manifest,zstd: give priority to zstd compressed images

While perform a pull from manifest list of type
`application/vnd.oci.image.index.v1+json` make sure that if more
than one image exists for the same platform then give priority to the
one compressed with `zstd`.

Annotation `io.github.containers.compression.zstd` is expected to be set
for the image compressed with `zstd`.

Signed-off-by: Aditya R <arajan@redhat.com>

internal/image/fixtures/schemazstdselection.json

@@ -0,0 +1,66 @@
+{
+    "schemaVersion": 2,
+    "mediaType": "application/vnd.oci.image.index.v1+json",
+    "manifests": [
+        {
+            "mediaType": "application/vnd.oci.image.manifest.v1+json",
+            "digest": "sha256:88cb27f194cce3b06994b4af2dd8a5642f4f294b71bda5b74a3fbb74a9522510",
+            "size": 758,
+            "platform": {
+                "architecture": "amd64",
+                "os": "linux"
+            }
+        },
+        {
+            "mediaType": "application/vnd.oci.image.manifest.v1+json",
+            "digest": "sha256:ac9bbf71dacfad2bfbe48f7d46ca279b1b3b351a6c814db760030015167405b8",
+            "size": 772,
+            "annotations": {
+                "io.github.containers.compression.zstd": "true"
+            },
+            "platform": {
+                "architecture": "amd64",
+                "os": "linux"
+            }
+        },
+        {
+            "mediaType": "application/vnd.oci.image.manifest.v1+json",
+            "digest": "sha256:88cb27f194cce3b06994b4af2dd8a5642f4f294b71bda5b74a3fbb74a9522511",
+            "size": 758,
+            "platform": {
+                "architecture": "arm64",
+                "os": "linux"
+            }
+        },
+        {
+            "mediaType": "application/vnd.oci.image.manifest.v1+json",
+            "digest": "sha256:88cb27f194cce3b06994b4af2dd8a5642f4f294b71bda5b74a3fbb74z9522511",
+            "size": 758,
+            "platform": {
+                "architecture": "arm64",
+                "os": "linux"
+            }
+        },
+        {
+            "mediaType": "application/vnd.oci.image.manifest.v1+json",
+            "digest": "sha256:88cb27f194cce3b06994b4af2dd8a5642f4f294b71bda5b74a3fbb74z9522511",
+            "size": 758,
+            "platform": {
+                "architecture": "s390x",
+                "os": "linux"
+            }
+        },
+        {
+            "mediaType": "application/vnd.oci.image.manifest.v1+json",
+            "digest": "sha256:ac9bbf71dacfad2bfbe48m7d46ca279b1b3b351a6c814db760030015167405b8",
+            "size": 772,
+            "annotations": {
+                "io.github.containers.compression.zstd": "true"
+            },
+            "platform": {
+                "architecture": "amd64",
+                "os": "linux"
+            }
+        }
+    ]
+}

internal/image/fixtures/schemazstdselection2.json

@@ -0,0 +1,57 @@
+{
+    "schemaVersion": 2,
+    "mediaType": "application/vnd.oci.image.index.v1+json",
+    "manifests": [
+        {
+            "mediaType": "application/vnd.oci.image.manifest.v1+json",
+            "digest": "sha256:88cb27f194cce3b06994b4af2dd8a5642f4f294b71bda5b74a3fbb74a9522510",
+            "size": 758,
+            "platform": {
+                "architecture": "amd64",
+                "os": "linux"
+            }
+        },
+        {
+            "mediaType": "application/vnd.oci.image.manifest.v1+json",
+            "digest": "sha256:88cb27f194cce3b06994b4af2dd8a5642f4f294b71bda5b74a3fbb74a9522513",
+            "size": 759,
+            "platform": {
+                "architecture": "amd64",
+                "os": "linux"
+            }
+        },
+        {
+            "mediaType": "application/vnd.oci.image.manifest.v1+json",
+            "digest": "sha256:ac9bbf71dacfad2bfbe48f7d46ca279b1b3b351a6c814db760030015167405b8",
+            "size": 772,
+            "annotations": {
+                "io.github.containers.compression.zstd": "true"
+            },
+            "platform": {
+                "architecture": "amd64",
+                "os": "linux"
+            }
+        },
+        {
+            "mediaType": "application/vnd.oci.image.manifest.v1+json",
+            "digest": "sha256:88cb27f194cce3b06994b4af2dd8a5642f4f294b71bda5b74a3fbb74a9522514",
+            "size": 759,
+            "platform": {
+                "architecture": "amd64",
+                "os": "linux"
+            }
+        },
+        {
+            "mediaType": "application/vnd.oci.image.manifest.v1+json",
+            "digest": "sha256:ac9bbf71dacfad2bfbe48f7d46ca279b1b3b351a6c814db760030015167405b6",
+            "size": 772,
+            "annotations": {
+                "io.github.containers.compression.zstd": "true"
+            },
+            "platform": {
+                "architecture": "amd64",
+                "os": "linux"
+            }
+        }
+    ]
+}

