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bpo-38319: Fix shutil._fastcopy_sendfile(): set sendfile() max block size #16491

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Oct 1, 2019
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bpo-38319: Fix shutil._fastcopy_sendfile(): set sendfile() max block size #16491

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d5fc0e9
set blocksize max 2G
giampaolo Sep 30, 2019
5ea0ee1
add news entry
giampaolo Sep 30, 2019
549e508
use power of 2; catch OSError
giampaolo Sep 30, 2019
49e6684
use binary prefixes
giampaolo Sep 30, 2019
222a267
only limit blocksize to 1GiB for 32 archs
giampaolo Sep 30, 2019
6614996
add comments
giampaolo Oct 1, 2019
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1 change: 1 addition & 0 deletions Lib/shutil.py
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Original file line number Diff line number Diff line change
Expand Up @@ -136,6 +136,7 @@ def _fastcopy_sendfile(fsrc, fdst):
# changes while being copied.
try:
blocksize = max(os.fstat(infd).st_size, 2 ** 23) # min 8MB
blocksize = min(blocksize, 2 ** 31 - 2) # max 2GB
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Why not 2 ** 31 - 1?

Would not be better to use a power of two, 2**30?

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Agreed, used 2**30.

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That sounds inefficient on 64-bit system: files larger than 2 GB will need multiple sendfile() syscalls, rather currently a single one is enough, no? Maybe use sys.maxsize instead?

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On my 64-bit Ubuntu 18.04 box sendfile() always copies up to 2GB per call, never more than that.

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Well ok, but the kernel can be updated to support larger copies. Why should Python put an arbitrary limit on a syscall? Well, you're the expert, it's up to you. It was just a comment ;-)

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OK, I think it makes sense. I added:

+        if sys.maxsize < 2 ** 32:  # 32-bit architecture
+            blocksize = min(blocksize, 2 ** 30)  # max 1GiB

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...I did that for shutil only. In socket.py I decided to always limit to max 1GiB: since we're dealing with a socket I want to exercise the loop more often (and hit the selector(), which implements the timeout logic).

except Exception:
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What exceptions besides OSError can be raised? I do not feel comfortable with silencing a wide range of exceptions.

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Agreed.

blocksize = 2 ** 27 # 128MB

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1 change: 1 addition & 0 deletions Lib/socket.py
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Expand Up @@ -357,6 +357,7 @@ def _sendfile_use_sendfile(self, file, offset=0, count=None):
if not fsize:
return 0 # empty file
blocksize = fsize if not count else count
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Suggested change
blocksize = fsize if not count else count
blocksize = count or fsize

blocksize = min(blocksize, 2 ** 31 - 2) # max 2GB

timeout = self.gettimeout()
if timeout == 0:
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