Keep track of whether the cache needs rebuilding (#24)

This is in addition to always rebuilding when the cache is empty
Fix NuGet/NuGetGallery#3277
(cherry picked from commit 466f321)

Author: joelverhagen

src/NuGet.Server.Core/Infrastructure/ServerPackageRepository.cs

@@ -34,7 +34,8 @@ public class ServerPackageRepository
 
         private readonly bool _runBackgroundTasks;
         private FileSystemWatcher _fileSystemWatcher;
-        private bool _isFileSystemWatcherSuppressed;
+        private bool _isFileSystemWatcherSuppressed = false;
+        private bool _needsRebuild = true;
 
         private Timer _persistenceTimer;
         private Timer _rebuildTimer;
@@ -375,11 +376,11 @@ private IEnumerable<ServerPackage> CachedPackages
         {
             get
             {
-                if (!_serverPackageStore.HasPackages())
+                if (_needsRebuild || !_serverPackageStore.HasPackages())
                 {
                     lock (_syncLock)
                     {
-                        if (!_serverPackageStore.HasPackages())
+                        if (_needsRebuild || !_serverPackageStore.HasPackages())
                         {
                             RebuildPackageStore();
                         }
@@ -420,6 +421,8 @@ private void RebuildPackageStore()
                 // Persist
                 _serverPackageStore.PersistIfDirty();
 
+                _needsRebuild = false;
+
                 _logger.Log(LogLevel.Info, "Finished rebuilding package store.");
             }
         }
@@ -557,6 +560,7 @@ public void ClearCache()
 
                 _serverPackageStore.Clear();
                 _serverPackageStore.Persist();
+                _needsRebuild = true;
                 _logger.Log(LogLevel.Info, "Cleared package cache.");
             }
         }