Correct check for whether a package exists in a package folder

src/Microsoft.NuGet.Build.Tasks.Tests/NuGetTestHelpers.cs

@@ -55,13 +55,23 @@ public static ResolvePackagesResult ResolvePackagesWithJsonFileContents(
                         filesInPackages.Add(fileInPackage);
                     }
                 }
+                else
+                {
+                    // We will assume there is a location in the lock file we're using
+                    var lockFile = JObject.Parse(projectLockJsonFileContents);
+                    var firstLocation = ((JObject)lockFile["packageFolders"]).Properties().First().Name;
+
+                    foreach (var fileInPackage in GetFakeFileNamesFromPackages(projectLockJsonFileContents, firstLocation))
+                    {
+                        filesInPackages.Add(fileInPackage);
+                    }
+                }
 
                 // Don't require the packages be restored on the machine
                 ResolveNuGetPackageAssets task = null;
-                DirectoryExists directoryExists = path => task.GetPackageFolders().Any(l => path.StartsWith(l)) || Directory.Exists(path);
                 FileExists fileExists = path => filesInPackages.Contains(path) || File.Exists(path);
 
-                task = new ResolveNuGetPackageAssets(directoryExists, fileExists, tryGetRuntimeVersion);
+                task = new ResolveNuGetPackageAssets(fileExists, tryGetRuntimeVersion);
                 var sw = new StringWriter();
                 task.BuildEngine = new MockBuildEngine(sw);
 
@@ -105,6 +115,10 @@ private static IEnumerable<string> GetFakeFileNamesFromPackages(string projectJs
                         yield return Path.Combine(packagesDirectory, library.Name, file).Replace('/', '\\');
                     }
                 }
+
+                // Some earlier versions of NuGet didn't include the hash file in the file list, so fake that
+                // in here.
+                yield return Path.Combine(packagesDirectory, library.Name.Replace('/', '\\'), library.Name.Replace('/', '.') + ".nupkg.sha512");
             }
         }
     }

src/Microsoft.NuGet.Build.Tasks/Delegates.cs

@@ -3,7 +3,6 @@
 
 namespace Microsoft.NuGet.Build.Tasks
 {
-    internal delegate bool DirectoryExists(string path);
     internal delegate bool FileExists(string path);
     internal delegate string TryGetRuntimeVersion(string path);
 }

src/Microsoft.NuGet.Build.Tasks/ResolveNuGetPackageAssets.cs

@@ -49,19 +49,13 @@ public sealed class ResolveNuGetPackageAssets : Task
         private readonly Dictionary<string, string> _projectReferencesToOutputBasePaths = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
 
         #region UnitTestSupport
-        private readonly DirectoryExists _directoryExists = new DirectoryExists(Directory.Exists);
         private readonly FileExists _fileExists = new FileExists(File.Exists);
         private readonly TryGetRuntimeVersion _tryGetRuntimeVersion = new TryGetRuntimeVersion(TryGetRuntimeVersion);
         private readonly bool _reportExceptionsToMSBuildLogger = true;
 
-        internal ResolveNuGetPackageAssets(DirectoryExists directoryExists, FileExists fileExists, TryGetRuntimeVersion tryGetRuntimeVersion)
+        internal ResolveNuGetPackageAssets(FileExists fileExists, TryGetRuntimeVersion tryGetRuntimeVersion)
             : this()
         {
-            if (directoryExists != null)
-            {
-                _directoryExists = directoryExists;
-            }
-
             if (fileExists != null)
             {
                 _fileExists = fileExists;
@@ -894,7 +888,9 @@ private string GetNuGetPackagePath(string packageId, string packageVersion)
             {
                 string packagePath = Path.Combine(packagesFolder, packageId, packageVersion);
 
-                if (_directoryExists(packagePath))
+                // The proper way to check if a package is available is to look for the hash file, since that's the last
+                // file written as a part of the restore process. If it's not there, it means something failed part way through.
+                if (_fileExists(Path.Combine(packagePath, $"{packageId}.{packageVersion}.nupkg.sha512")))
                 {
                     return packagePath;
                 }