Re: [PATCH] Fix in Git.pm cat_blob crashes on large files (resubmit with reviewed-by)

Thanks for all the input and patience.

On Fri, Feb 22, 2013 at 10:34 AM, Jeff King [off-list ref] wrote:

On Fri, Feb 22, 2013 at 09:30:57AM -0800, Joshua Clayton wrote:

quoted

Read and write each 1024 byte buffer, rather than trying to buffer
the entire content of the file.

OK. Did you ever repeat your timing with a larger symmetric buffer? That
should probably be a separate patch on top, but it might be worth doing
while we are thinking about it.

quoted

Previous code would crash on all files > 2 Gib, when the offset variable
became negative (perhaps below the level of perl), resulting in a crash.

I'm still slightly dubious of this, just because it doesn't match my
knowledge of perl (which is admittedly imperfect). I'm curious how you
diagnosed it?

I first had the memory exhaustion problem running my git repo on a 32 vm.
After bumping the memory from 512 to 4 GiB, and that failing to fix it
I moved to my workstation with 16 GiB
...reproduced
After the initial crash, I added

print $size, " ", $bytesToRead, " ", $bytesRead, "\n";

right before the read command, and it does indeed crash right after
the $bytesRead variable crosses LONG_MAX
...
2567089913 1024 2147482624
2567089913 1024 2147483648
2567089913 1024 2147484672
Offset outside string at /usr/share/perl5/Git.pm line 901, <GEN36> line 2604.

Note that $bytesRead is still positive.
I know very little perl, but that symptom seems pretty clear

quoted

On a 32 bit system, or a system with low memory it might crash before
reaching 2 GiB due to memory exhaustion.

Signed-off-by: Joshua Clayton <redacted>
Reviewed-by: Jeff King <redacted>

The commit message is a good place to mention any side effects, and why
they are not a problem. Something like:

  The previous code buffered the whole blob before writing, so any error
  reading from cat-file would result in zero bytes being written to the
  output stream.  After this change, the output may be left in a
  partially written state (or even fully written, if we fail when
  parsing the final newline from cat-file). However, it's not reasonable
  for callers to expect anything about the state of the output when we
  return an error (after all, even with full buffering, we might fail
  during the writing process).  So any caller which cares about this is
  broken already, and we do not have to worry about them.

quoted

---
 perl/Git.pm |   12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

The patch itself looks fine to me.

-Peff