ClientCache: made it a new-style class.

FileCache:  Removed the reuse= argument.  IIRC, it made some sense
when different cache schemes were first being tried, but complicated
the logic now to no good end.

Nuked the maddening log warnings about reuse=True.  They were never
helpful and were often confusing.

Added info-level log messages to record the path of the client cache
file, whether it's persistent or temporary, and if it's persistent
whether we're creating or reusing it.

Minor changes to comments.

src/ZEO/cache.py

@@ -64,7 +64,7 @@ logger = logging.getLogger("ZEO.cache")
 # full verification
 # <p>
 
-class ClientCache:
+class ClientCache(object):
     """A simple in-memory cache."""
 
     ##
@@ -689,17 +689,15 @@ def sync(f):
 
 class FileCache(object):
 
-    def __init__(self, maxsize, fpath, parent, reuse=True):
+    def __init__(self, maxsize, fpath, parent):
         # - `maxsize`:  total size of the cache file, in bytes; this is
-        #   ignored if reuse is true and fpath names an existing file;
-        #   perhaps we should attempt to change the cache size in that
-        #   case
-        # - `fpath`:  filepath for the cache file, or None; see `reuse`
-        # - `parent`:  the ClientCache this FileCache is part of
-        # - `reuse`:  If true, and fpath is not None, and fpath names a
-        #    file that exists, that pre-existing file is used (persistent
-        #    cache).  In all other cases a new file is created:  a temp
-        #    file if fpath is None, else with path fpath.
+        #   ignored path names an existing file; perhaps we should attempt
+        #   to change the cache size in that case
+        # - `fpath`:  filepath for the cache file, or None (in which case
+        #   a temp file will be created)
+        # - `parent`:  the ClientCache instance; its `_evicted()` method
+        #   is called whenever we need to evict an object to make room in
+        #   the file
         self.maxsize = maxsize
         self.parent = parent
 
@@ -743,17 +741,17 @@ class FileCache(object):
         # (and it sets self.f).
 
         self.fpath = fpath
-        if reuse and fpath and os.path.exists(fpath):
+        if fpath and os.path.exists(fpath):
             # Reuse an existing file.  scan() will open & read it.
             self.f = None
+            logger.info("reusing persistent cache file %r", fpath)
         else:
-            if reuse:
-                logger.warning("reuse=True but the given file path %r "
-                               "doesn't exist; ignoring reuse=True", fpath)
             if fpath:
                 self.f = open(fpath, 'wb+')
+                logger.info("created persistent cache file %r", fpath)
             else:
                 self.f = tempfile.TemporaryFile()
+                logger.info("created temporary cache file %r", self.f.name)
             # Make sure the OS really saves enough bytes for the file.
             self.f.seek(self.maxsize - 1)
             self.f.write('x')
@@ -779,11 +777,11 @@ class FileCache(object):
     # for each object found in the cache.  This method should only
     # be called once to initialize the cache from disk.
     def scan(self, install):
-        if self.f is not None:
+        if self.f is not None:  # we're not (re)using a pre-existing file
             return
         fsize = os.path.getsize(self.fpath)
         if fsize != self.maxsize:
-            logger.warning("existing cache file %s has size %d; "
+            logger.warning("existing cache file %r has size %d; "
                            "requested size %d ignored", self.fpath,
                            fsize, self.maxsize)
             self.maxsize = fsize
@@ -797,8 +795,8 @@ class FileCache(object):
 
         # Populate .filemap and .key2entry to reflect what's currently in the
         # file, and tell our parent about it too (via the `install` callback).
-        # Remember the location of the largest free block  That seems a decent
-        # place to start currentofs.
+        # Remember the location of the largest free block.  That seems a
+        # decent place to start currentofs.
         max_free_size = max_free_offset = 0
         ofs = ZEC3_HEADER_SIZE
         while ofs < fsize: