Fix source-flow PRs removing required darc-* feeds from NuGet.config

src/Microsoft.DotNet.Darc/DarcLib/VirtualMonoRepo/ForwardFlowConflictResolver.cs

@@ -339,9 +339,68 @@ await _jsonFileMerger.MergeJsonsAsync(
             await vmr.StageAsync([relativeSourceMappingPath / VersionFiles.VersionDetailsProps], cancellationToken);
         }
 
+        // Recompute NuGet.config feeds based on the current dependencies in Version.Details.xml
+        // This ensures that source-flow PRs don't remove feeds that are still required by dependencies
+        await UpdateNugetConfigFeedsAsync(vmr, relativeSourceMappingPath, cancellationToken);
+
         if (!await vmr.HasWorkingTreeChangesAsync() && !await vmr.HasStagedChangesAsync())
         {
             _logger.LogInformation("No changes to dependencies in this forward flow update");
         }
     }
+
+    /// <summary>
+    /// Updates NuGet.config with feeds required by the current dependencies in Version.Details.xml.
+    /// This ensures that source-flow PRs don't remove feeds that are still needed.
+    /// </summary>
+    private async Task UpdateNugetConfigFeedsAsync(
+        ILocalGitRepo vmr,
+        UnixPath relativeSourceMappingPath,
+        CancellationToken cancellationToken)
+    {
+        // Read the current Version.Details.xml to get all dependencies
+        var versionDetailsXml = await _dependencyFileManager.ReadVersionDetailsXmlAsync(
+            vmr.Path,
+            branch: null!, // reads the working tree
+            relativeSourceMappingPath);
+        var versionDetails = _versionDetailsParser.ParseVersionDetailsXml(versionDetailsXml);
+
+        // Build the asset location mapping from all dependencies
+        var assetLocationMap = new Dictionary<string, HashSet<string>>();
+        foreach (var dependency in versionDetails.Dependencies)
+        {
+            if (!assetLocationMap.TryGetValue(dependency.Name, out HashSet<string>? value))
+            {
+                value = [];
+                assetLocationMap[dependency.Name] = value;
+            }
+
+            if (dependency.Locations != null)
+            {
+                value.UnionWith(dependency.Locations);
+            }
+        }
+
+        // Flatten the locations and split into groups
+        var managedFeeds = _dependencyFileManager.FlattenLocationsAndSplitIntoGroups(assetLocationMap);
+
+        // Read the current NuGet.config
+        (string nugetConfigName, XmlDocument nugetConfig) = await _dependencyFileManager.ReadNugetConfigAsync(
+            vmr.Path,
+            branch: null!, // reads the working tree
+            relativeSourceMappingPath);
+
+        // Update package sources based on the managed feeds
+        var updatedNugetConfig = _dependencyFileManager.UpdatePackageSources(nugetConfig, managedFeeds);
+
+        // Write the updated NuGet.config back
+        _fileSystem.WriteToFile(
+            vmr.Path / relativeSourceMappingPath / nugetConfigName,
+            GitFile.GetIndentedXmlBody(updatedNugetConfig));
+
+        // Stage the NuGet.config file
+        await vmr.StageAsync([relativeSourceMappingPath / nugetConfigName], cancellationToken);
+
+        _logger.LogInformation("Updated NuGet.config feeds based on Version.Details.xml dependencies");
+    }
 }

test/Microsoft.DotNet.DarcLib.Tests/VirtualMonoRepo/ForwardFlowConflictResolverTests.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
+using System.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
 using System.Xml;
@@ -122,6 +123,27 @@ public async Task ForwardFlowConflictResolverMergesDependenciesCorrectly()
         dependencyFileManagerMock.Setup(m => m.ReadVersionDetailsXmlAsync(
                 vmrPath, It.IsAny<string>(), It.IsAny<UnixPath>()))
             .ReturnsAsync(doc);
+
+        // Setup for NuGet.config feed update
+        var nugetConfigDoc = new XmlDocument();
+        nugetConfigDoc.LoadXml(@"<configuration><packageSources></packageSources></configuration>");
+        dependencyFileManagerMock.Setup(m => m.ReadNugetConfigAsync(
+                vmrPath, It.IsAny<string>(), It.IsAny<UnixPath>()))
+            .ReturnsAsync(("NuGet.config", nugetConfigDoc));
+        dependencyFileManagerMock.Setup(m => m.FlattenLocationsAndSplitIntoGroups(
+                It.IsAny<Dictionary<string, HashSet<string>>>()))
+            .Returns(new Dictionary<string, HashSet<string>>());
+        dependencyFileManagerMock.Setup(m => m.UpdatePackageSources(
+                It.IsAny<XmlDocument>(), It.IsAny<Dictionary<string, HashSet<string>>>()))
+            .Returns(nugetConfigDoc);
+
+        // Setup version details parser to return an empty version details
+        versionDetailsParserMock.Setup(m => m.ParseVersionDetailsXml(
+                It.IsAny<XmlDocument>(),
+                It.IsAny<bool>()))
+            .Returns(new VersionDetails([], null));
+
+        var fileSystemMock = new Mock<IFileSystem>();
 
         ForwardFlowConflictResolver resolver = new(
             new Mock<IVmrInfo>().Object,
@@ -132,7 +154,7 @@ public async Task ForwardFlowConflictResolverMergesDependenciesCorrectly()
             jsonMergerMock.Object,
             versionDetailsFileMergerMock.Object,
             versionDetailsParserMock.Object,
-            new Mock<IFileSystem>().Object,
+            fileSystemMock.Object,
             NullLogger<ForwardFlowConflictResolver>.Instance);
 
         await resolver.MergeDependenciesAsync(
@@ -185,5 +207,16 @@ await resolver.MergeDependenciesAsync(
                 It.IsAny<XmlDocument>(),
                 newSourceDependency),
             Times.Once);
+
+        // Verify that NuGet.config feeds are recomputed based on Version.Details.xml
+        dependencyFileManagerMock.Verify(m => m.ReadNugetConfigAsync(
+                vmrPath, null!, It.IsAny<UnixPath>()),
+            Times.Once);
+        dependencyFileManagerMock.Verify(m => m.FlattenLocationsAndSplitIntoGroups(
+                It.IsAny<Dictionary<string, HashSet<string>>>()),
+            Times.Once);
+        dependencyFileManagerMock.Verify(m => m.UpdatePackageSources(
+                It.IsAny<XmlDocument>(), It.IsAny<Dictionary<string, HashSet<string>>>()),
+            Times.Once);
     }
 }