http_wsgibase: Do not read entire chunks.

Chunk size is not bounded. So instead of remembering chunk tail, remember
how much there is to read in current chunk.

caucase/http_wsgibase.py

@@ -42,17 +42,21 @@ class ChunkedFile(ProxyFile):
 
   def __init__(self, actual_file):
     super(ChunkedFile, self).__init__(actual_file)
-    self._recv_buf = ''
+    self._chunk_remaining_length = 0
 
   def read(self, length=None):
     """
     Read chunked data.
     """
-    result = self._recv_buf
+    result = ''
     if not self._at_eof:
       readline = self.readline
       read = self.__getattr__('read')
-      while length is None or len(result) < length:
+      while True:
+        if self._chunk_remaining_length:
+          chunk_length = self._chunk_remaining_length
+          self._chunk_remaining_length = 0
+        else:
           chunk_header = readline(MAX_CHUNKED_HEADER_LENGTH + 1)
           if len(chunk_header) > MAX_CHUNKED_HEADER_LENGTH:
             raise ValueError('Chunked encoding header too long')
@@ -66,14 +70,17 @@ class ChunkedFile(ProxyFile):
               raise ValueError('Chunked encoding trailer too long')
             self._at_eof = True
             break
-        result += read(chunk_length)
+        if length is None:
+          to_read = chunk_length
+        else:
+          to_read = min(chunk_length, length - len(result))
+        result += read(to_read)
+        if to_read != chunk_length:
+          self._chunk_remaining_length = chunk_length - to_read
+          break
         if read(2) != '\r\n':
           raise ValueError('Invalid chunked encoding separator')
-    if length is None:
-      self._recv_buf = ''
     return result
-    self._recv_buf = result[length:]
-    return result[:length]
 
 class HookFirstReadFile(ProxyFile):
   """