client/dotnet: make LazyBrowseNode.ExpandAsync thread-safe

clients/dotnet/ZB.MOM.WW.MxGateway.Client.Tests/LazyBrowseNodeTests.cs

@@ -142,6 +142,37 @@ public sealed class LazyBrowseNodeTests
         Assert.Equal("7:abc:2", transport.BrowseChildrenCalls[2].Request.PageToken);
     }
 
+    /// <summary>
+    /// Verifies that ten concurrent ExpandAsync calls issue exactly one RPC, not ten.
+    /// </summary>
+    [Fact]
+    public async Task Expand_CalledConcurrently_OnlyFiresOneRpc()
+    {
+        FakeGalaxyRepositoryTransport transport = CreateTransport();
+        transport.BrowseChildrenReplies.Enqueue(BuildReply(
+            children: [BuildObject(1, "Plant", isArea: true)],
+            childHasChildren: [true],
+            cacheSequence: 7));
+        transport.BrowseChildrenReplies.Enqueue(BuildReply(
+            children: [BuildObject(2, "Mixer_001")],
+            childHasChildren: [false],
+            cacheSequence: 7));
+
+        await using GalaxyRepositoryClient client = CreateClient(transport);
+        IReadOnlyList<LazyBrowseNode> roots = await client.BrowseAsync();
+
+        // Fire ten concurrent expands of the same node.
+        Task[] tasks = Enumerable.Range(0, 10)
+            .Select(_ => roots[0].ExpandAsync())
+            .ToArray();
+        await Task.WhenAll(tasks);
+
+        Assert.True(roots[0].IsExpanded);
+        Assert.Single(roots[0].Children);
+        // 1 roots fetch + exactly 1 expand fetch = 2 total
+        Assert.Equal(2, transport.BrowseChildrenCalls.Count);
+    }
+
     /// <summary>
     /// Verifies that BrowseChildrenOptions filter fields are forwarded to the BrowseChildren request.
     /// </summary>

clients/dotnet/ZB.MOM.WW.MxGateway.Client/LazyBrowseNode.cs

@@ -13,6 +13,7 @@ public sealed class LazyBrowseNode
     private readonly GalaxyRepositoryClient _client;
     private readonly BrowseChildrenOptions _options;
     private readonly List<LazyBrowseNode> _children = [];
+    private readonly SemaphoreSlim _expandLock = new(1, 1);
     private bool _isExpanded;
 
     internal LazyBrowseNode(
@@ -43,6 +44,10 @@ public sealed class LazyBrowseNode
     ///     Fetches direct children from the gateway and populates <see cref="Children"/>.
     ///     Idempotent: subsequent calls are no-ops.
     /// </summary>
+    /// <remarks>
+    ///     Thread-safe: concurrent callers see exactly one fetch; subsequent callers
+    ///     (after the first completes) return immediately.
+    /// </remarks>
     /// <param name="cancellationToken">Token to observe for cancellation.</param>
     public async Task ExpandAsync(CancellationToken cancellationToken = default)
     {
@@ -51,33 +56,46 @@ public sealed class LazyBrowseNode
             return;
         }
 
-        string pageToken = string.Empty;
+        await _expandLock.WaitAsync(cancellationToken).ConfigureAwait(false);
-        HashSet<string> seenPageTokens = new(StringComparer.Ordinal);
+        try
-        do
         {
-            BrowseChildrenRequest request = GalaxyRepositoryClient.BuildBrowseChildrenRequest(_options);
+            if (_isExpanded)
-            request.ParentGobjectId = Object.GobjectId;
-            request.PageToken = pageToken;
-
-            BrowseChildrenReply reply = await _client
-                .BrowseChildrenRawAsync(request, cancellationToken)
-                .ConfigureAwait(false);
-
-            for (int i = 0; i < reply.Children.Count; i++)
             {
-                bool hint = i < reply.ChildHasChildren.Count && reply.ChildHasChildren[i];
+                return;
-                _children.Add(new LazyBrowseNode(_client, reply.Children[i], hint, _options));
             }
 
-            pageToken = reply.NextPageToken;
+            string pageToken = string.Empty;
-            if (!string.IsNullOrWhiteSpace(pageToken) && !seenPageTokens.Add(pageToken))
+            HashSet<string> seenPageTokens = new(StringComparer.Ordinal);
+            do
             {
-                throw new MxGatewayException(
+                BrowseChildrenRequest request = GalaxyRepositoryClient.BuildBrowseChildrenRequest(_options);
-                    $"Galaxy BrowseChildren returned a repeated page token '{pageToken}'.");
+                request.ParentGobjectId = Object.GobjectId;
+                request.PageToken = pageToken;
+
+                BrowseChildrenReply reply = await _client
+                    .BrowseChildrenRawAsync(request, cancellationToken)
+                    .ConfigureAwait(false);
+
+                for (int i = 0; i < reply.Children.Count; i++)
+                {
+                    bool hint = i < reply.ChildHasChildren.Count && reply.ChildHasChildren[i];
+                    _children.Add(new LazyBrowseNode(_client, reply.Children[i], hint, _options));
+                }
+
+                pageToken = reply.NextPageToken;
+                if (!string.IsNullOrWhiteSpace(pageToken) && !seenPageTokens.Add(pageToken))
+                {
+                    throw new MxGatewayException(
+                        $"Galaxy BrowseChildren returned a repeated page token '{pageToken}'.");
+                }
             }
+            while (!string.IsNullOrWhiteSpace(pageToken));
+
+            _isExpanded = true;
+        }
+        finally
+        {
+            _expandLock.Release();
         }
-        while (!string.IsNullOrWhiteSpace(pageToken));
-
-        _isExpanded = true;
     }
 }