semi implemented and features implemented save checkpoint

src/Scanner/__Libraries/StellaOps.Scanner.Storage.Oci/OciArtifactPusher.cs

@@ -60,16 +60,14 @@ public sealed class OciArtifactPusher
 
         try
         {
-            var configDigest = await PushBlobAsync(reference, EmptyConfigBlob, OciMediaTypes.EmptyConfig, auth, cancellationToken)
-                .ConfigureAwait(false);
+            var configDigest = ComputeDigest(EmptyConfigBlob);
 
             var layerDescriptors = new List<OciDescriptor>();
             var layerDigests = new List<string>();
 
             foreach (var layer in request.Layers)
             {
-                var digest = await PushBlobAsync(reference, layer.Content, layer.MediaType, auth, cancellationToken)
-                    .ConfigureAwait(false);
+                var digest = ComputeDigest(layer.Content);
 
                 layerDescriptors.Add(new OciDescriptor
                 {
@@ -86,7 +84,45 @@ public sealed class OciArtifactPusher
             var manifestBytes = JsonSerializer.SerializeToUtf8Bytes(manifest, SerializerOptions);
             var manifestDigest = ComputeDigest(manifestBytes);
 
-            var tag = reference.Tag ?? manifestDigest.Replace("sha256:", string.Empty, StringComparison.Ordinal);
+            var tag = request.Tag;
+            if (string.IsNullOrWhiteSpace(tag))
+            {
+                tag = reference.Tag ?? manifestDigest.Replace("sha256:", string.Empty, StringComparison.Ordinal);
+            }
+
+            if (request.SkipIfTagExists)
+            {
+                var existingDigest = await TryGetExistingManifestDigestAsync(reference, tag, auth, cancellationToken)
+                    .ConfigureAwait(false);
+                if (existingDigest is not null)
+                {
+                    if (string.IsNullOrWhiteSpace(existingDigest))
+                    {
+                        existingDigest = manifestDigest;
+                    }
+
+                    var existingReference = $"{reference.Registry}/{reference.Repository}@{existingDigest}";
+                    _logger.LogInformation("OCI artifact already exists for tag {Tag}: {Reference}", tag, existingReference);
+                    return new OciArtifactPushResult
+                    {
+                        Success = true,
+                        AlreadyExists = true,
+                        ManifestDigest = existingDigest,
+                        ManifestReference = existingReference,
+                        LayerDigests = layerDigests
+                    };
+                }
+            }
+
+            await PushBlobAsync(reference, EmptyConfigBlob, OciMediaTypes.EmptyConfig, auth, cancellationToken)
+                .ConfigureAwait(false);
+
+            foreach (var layer in request.Layers)
+            {
+                await PushBlobAsync(reference, layer.Content, layer.MediaType, auth, cancellationToken)
+                    .ConfigureAwait(false);
+            }
+
             await PushManifestAsync(reference, manifestBytes, tag, auth, cancellationToken).ConfigureAwait(false);
 
             var manifestReference = $"{reference.Registry}/{reference.Repository}@{manifestDigest}";
@@ -94,11 +130,12 @@ public sealed class OciArtifactPusher
             _logger.LogInformation("Pushed OCI artifact {Reference}", manifestReference);
 
             return new OciArtifactPushResult
-            {
-                Success = true,
-                ManifestDigest = manifestDigest,
-                ManifestReference = manifestReference,
-                LayerDigests = layerDigests
+                {
+                    Success = true,
+                    AlreadyExists = false,
+                    ManifestDigest = manifestDigest,
+                    ManifestReference = manifestReference,
+                    LayerDigests = layerDigests
             };
         }
         catch (OciRegistryException ex)
@@ -113,6 +150,40 @@ public sealed class OciArtifactPusher
         }
     }
 
+    private async Task<string?> TryGetExistingManifestDigestAsync(
+        OciImageReference reference,
+        string tag,
+        OciRegistryAuthorization auth,
+        CancellationToken cancellationToken)
+    {
+        var manifestUri = BuildRegistryUri(reference, $"manifests/{tag}");
+        using var request = new HttpRequestMessage(HttpMethod.Head, manifestUri);
+        request.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue(OciMediaTypes.ImageManifest));
+        auth.ApplyTo(request);
+
+        using var response = await _httpClient.SendAsync(request, cancellationToken).ConfigureAwait(false);
+        if (response.StatusCode == HttpStatusCode.NotFound)
+        {
+            return null;
+        }
+
+        if (!response.IsSuccessStatusCode)
+        {
+            throw new OciRegistryException($"Manifest HEAD failed with {response.StatusCode}", "ERR_OCI_MANIFEST_HEAD");
+        }
+
+        if (response.Headers.TryGetValues("Docker-Content-Digest", out var digestValues))
+        {
+            var digest = digestValues.FirstOrDefault();
+            if (!string.IsNullOrWhiteSpace(digest))
+            {
+                return digest;
+            }
+        }
+
+        return string.Empty;
+    }
+
     private OciArtifactManifest BuildManifest(
         OciArtifactPushRequest request,
         string configDigest,