grpc: implement BrowseChildren handler + metadata:read scope

src/ZB.MOM.WW.MxGateway.Server/Grpc/GalaxyRepositoryGrpcService.cs

@@ -26,6 +26,8 @@ public sealed class GalaxyRepositoryGrpcService(
     private static readonly TimeSpan FirstLoadWaitBudget = TimeSpan.FromSeconds(5);
     private const int DefaultDiscoverPageSize = 1000;
     private const int MaxDiscoverPageSize = 5000;
+    private const int DefaultBrowsePageSize = 500;
+    // MaxBrowsePageSize reuses MaxDiscoverPageSize (5000) — same cap.
 
     /// <inheritdoc />
     public override async Task<TestConnectionReply> TestConnection(
@@ -107,6 +109,62 @@ public sealed class GalaxyRepositoryGrpcService(
         return reply;
     }
 
+    /// <inheritdoc />
+    public override async Task<BrowseChildrenReply> BrowseChildren(
+        BrowseChildrenRequest request,
+        ServerCallContext context)
+    {
+        await WaitForCacheBootstrap(context.CancellationToken).ConfigureAwait(false);
+        GalaxyDb.GalaxyHierarchyCacheEntry entry = cache.Current;
+
+        if (!entry.HasData)
+        {
+            throw new RpcException(new Status(
+                StatusCode.Unavailable,
+                ResolveUnavailableMessage(entry)));
+        }
+
+        int pageSize = ResolveBrowsePageSize(request.PageSize);
+        IReadOnlyList<string> browseSubtrees = ResolveBrowseSubtrees();
+
+        // Resolve the parent id once so the page-token signature can include it
+        // and the projector sees the same resolved id when memoizing.
+        int parentId = ResolveParentIdForToken(entry, request);
+        string filterSignature = GalaxyDb.GalaxyBrowseProjector.ComputeFilterSignature(
+            request, browseSubtrees, parentId);
+        PageToken pageToken = ParsePageToken(request.PageToken, entry.Sequence, filterSignature);
+
+        GalaxyDb.GalaxyBrowseChildrenResult result = GalaxyDb.GalaxyBrowseProjector.ProjectChildren(
+            entry,
+            request,
+            browseSubtrees,
+            pageToken.Offset,
+            pageSize);
+
+        if (pageToken.Offset > result.TotalChildCount)
+        {
+            throw new RpcException(new Status(
+                StatusCode.InvalidArgument,
+                "BrowseChildren page_token is outside the current children set."));
+        }
+
+        BrowseChildrenReply reply = new()
+        {
+            TotalChildCount = result.TotalChildCount,
+            CacheSequence = (ulong)entry.Sequence,
+        };
+        reply.Children.Add(result.Children);
+        reply.ChildHasChildren.Add(result.ChildHasChildren);
+
+        int nextOffset = pageToken.Offset + result.Children.Count;
+        if (nextOffset < result.TotalChildCount)
+        {
+            reply.NextPageToken = FormatPageToken(entry.Sequence, result.FilterSignature, nextOffset);
+        }
+
+        return reply;
+    }
+
     /// <inheritdoc />
     public override async Task WatchDeployEvents(
         WatchDeployEventsRequest request,
@@ -213,6 +271,44 @@ public sealed class GalaxyRepositoryGrpcService(
         return Math.Min(pageSize, MaxDiscoverPageSize);
     }
 
+    private static int ResolveBrowsePageSize(int requested)
+    {
+        if (requested < 0)
+        {
+            throw new RpcException(new Status(
+                StatusCode.InvalidArgument,
+                "BrowseChildren page_size must be greater than zero when provided."));
+        }
+        int pageSize = requested == 0 ? DefaultBrowsePageSize : requested;
+        return Math.Min(pageSize, MaxDiscoverPageSize);
+    }
+
+    // Lightweight parent resolver used only for signature computation. Re-throws
+    // NotFound consistently with the projector so the error surface matches.
+    private static int ResolveParentIdForToken(
+        GalaxyDb.GalaxyHierarchyCacheEntry entry,
+        BrowseChildrenRequest request)
+    {
+        return request.ParentCase switch
+        {
+            BrowseChildrenRequest.ParentOneofCase.None => 0,
+            BrowseChildrenRequest.ParentOneofCase.ParentGobjectId =>
+                request.ParentGobjectId == 0 ? 0
+                : entry.Index.ObjectViewsById.ContainsKey(request.ParentGobjectId)
+                    ? request.ParentGobjectId
+                    : throw new RpcException(new Status(StatusCode.NotFound, "BrowseChildren parent was not found.")),
+            BrowseChildrenRequest.ParentOneofCase.ParentTagName =>
+                entry.Index.ObjectViews.FirstOrDefault(
+                    v => string.Equals(v.Object.TagName, request.ParentTagName, StringComparison.OrdinalIgnoreCase))?.Object.GobjectId
+                ?? throw new RpcException(new Status(StatusCode.NotFound, "BrowseChildren parent was not found.")),
+            BrowseChildrenRequest.ParentOneofCase.ParentContainedPath =>
+                entry.Index.ObjectViews.FirstOrDefault(
+                    v => string.Equals(v.ContainedPath, request.ParentContainedPath, StringComparison.OrdinalIgnoreCase))?.Object.GobjectId
+                ?? throw new RpcException(new Status(StatusCode.NotFound, "BrowseChildren parent was not found.")),
+            _ => 0,
+        };
+    }
+
     private IReadOnlyList<string> ResolveBrowseSubtrees()
     {
         ApiKeyConstraints constraints = identityAccessor.Current?.EffectiveConstraints ?? ApiKeyConstraints.Empty;