From a98c5cf627071292de92d6a0b3e61ab916308c21 Mon Sep 17 00:00:00 2001
From: Robey Pointer <robey@lag.net>
Date: Thu, 9 Mar 2006 00:28:30 -0800
Subject: [PATCH] [project @ robey@lag.net-20060309082830-ea89d2b2259098a2]
 revise the prefetch machinery so that it doesn't assert if you seek to
 earlier than the prefetched buffers.  also, keep around any prefetched data
 that has been seeked around, so that jumping around in the file will still
 get the benefit of prefetch, though only the first time any chunk is read.

---
 paramiko/sftp_file.py | 43 +++++++++++++++++++++++++++++++++----------
 1 file changed, 33 insertions(+), 10 deletions(-)

diff --git a/paramiko/sftp_file.py b/paramiko/sftp_file.py
index 02c6e76..7d9b8f4 100644
--- a/paramiko/sftp_file.py
+++ b/paramiko/sftp_file.py
@@ -44,6 +44,7 @@ class SFTPFile (BufferedFile):
         BufferedFile._set_mode(self, mode, bufsize)
         self.pipelined = False
         self._prefetching = False
+        self._prefetch_done = False
         self._prefetch_so_far = 0
         self._prefetch_size = 0
         self._prefetch_data = {}
@@ -83,29 +84,48 @@ class SFTPFile (BufferedFile):
             pass
 
     def _read_prefetch(self, size):
+        """
+        read data out of the prefetch buffer, if possible.  if the data isn't
+        in the buffer, return None.  otherwise, behaves like a normal read.
+        """
         # while not closed, and haven't fetched past the current position, and haven't reached EOF...
         while (self._prefetch_so_far <= self._realpos) and \
             (self._prefetch_so_far < self._prefetch_size) and not self._closed:
+            if self._prefetch_done:
+                return None
             self.sftp._read_response()
         self._check_exception()
         k = self._prefetch_data.keys()
-        k.sort()
-        while (len(k) > 0) and (k[0] + len(self._prefetch_data[k[0]]) <= self._realpos):
-            # done with that block
-            del self._prefetch_data[k[0]]
-            k.pop(0)
         if len(k) == 0:
             self._prefetching = False
             return ''
-        assert k[0] <= self._realpos
-        buf_offset = self._realpos - k[0]
-        buf_length = len(self._prefetch_data[k[0]]) - buf_offset
-        return self._prefetch_data[k[0]][buf_offset : buf_offset + buf_length]
+
+        # find largest offset < realpos
+        pos_list = [i for i in k if i <= self._realpos]
+        if len(pos_list) == 0:
+            return None
+        index = max(pos_list)
+        prefetch = self._prefetch_data[index]
+        del self._prefetch_data[index]
+        
+        buf_offset = self._realpos - index
+        if buf_offset > 0:
+            self._prefetch_data[index] = prefetch[:buf_offset]
+            prefetch = prefetch[buf_offset:]
+        if buf_offset >= len(prefetch):
+            # it's not here.
+            return None
+        if size < len(prefetch):
+            self._prefetch_data[self._realpos + size] = prefetch[size:]
+            prefetch = prefetch[:size]
+        return prefetch
         
     def _read(self, size):
         size = min(size, self.MAX_REQUEST_SIZE)
         if self._prefetching:
-            return self._read_prefetch(size)
+            data = self._read_prefetch(size)
+            if data is not None:
+                return data
         t, msg = self.sftp._request(CMD_READ, self.handle, long(self._realpos), int(size))
         if t != CMD_DATA:
             raise SFTPError('Expected data')
@@ -333,6 +353,7 @@ class SFTPFile (BufferedFile):
         size = self.stat().st_size
         # queue up async reads for the rest of the file
         self._prefetching = True
+        self._prefetch_done = False
         self._prefetch_so_far = self._realpos
         self._prefetch_size = size
         self._prefetch_data = {}
@@ -371,6 +392,8 @@ class SFTPFile (BufferedFile):
         data = msg.get_string()
         self._prefetch_data[self._prefetch_so_far] = data
         self._prefetch_so_far += len(data)
+        if self._prefetch_so_far == self._prefetch_size:
+            self._prefetch_done = True
     
     def _check_exception(self):
         "if there's a saved exception, raise & clear it"