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import struct
class ArReader(object):
"""Streaming AR file reader. After constructing an object, you usually
call read_magic once. Then you call read_entry in a loop and use the
ArReader object as file-like only providing read() to read the respective
file contents until you get EOFError from read_entry.
"""
global_magic = b"!<arch>\n"
file_magic = b"`\n"
def __init__(self, fileobj):
"""
@param fileobj: a file-like object providing nothing but read(length)
"""
self.fileobj = fileobj
self.remaining = None
self.padding = 0
def read_magic(self):
"""Consume the AR magic marker at the beginning of an AR file. You
must not call any other method before calling this method.
@raises ValueError: if the magic is not found
"""
data = self.fileobj.read(len(self.global_magic))
if data != self.global_magic:
raise ValueError("ar global header not found")
self.remaining = 0
def read_entry(self):
"""Read the next file header, return the filename and record the
length of the next file, so that the read method can be used to
exhaustively read the current file.
@rtype: bytes
@returns: the name of the next file
@raises ValueError: if the data format is wrong
@raises EOFError: when the end f the stream is reached
"""
self.skip_current_entry()
if self.padding:
if self.fileobj.read(1) != b'\n':
raise ValueError("missing ar padding")
self.padding = 0
file_header = self.fileobj.read(60)
if not file_header:
raise EOFError("end of archive found")
parts = struct.unpack("16s 12s 6s 6s 8s 10s 2s", file_header)
parts = [p.rstrip(b" ") for p in parts]
if parts.pop() != self.file_magic:
raise ValueError("ar file header not found")
self.remaining = int(parts[5])
self.padding = self.remaining % 2
return parts[0] # name
def skip_current_entry(self):
"""Skip the remainder of the current file. This method must not be
called before calling read_entry.
@raises ValueError: if the archive appears truncated
"""
while self.remaining:
data = self.fileobj.read(min(4096, self.remaining))
if not data:
raise ValueError("archive truncated")
self.remaining -= len(data)
def read(self, length=None):
"""
@type length: int or None
@param length: number of bytes to read from the current file
@rtype: bytes
@returns: length or fewer bytes from the current file
"""
if length is None:
length = self.remaining
else:
length = min(self.remaining, length)
data = self.fileobj.read(length)
self.remaining -= len(data)
return data
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