manifest/list_test.go

@@ -87,6 +87,29 @@ func TestChooseInstance(t *testing.T) {
 				"unmatched",
 			},
 		},
+		{
+			listFile: "schemazstdselection.json",
+			matchedInstances: []expectedMatch{
+				// out of gzip and zstd in amd64 select the first zstd image
+				{"amd64", "", "sha256:ac9bbf71dacfad2bfbe48f7d46ca279b1b3b351a6c814db760030015167405b8"},
+				// out of multiple gzip in arm64 select the first one to ensure original logic is prevented
+				{"arm64", "", "sha256:88cb27f194cce3b06994b4af2dd8a5642f4f294b71bda5b74a3fbb74a9522511"},
+				// select a signle gzip s390x image
+				{"s390x", "", ""},
+			},
+			unmatchedInstances: []string{
+				"unmatched",
+			},
+		},
+		{
+			listFile: "schemazstdselection2.json",
+			matchedInstances: []expectedMatch{
+				{"amd64", "", "sha256:ac9bbf71dacfad2bfbe48f7d46ca279b1b3b351a6c814db760030015167405b8"},
+			},
+			unmatchedInstances: []string{
+				"unmatched",
+			},
+		},
 		{ // Focus on ARM variant field testing
 			listFile: "schema2list-variants.json",
 			matchedInstances: []expectedMatch{

manifest/oci_index.go

@@ -12,6 +12,13 @@ import (
 	imgspecv1 "github.com/opencontainers/image-spec/specs-go/v1"
 )
 
+const (
+	// Following annotation is used while performing instance selection.
+	// If more than one image exists for the same platform then the image
+	// compressed with `zstd` will be given priority.
+	OCI1InstanceAnnotationCompressionZSTD = "io.github.containers.compression.zstd"
+)
+
 // OCI1Index is just an alias for the OCI index type, but one which we can
 // provide methods for.
 type OCI1Index struct {
@@ -76,6 +83,8 @@ func (index *OCI1Index) ChooseInstance(ctx *types.SystemContext) (digest.Digest,
 	if err != nil {
 		return "", fmt.Errorf("getting platform information %#v: %w", ctx, err)
 	}
+	var matchedDigest digest.Digest
+	matchedDigest = ""
 	for _, wantedPlatform := range wantedPlatforms {
 		for _, d := range index.Manifests {
 			if d.Platform == nil {
@@ -89,16 +98,45 @@ func (index *OCI1Index) ChooseInstance(ctx *types.SystemContext) (digest.Digest,
 				Variant:      d.Platform.Variant,
 			}
 			if platform.MatchesPlatform(imagePlatform, wantedPlatform) {
-				return d.Digest, nil
+				if matchedDigest == "" {
+					matchedDigest = d.Digest
+				}
+				// Always give priority to zstd if more than one image
+				// exists for the same platform.
+				// TODO: In future this check will be configurable and
+				// will be relayed via `ctx`.
+				if _, ok := d.Annotations[OCI1InstanceAnnotationCompressionZSTD]; ok {
+					matchedDigest = d.Digest
+					return matchedDigest, nil
+				}
 			}
 		}
 	}
 
+	if matchedDigest != "" {
+		return matchedDigest, nil
+	}
+
 	for _, d := range index.Manifests {
 		if d.Platform == nil {
-			return d.Digest, nil
+			if matchedDigest == "" {
+				matchedDigest = d.Digest
+			}
+			// Always give priority to zstd if more than one image
+			// exists for the same platform.
+			// TODO: In future this check will be configurable and
+			// will be relayed via `ctx`.
+			if _, ok := d.Annotations[OCI1InstanceAnnotationCompressionZSTD]; ok {
+				matchedDigest = d.Digest
+				return matchedDigest, nil
+			}
 		}
 	}
+
+	if matchedDigest != "" {
+		return matchedDigest, nil
+	}
+
 	return "", fmt.Errorf("no image found in image index for architecture %s, variant %q, OS %s", wantedPlatforms[0].Architecture, wantedPlatforms[0].Variant, wantedPlatforms[0].OS)
 }