Fix "Array dimensions exceeded supported range" exception (#5343)

Hitting this on a quickbuild repo with ~550 projects and ~19K edges (due to transitive edges).

The problem was using an intermediary list to aggregate intermediary results, which exploded exponentially in size due to duplicates. Fix is to use a hash set.

The failure can't really be tested (without stress tests) because it appears in an intermediary list which finally gets dumped into a set, so there's no observable trace of the problem at the end of graph construction.

This fix avoids the exception and takes graph construction down from 5 minutes to 30 seconds on that specific repo.

src/Build.UnitTests/Graph/GraphTestingUtilities.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
+using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.IO;
@@ -218,13 +219,18 @@ internal static TransientTestFile CreateProjectFile(
 
         internal static IEnumerable<ProjectGraphNode> ComputeClosure(ProjectGraphNode node)
         {
-            foreach (var reference in node.ProjectReferences)
-            {
-                yield return reference;
+            return ComputeClosureRecursive(node).ToHashSet();
 
-                foreach (var closureReference in ComputeClosure(reference))
+            IEnumerable<ProjectGraphNode> ComputeClosureRecursive(ProjectGraphNode projectGraphNode)
+            {
+                foreach (var reference in projectGraphNode.ProjectReferences)
                 {
-                    yield return closureReference;
+                    yield return reference;
+
+                    foreach (var closureReference in ComputeClosureRecursive(reference))
+                    {
+                        yield return closureReference;
+                    }
                 }
             }
         }

src/Build.UnitTests/Graph/ProjectGraph_Tests.cs

@@ -1405,6 +1405,19 @@ public static IEnumerable<object[]> Graphs
                     }
                 };
 
+                yield return new object[]
+                {
+                    new Dictionary<int, int[]>
+                    {
+                        {1, new []{2, 4, 3, 5} },
+                        {2, new []{5} },
+                        {3, new []{5} },
+                        {4, new []{6} },
+                        {5, new []{7} },
+                        {6, new []{5} }
+                    },
+                };
+
                 yield return new object[]
                 {
                     new Dictionary<int, int[]>
@@ -1996,11 +2009,9 @@ public void GraphShouldSupportTransitiveReferences(Dictionary<int, int[]> edges)
 
             foreach (var node in graph.ProjectNodes)
             {
-                foreach (var closureReference in ComputeClosure(node))
-                {
-                    node.ProjectReferences.ShouldContain(closureReference);
-                    closureReference.ReferencingProjects.ShouldContain(node);
-                }
+                var expectedClosure = ComputeClosure(node);
+
+                node.ProjectReferences.ShouldBeSameIgnoringOrder(expectedClosure);
             }
         }
 

src/Build/Graph/GraphBuilder.cs

@@ -124,7 +124,7 @@ private void AddEdges(Dictionary<ConfigurationMetadata, ParsedProject> allParsed
 
         private void AddEdgesFromProjectReferenceItems(Dictionary<ConfigurationMetadata, ParsedProject> allParsedProjects, GraphEdges edges)
         {
-            var transitiveReferenceCache = new Dictionary<ProjectGraphNode, List<ProjectGraphNode>>(allParsedProjects.Count);
+            var transitiveReferenceCache = new Dictionary<ProjectGraphNode, HashSet<ProjectGraphNode>>(allParsedProjects.Count);
 
             foreach (var parsedProject in allParsedProjects)
             {
@@ -143,7 +143,7 @@ private void AddEdgesFromProjectReferenceItems(Dictionary<ConfigurationMetadata,
                     // Add transitive references only if the project requires it.
                     if (requiresTransitiveProjectReferences)
                     {
-                        foreach (var transitiveProjectReference in GetTransitiveProjectReferences(allParsedProjects[referenceInfo.ReferenceConfiguration]))
+                        foreach (var transitiveProjectReference in GetTransitiveProjectReferencesExcludingSelf(allParsedProjects[referenceInfo.ReferenceConfiguration]))
                         {
                             currentNode.AddProjectReference(
                                 transitiveProjectReference,
@@ -160,20 +160,24 @@ private void AddEdgesFromProjectReferenceItems(Dictionary<ConfigurationMetadata,
                 }
             }
 
-            List<ProjectGraphNode> GetTransitiveProjectReferences(ParsedProject parsedProject)
+            HashSet<ProjectGraphNode> GetTransitiveProjectReferencesExcludingSelf(ParsedProject parsedProject)
             {
-                if (transitiveReferenceCache.TryGetValue(parsedProject.GraphNode, out List<ProjectGraphNode> cachedTransitiveReferences))
+                if (transitiveReferenceCache.TryGetValue(parsedProject.GraphNode, out HashSet<ProjectGraphNode> cachedTransitiveReferences))
                 {
                     return cachedTransitiveReferences;
                 }
                 else
                 {
-                    var transitiveReferences = new List<ProjectGraphNode>();
+                    var transitiveReferences = new HashSet<ProjectGraphNode>();
 
                     foreach (var referenceInfo in parsedProject.ReferenceInfos)
                     {
                         transitiveReferences.Add(allParsedProjects[referenceInfo.ReferenceConfiguration].GraphNode);
-                        transitiveReferences.AddRange(GetTransitiveProjectReferences(allParsedProjects[referenceInfo.ReferenceConfiguration]));
+
+                        foreach (var transitiveReference in GetTransitiveProjectReferencesExcludingSelf(allParsedProjects[referenceInfo.ReferenceConfiguration]))
+                        {
+                            transitiveReferences.Add(transitiveReference);
+                        }
                     }
 
                     transitiveReferenceCache.Add(parsedProject.GraphNode, transitiveReferences);