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Make mode descriptor cache initialization thread-safe.

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Initializing mode descriptor cache in-place is racy and may cause a thread to
observe a partially constructed cache if another thread is pre-empted while
it's still constructing the cache. In this change, the mode descriptor cache is
constructed into a local variable instead and then set globally in a single
atomic operation, preventing any possibility of observing an incomplete cache.
This commit is contained in:
Jarkko Pöyry 2017-01-10 23:20:05 +02:00
parent b7a27533b1
commit 3bdd15e55e
1 changed files with 13 additions and 9 deletions
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22
PIL/ImageMode.py
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@ -14,7 +14,7 @@
# #
# mode descriptor cache # mode descriptor cache
_modes = {} _modes = None
class ModeDescriptor(object): class ModeDescriptor(object):
@ -32,19 +32,23 @@ class ModeDescriptor(object):
def getmode(mode): def getmode(mode):
"""Gets a mode descriptor for the given mode.""" """Gets a mode descriptor for the given mode."""
global _modes
if not _modes: if not _modes:
# initialize mode cache # initialize mode cache
from PIL import Image from PIL import Image
modes = {}
# core modes # core modes
for m, (basemode, basetype, bands) in Image._MODEINFO.items(): for m, (basemode, basetype, bands) in Image._MODEINFO.items():
_modes[m] = ModeDescriptor(m, bands, basemode, basetype) modes[m] = ModeDescriptor(m, bands, basemode, basetype)
# extra experimental modes # extra experimental modes
_modes["RGBa"] = ModeDescriptor("RGBa", ("R", "G", "B", "a"), "RGB", "L") modes["RGBa"] = ModeDescriptor("RGBa", ("R", "G", "B", "a"), "RGB", "L")
_modes["LA"] = ModeDescriptor("LA", ("L", "A"), "L", "L") modes["LA"] = ModeDescriptor("LA", ("L", "A"), "L", "L")
_modes["La"] = ModeDescriptor("La", ("L", "a"), "L", "L") modes["La"] = ModeDescriptor("La", ("L", "a"), "L", "L")
_modes["PA"] = ModeDescriptor("PA", ("P", "A"), "RGB", "L") modes["PA"] = ModeDescriptor("PA", ("P", "A"), "RGB", "L")
# mapping modes # mapping modes
_modes["I;16"] = ModeDescriptor("I;16", "I", "L", "L") modes["I;16"] = ModeDescriptor("I;16", "I", "L", "L")
_modes["I;16L"] = ModeDescriptor("I;16L", "I", "L", "L") modes["I;16L"] = ModeDescriptor("I;16L", "I", "L", "L")
_modes["I;16B"] = ModeDescriptor("I;16B", "I", "L", "L") modes["I;16B"] = ModeDescriptor("I;16B", "I", "L", "L")
# set global mode cache atomically
_modes = modes
return _modes[mode] return _modes[mode]