource, image.OciDirectorySource, image.OciTarballSource, image.SingularitySource:
-		if cfg.Platform != nil {
-			return fmt.Errorf("specified platform=%q however image source=%q does not support selecting platform", cfg.Platform.String(), source.String())
-		}
-
-	case image.DockerDaemonSource, image.PodmanDaemonSource, image.OciRegistrySource:
-		if cfg.Platform == nil {
-			cfg.Platform = defaultPlatform
+// defaultPlatformIfNil sets the platform to use the host's architecture
+// if no platform was specified. The OCI registry provider uses "linux/amd64"
+// as a hard-coded default platform, which has surprised customers
+// running stereoscope on non-amd64 hosts. If platform is already
+// set on the config, or the code can't generate a matching platform,
+// do nothing.
+func defaultPlatformIfNil(cfg *config) {
+	if cfg.Platform == nil {
+		p, err := image.NewPlatform(fmt.Sprintf("linux/%s", runtime.GOARCH))
+		if err == nil {
+			cfg.Platform = p
 		}
-
-	default:
-		return fmt.Errorf("unable to determine image source to select platform")
 	}
-	return nil
 }
 
 // GetImage parses the user provided image string and provides an image object;