Fix bug

Author: javiercn

src/StaticWebAssetsSdk/Tasks/ApplyCompressionNegotiation.cs

@@ -22,6 +22,7 @@ public class ApplyCompressionNegotiation : Task
 
     private readonly List<StaticWebAssetEndpointSelector> _selectorsList = new();
     private readonly List<StaticWebAssetEndpointResponseHeader> _headersList = new();
+    private readonly List<StaticWebAssetEndpointResponseHeader> _tempHeadersList = new();
     private readonly List<StaticWebAssetEndpointProperty> _propertiesList = new();
     private const int ExpectedCompressionHeadersCount = 2;
 
@@ -326,9 +327,9 @@ private static StaticWebAssetEndpointProperty ResolveFingerprint(StaticWebAssetE
 
     private void ApplyCompressedEndpointHeaders(List<StaticWebAssetEndpointResponseHeader> headers, StaticWebAssetEndpoint compressedEndpoint, string relatedEndpointCandidateRoute)
     {
-        StaticWebAssetEndpointResponseHeader.PopulateFromMetadataValue(compressedEndpoint.ResponseHeadersString, _headersList);
+        StaticWebAssetEndpointResponseHeader.PopulateFromMetadataValue(compressedEndpoint.ResponseHeadersString, _tempHeadersList);
 
-        foreach (var header in _headersList)
+        foreach (var header in _tempHeadersList)
         {
             if (string.Equals(header.Name, "Content-Type", StringComparison.Ordinal))
             {
@@ -346,9 +347,9 @@ private void ApplyCompressedEndpointHeaders(List<StaticWebAssetEndpointResponseH
 
     private void ApplyRelatedEndpointCandidateHeaders(List<StaticWebAssetEndpointResponseHeader> headers, StaticWebAssetEndpoint relatedEndpointCandidate, HashSet<string> compressedHeaders)
     {
-        StaticWebAssetEndpointResponseHeader.PopulateFromMetadataValue(relatedEndpointCandidate.ResponseHeadersString, _headersList);
+        StaticWebAssetEndpointResponseHeader.PopulateFromMetadataValue(relatedEndpointCandidate.ResponseHeadersString, _tempHeadersList);
 
-        foreach (var header in _headersList)
+        foreach (var header in _tempHeadersList)
         {
             // We need to keep the headers that are specific to the compressed asset like Content-Length,
             // Last-Modified and ETag. Any other header we should add it.

test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssets/ApplyCompressionNegotiationTest.cs

@@ -930,6 +930,65 @@ public void AppliesContentNegotiationRules_ToAllRelatedAssetEndpoints()
         ]);
     }
 
+    [Fact]
+    public void AppliesContentNegotiationRules_DoesNotFailWithCollectionModifiedException()
+    {
+        // This test reproduces the bug where shared lists were being modified during enumeration
+        var errorMessages = new List<string>();
+        var buildEngine = new Mock<IBuildEngine>();
+        buildEngine.Setup(e => e.LogErrorEvent(It.IsAny<BuildErrorEventArgs>()))
+            .Callback<BuildErrorEventArgs>(args => errorMessages.Add(args.Message));
+
+        var task = new ApplyCompressionNegotiation
+        {
+            BuildEngine = buildEngine.Object,
+            CandidateAssets =
+            [
+                CreateCandidate(
+                    Path.Combine("wwwroot", "candidate.js"),
+                    "MyPackage",
+                    "Discovered",
+                    "candidate.js",
+                    "All",
+                    "All",
+                    "original-fingerprint",
+                    "original",
+                    fileLength: 20
+                ),
+                CreateCandidate(
+                    Path.Combine("compressed", "candidate.js.gz"),
+                    "MyPackage",
+                    "Discovered",
+                    "candidate.js",
+                    "All",
+                    "All",
+                    "compressed-fingerprint",
+                    "compressed",
+                    Path.Combine("wwwroot", "candidate.js"),
+                    "Content-Encoding",
+                    "gzip",
+                    9
+                )
+            ],
+            CandidateEndpoints =
+            [
+                CreateCandidateEndpoint(
+                    "candidate.js",
+                    Path.Combine("wwwroot", "candidate.js"),
+                    CreateHeaders("text/javascript", [("Content-Length", "20"), ("Cache-Control", "max-age=3600"), ("ETag", "\"original-etag\""), ("Last-Modified", "Wed, 01 Jan 2020 00:00:00 GMT")])),
+
+                CreateCandidateEndpoint(
+                    "candidate.js.gz",
+                    Path.Combine("compressed", "candidate.js.gz"),
+                    CreateHeaders("text/javascript", [("Content-Length", "9"), ("Cache-Control", "max-age=3600"), ("ETag", "\"compressed-etag\""), ("Last-Modified", "Wed, 01 Jan 2020 00:00:00 GMT")]))
+            ],
+        };
+
+        // Act & Assert - This should not throw a CollectionModifiedException
+        var result = task.Execute();
+        result.Should().Be(true);
+    }
+
     [Fact]
     public void AppliesContentNegotiationRules_IgnoresAlreadyProcessedEndpoints()
     {