From 9b307684333fcae533e01785fceb059d82f1b107 Mon Sep 17 00:00:00 2001
From: Florian Festi <ffesti@redhat.com>
Date: Wed, 22 Sep 2010 11:03:53 +0200
Subject: [PATCH] Avoid the need of seek()ing on the file read
 using a new "PaddedFile" class that returns the prepended data before
 calling read() of the underlying file object

---
 gzip.py |   73 +++++++++++++++++++++++++++++++++++++++++++++++---------------
 1 files changed, 55 insertions(+), 18 deletions(-)

diff --git a/gzip.py b/gzip.py
index 83311cc..0bc2c19 100644
--- a/gzip.py
+++ b/gzip.py
@@ -45,6 +45,43 @@ def open(filename, mode="rb", compresslevel=9):
     """
     return GzipFile(filename, mode, compresslevel)
 
+class PaddedFile:
+    """readonly file object that prepends string the content of f"""
+    def __init__(self, f, prepend=b''):
+        self._buffer = prepend
+        self._length = len(prepend)
+        self.file = f
+        self._read = 0
+
+    def read(self, size):
+        if self._read is None:
+            return self.file.read(size)
+        if self._read + size <= self._length:
+            read = self._read
+            self._read += size
+            return self._buffer[read:self._read]
+        else:
+            read = self._read
+            self._read = None
+            return self._buffer[read:] + \
+                   self.file.read(size-self._length+read)
+
+    def prepend(self, prepend=b'', readprevious=False):
+        if self._read is None:
+            self._buffer = prepend
+        elif readprevious and len(prepend) <= self._read:
+            self._read -= len(prepend)
+            return
+        else:
+            self._buffer = self._buffer[read:] + prepend
+        self._length = len(self._buffer)
+        self._read = 0
+
+    def unused(self):
+        if self._read is None:
+            return b''
+        return self._buffer[self._read:]
+
 class GzipFile(io.BufferedIOBase):
     """The GzipFile class simulates most of the methods of a file object with
     the exception of the readinto() and truncate() methods.
@@ -188,6 +225,9 @@ class GzipFile(io.BufferedIOBase):
 
     def _read_gzip_header(self):
         magic = self.fileobj.read(2)
+        if magic == b'':
+            raise EOFError("Reached EOF")
+
         if magic != b'\037\213':
             raise IOError('Not a gzipped file')
         method = ord( self.fileobj.read(1) )
@@ -219,6 +259,13 @@ class GzipFile(io.BufferedIOBase):
         if flag & FHCRC:
             self.fileobj.read(2)     # Read & discard the 16-bit header CRC
 
+        if isinstance(self.fileobj, PaddedFile):
+            unused = self.fileobj.unused()
+            if unused:
+                uncompress = self.decompress.decompress(unused)
+                self._add_read_data(uncompress)
+            self.fileobj = self.fileobj.file
+
     def write(self,data):
         if self.mode != WRITE:
             import errno
@@ -282,16 +329,6 @@ class GzipFile(io.BufferedIOBase):
         if self._new_member:
             # If the _new_member flag is set, we have to
             # jump to the next member, if there is one.
-            #
-            # First, check if we're at the end of the file;
-            # if so, it's time to stop; no more members to read.
-            pos = self.fileobj.tell()   # Save current position
-            self.fileobj.seek(0, 2)     # Seek to end of file
-            if pos == self.fileobj.tell():
-                raise EOFError("Reached EOF")
-            else:
-                self.fileobj.seek( pos ) # Return to original position
-
             self._init_read()
             self._read_gzip_header()
             self.decompress = zlib.decompressobj(-zlib.MAX_WBITS)
@@ -305,6 +342,9 @@ class GzipFile(io.BufferedIOBase):
 
         if buf == b"":
             uncompress = self.decompress.flush()
+            # Prepend the already read bytes to the fileobj to they can be
+            # seen by _read_eof()
+            self.fileobj = PaddedFile(self.fileobj, self.decompress.unused_data)
             self._read_eof()
             self._add_read_data( uncompress )
             raise EOFError('Reached EOF')
@@ -316,10 +356,9 @@ class GzipFile(io.BufferedIOBase):
             # Ending case: we've come to the end of a member in the file,
             # so seek back to the start of the unused data, finish up
             # this member, and read a new gzip header.
-            # (The number of bytes to seek back is the length of the unused
-            # data, minus 8 because _read_eof() will rewind a further 8 bytes)
-            self.fileobj.seek( -len(self.decompress.unused_data)+8, 1)
-
+            # Prepend the already read bytes to the fileobj to they can be
+            # seen by _read_eof() and _read_gzip_header()
+            self.fileobj = PaddedFile(self.fileobj, self.decompress.unused_data)
             # Check the CRC and file size, and set the flag so we read
             # a new member on the next call
             self._read_eof()
@@ -334,12 +373,10 @@ class GzipFile(io.BufferedIOBase):
         self.size = self.size + len(data)
 
     def _read_eof(self):
-        # We've read to the end of the file, so we have to rewind in order
-        # to reread the 8 bytes containing the CRC and the file size.
+        # We've read to the end of the file
         # We check the that the computed CRC and size of the
         # uncompressed data matches the stored values.  Note that the size
         # stored is the true file size mod 2**32.
-        self.fileobj.seek(-8, 1)
         crc32 = read32(self.fileobj)
         isize = read32(self.fileobj)  # may exceed 2GB
         if crc32 != self.crc:
@@ -355,7 +392,7 @@ class GzipFile(io.BufferedIOBase):
         while c == b"\x00":
             c = self.fileobj.read(1)
         if c:
-            self.fileobj.seek(-1, 1)
+            self.fileobj.prepend(c, True)
 
     @property
     def closed(self):
-- 
1.7.3