directories: make initialization of the download cache lazy

If you open up a directory containing thousands of files, it currently computes the cache filename and checks for the cache file on disk immediately for each immutble file in that directory.  With this patch, it delays those steps until you try to do something with an immutable file that could use the cache.

src/allmydata/client.py

@@ -428,9 +428,7 @@ class Client(node.Node, pollmixin.PollMixin):
                 if isinstance(u, LiteralFileURI):
                     node = LiteralFileNode(u, self) # LIT
                 else:
-                    key = base32.b2a(u.storage_index)
-                    cachefile = self.download_cache_dirman.get_file(key)
-                    node = FileNode(u, self, cachefile) # CHK
+                    node = FileNode(u, self, self.download_cache_dirman) # CHK
             else:
                 assert IMutableFileURI.providedBy(u), u
                 node = MutableFileNode(self).init_from_uri(u)

src/allmydata/immutable/filenode.py

@@ -68,12 +68,13 @@ class PortionOfFile:
 class DownloadCache:
     implements(IDownloadTarget)
 
-    def __init__(self, node, cachefile):
+    def __init__(self, node, cachedirectorymanager):
         self._downloader = node._client.getServiceNamed("downloader")
         self._uri = node.get_uri()
         self._storage_index = node.get_storage_index()
         self.milestones = set() # of (offset,size,Deferred)
-        self.cachefile = cachefile
+        self.cachedirectorymanager = cachedirectorymanager
+        self.cachefile = None
         self.download_in_progress = False
         # five states:
         #  new FileNode, no downloads ever performed
@@ -103,6 +104,8 @@ class DownloadCache:
 
     def read(self, consumer, offset, size):
         assert offset+size <= self.get_filesize()
+        if not self.cachefile:
+            self.cachefile = self.cachedirectorymanager.get_file(self._storage_index)
         f = PortionOfFile(self.cachefile.get_filename(), offset, size)
         d = basic.FileSender().beginFileTransfer(f, consumer)
         d.addCallback(lambda lastSent: consumer)
@@ -142,6 +145,8 @@ class DownloadCache:
                         umid="8PKOhg", level=log.NOISY)
 
     def get_filesize(self):
+        if not self.cachefile:
+            self.cachefile = self.cachedirectorymanager.get_file(self._storage_index)
         try:
             filesize = os.stat(self.cachefile.get_filename())[stat.ST_SIZE]
         except OSError:
@@ -150,6 +155,8 @@ class DownloadCache:
 
 
     def open(self, size):
+        if not self.cachefile:
+            self.cachefile = self.cachedirectorymanager.get_file(self._storage_index)
         self.f = open(self.cachefile.get_filename(), "wb")
 
     def write(self, data):
@@ -176,9 +183,9 @@ class DownloadCache:
 
 
 class FileNode(_ImmutableFileNodeBase, log.PrefixingLogMixin):
-    def __init__(self, uri, client, cachefile):
+    def __init__(self, uri, client, cachedirectorymanager):
         _ImmutableFileNodeBase.__init__(self, uri, client)
-        self.download_cache = DownloadCache(self, cachefile)
+        self.download_cache = DownloadCache(self, cachedirectorymanager)
         prefix = uri.get_verify_cap().to_string()
         log.PrefixingLogMixin.__init__(self, "allmydata.immutable.filenode", prefix=prefix)
         self.log("starting", level=log.OPERATIONAL)