[gentoo-portage-dev] [PATCH 10/14] portage.checksum: Reorder to avoid loading redundant impls

[Michał Górny]

Reorder the checksum implementations to start with the most preferred
implementation, and try fallbacks only if the most preferred
implementation is not available. Most importantly, this means that
Portage will no longer attempt to load all hash libraries in the system,
especially when it can satisfy its requirements with hashlib.
---
 pym/portage/checksum.py | 145 +++++++++++++++++++++++++++++-------------------
 1 file changed, 87 insertions(+), 58 deletions(-)

diff --git a/pym/portage/checksum.py b/pym/portage/checksum.py
index 659012cdc..2c482f5e7 100644
--- a/pym/portage/checksum.py
+++ b/pym/portage/checksum.py
@@ -93,66 +93,11 @@ class _generate_hash_function(object):
 
 		return (checksum.hexdigest(), size)
 
-# Define hash functions, try to use the best module available. Later definitions
-# override earlier ones
 
-# Try to use mhash if available
-# mhash causes GIL presently, so it gets less priority than hashlib and
-# pycrypto. However, it might be the only accelerated implementation of
-# WHIRLPOOL available.
-try:
-	import mhash
-	for local_name, hash_name in (("RMD160", "RIPEMD160"), ("WHIRLPOOL", "WHIRLPOOL")):
-		if hasattr(mhash, 'MHASH_%s' % hash_name):
-			_generate_hash_function(local_name,
-				functools.partial(mhash.MHASH, getattr(mhash, 'MHASH_%s' % hash_name)),
-				origin='mhash')
-except ImportError:
-	pass
-
-# Use pycrypto when available, prefer it over the internal fallbacks
-# Check for 'new' attributes, since they can be missing if the module
-# is broken somehow.
-try:
-	from Crypto.Hash import RIPEMD
-	rmd160hash_ = getattr(RIPEMD, 'new', None)
-	if rmd160hash_ is not None:
-		_generate_hash_function("RMD160",
-			rmd160hash_, origin="pycrypto")
-except ImportError:
-	pass
-
-try:
-	# added in pycryptodome
-	from Crypto.Hash import BLAKE2b, BLAKE2s, SHA3_256, SHA3_512
-
-	blake2bhash_ = getattr(BLAKE2b, 'new', None)
-	if blake2bhash_ is not None:
-		_generate_hash_function("BLAKE2B",
-			functools.partial(blake2bhash_, digest_bytes=64), origin="pycrypto")
-	blake2shash_ = getattr(BLAKE2s, 'new', None)
-	if blake2shash_ is not None:
-		_generate_hash_function("BLAKE2S",
-			functools.partial(blake2shash_, digest_bytes=32), origin="pycrypto")
-	sha3_256hash_ = getattr(SHA3_256, 'new', None)
-	if sha3_256hash_ is not None:
-		_generate_hash_function("SHA3_256",
-			sha3_256hash_, origin="pycrypto")
-	sha3_512hash_ = getattr(SHA3_512, 'new', None)
-	if sha3_512hash_ is not None:
-		_generate_hash_function("SHA3_512",
-			sha3_512hash_, origin="pycrypto")
-except ImportError:
-	pass
-
-# Support using pysha3 as fallback for python<3.6
-try:
-	import sha3
+# Define hash functions, try to use the best module available. Preferred
+# modules should go first, latter ones should check if the hashes aren't
+# already defined.
 
