From 332ac9eafb235443f163c606ced95dcbd615815e Mon Sep 17 00:00:00 2001
From: Helmut Grohne <helmut@subdivi.de>
Date: Wed, 19 Feb 2014 14:21:20 +0100
Subject: blacklist content rather than hashes

Otherwise the gzip hash cannot tell the empty stream and the
compressed empty stream apart.
---
 dedup/hashing.py | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
 importpkg.py     | 15 +++++----------
 2 files changed, 55 insertions(+), 10 deletions(-)

diff --git a/dedup/hashing.py b/dedup/hashing.py
index 5f015b2..70f6268 100644
--- a/dedup/hashing.py
+++ b/dedup/hashing.py
@@ -1,3 +1,5 @@
+import itertools
+
 class HashBlacklist(object):
     """Turn a hashlib-like object into a hash that returns None for some
     blacklisted hashes instead of the real hash value.
@@ -29,6 +31,54 @@ class HashBlacklist(object):
     def copy(self):
         return HashBlacklist(self.hashobj.copy(), self.blacklist)
 
+class HashBlacklistContent(object):
+    """Turn a hashlib-like object into a hash that returns None for some
+    blacklisted content instead of the real hash value. Unlike HashBlacklist,
+    not the output of the hash is considered, but its input."""
+
+    def __init__(self, hashobj, blacklist=(), maxlen=None):
+        """
+        @param hashobj: a hashlib-like object
+        @param blacklist: an object providing __contains__.
+            hash inputs which are contained in the blacklist
+            are turned into None values
+        @param maxlen: the maximum length of a blacklisted input.
+            Defaults to max(map(len, blacklist)), so if it is absent,
+            the blacklist must support iteration.
+        """
+        self.hashobj = hashobj
+        self.blacklist = blacklist
+        if maxlen is None:
+            # the chain avoids passing the empty sequence to max
+            maxlen = max(itertools.chain((0,), itertools.imap(len, blacklist)))
+        self.maxlen = maxlen
+        self.stored = ""
+
+    @property
+    def name(self):
+        return self.hashobj.name
+
+    def update(self, data):
+        if self.stored is not None:
+            self.stored += data
+            if len(self.stored) > self.maxlen:
+                self.stored = None
+        self.hashobj.update(data)
+
+    def digest(self):
+        if self.stored is not None and self.stored in self.blacklist:
+            return None
+        return self.hashobj.digest()
+
+    def hexdigest(self):
+        if self.stored is not None and self.stored in self.blacklist:
+            return None
+        return self.hashobj.hexdigest()
+
+    def copy(self):
+        return HashBlacklistContent(self.hashobj.copy(), self.blacklist,
+                                    self.maxlen)
+
 class DecompressedHash(object):
     """Apply a decompression function before the hash. This class provides the
     hashlib interface (update, hexdigest, copy) excluding digest and name."""
diff --git a/importpkg.py b/importpkg.py
index 54f6181..cb16f97 100755
--- a/importpkg.py
+++ b/importpkg.py
@@ -16,26 +16,21 @@ import yaml
 
 from dedup.arreader import ArReader
 from dedup.debpkg import process_control, get_tar_hashes
-from dedup.hashing import HashBlacklist, DecompressedHash, SuppressingHash, \
-    HashedStream
+from dedup.hashing import DecompressedHash, SuppressingHash, HashedStream, \
+        HashBlacklistContent
 from dedup.compression import GzipDecompressor, DecompressedStream
 from dedup.image import GIFHash, PNGHash
 
-boring_sha512_hashes = set((
-    # ""
-    "cf83e1357eefb8bdf1542850d66d8007d620e4050b5715dc83f4a921d36ce9ce47d0d13c5d85f2b0ff8318d2877eec2f63b931bd47417a81a538327af927da3e",
-    # "\n"
-    "be688838ca8686e5c90689bf2ab585cef1137c999b48c70b92f67a5c34dc15697b5d11c982ed6d71be1e1e7f7b4e0733884aa97c3f7a339a8ed03577cf74be09"))
+boring_content = set(("", "\n"))
 
 def sha512_nontrivial():
-    return HashBlacklist(hashlib.sha512(), boring_sha512_hashes)
+    return HashBlacklistContent(hashlib.sha512(), boring_content)
 
 def gziphash():
     hashobj = DecompressedHash(GzipDecompressor(), hashlib.sha512())
     hashobj = SuppressingHash(hashobj, (ValueError, zlib.error))
     hashobj.name = "gzip_sha512"
-    # don't blacklist boring hashes for gzip to get gzip issues right
-    return hashobj
+    return HashBlacklistContent(hashobj, boring_content)
 
 def pnghash():
     hashobj = PNGHash(hashlib.sha512())
-- 
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