-	_generate_hash_function("SHA3_256", sha3.sha3_256, origin="pysha3")
-	_generate_hash_function("SHA3_512", sha3.sha3_512, origin="pysha3")
-except ImportError:
-	pass
 
 # Use hashlib from python-2.5 if available and prefer it over pycrypto and internal fallbacks.
 # Need special handling for RMD160/WHIRLPOOL as they may not always be provided by hashlib.
@@ -178,6 +123,89 @@ for local_name, hash_name in (
 			functools.partial(hashlib.new, hash_name),
 			origin='hashlib')
 
+
+# Support using pysha3 as fallback for python<3.6
+if "SHA3_256" not in hashfunc_map or "SHA3_512" not in hashfunc_map:
+	try:
+		import sha3
+
+		_generate_hash_function("SHA3_256", sha3.sha3_256, origin="pysha3")
+		_generate_hash_function("SHA3_512", sha3.sha3_512, origin="pysha3")
+	except ImportError:
+		pass
+
+
+# Use pycrypto when available, prefer it over the internal fallbacks
+# Check for 'new' attributes, since they can be missing if the module
+# is broken somehow.
+if 'RMD160' not in hashfunc_map:
+	try:
+		from Crypto.Hash import RIPEMD
+		rmd160hash_ = getattr(RIPEMD, 'new', None)
+		if rmd160hash_ is not None:
+			_generate_hash_function("RMD160",
+				rmd160hash_, origin="pycrypto")
+	except ImportError:
+		pass
+
+# The following hashes were added in pycryptodome (pycrypto fork)
+if 'BLAKE2B' not in hashfunc_map:
+	try:
+		from Crypto.Hash import BLAKE2b
+		blake2bhash_ = getattr(BLAKE2b, 'new', None)
+		if blake2bhash_ is not None:
+			_generate_hash_function("BLAKE2B",
+				functools.partial(blake2bhash_, digest_bytes=64), origin="pycrypto")
+	except ImportError:
+		pass
+
+if 'BLAKE2S' not in hashfunc_map:
+	try:
+		from Crypto.Hash import BLAKE2s
+		blake2shash_ = getattr(BLAKE2s, 'new', None)
+		if blake2shash_ is not None:
+			_generate_hash_function("BLAKE2S",
+				functools.partial(blake2shash_, digest_bytes=32), origin="pycrypto")
+	except ImportError:
+		pass
+
+if 'SHA3_256' not in hashfunc_map:
+	try:
+		from Crypto.Hash import SHA3_256
+		sha3_256hash_ = getattr(SHA3_256, 'new', None)
+		if sha3_256hash_ is not None:
+			_generate_hash_function("SHA3_256",
+				sha3_256hash_, origin="pycrypto")
+	except ImportError:
+		pass
+
+if 'SHA3_512' not in hashfunc_map:
+	try:
+		from Crypto.Hash import SHA3_512
+		sha3_512hash_ = getattr(SHA3_512, 'new', None)
+		if sha3_512hash_ is not None:
+			_generate_hash_function("SHA3_512",
+				sha3_512hash_, origin="pycrypto")
+	except ImportError:
+		pass
+
+
+# Try to use mhash if available
+# mhash causes GIL presently, so it gets less priority than hashlib and
+# pycrypto. However, it might be the only accelerated implementation of
+# WHIRLPOOL available.
+if 'RMD160' not in hashfunc_map or 'WHIRLPOOL' not in hashfunc_map:
+	try:
+		import mhash
+		for local_name, hash_name in (("RMD160", "RIPEMD160"), ("WHIRLPOOL", "WHIRLPOOL")):
+			if local_name not in hashfunc_map and hasattr(mhash, 'MHASH_%s' % hash_name):
+				_generate_hash_function(local_name,
+					functools.partial(mhash.MHASH, getattr(mhash, 'MHASH_%s' % hash_name)),
+					origin='mhash')
+	except ImportError:
+		pass
+
+
 _whirlpool_unaccelerated = False
 if "WHIRLPOOL" not in hashfunc_map:
 	# Bundled WHIRLPOOL implementation
@@ -196,6 +224,7 @@ hashfunc_map["size"] = SizeHash()
 
 # end actual hash functions
 
+
 prelink_capable = False
 if os.path.exists(PRELINK_BINARY):
 	cmd = [PRELINK_BINARY, "--version"]
-- 
2.12.0