If tmp2 and final files are in same filesystem, rename() not copy()

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This avoids potentially painful copies of large files within the same filesystem.
To note:
a) I'm a UNIX person, so I'm not sure if this will compile on Windows as coded. Directions welcome.
b) I didn't write a test for this, it appears all of the existing test only use files in the same directory. Is the copy codepath in CreatePlotDisk() tested somewhere?

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This fixes #313 , waiting on workflows to test windows applicability.

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Live-testing this change yields the following result (successful):

Starting plotting progress into temporary dirs: /diskc/tmp and /diska/tmp
ID: IDSTR
Plot size is: 33
[...]
Final Directory is: /diska/chia/plots/k33
[...][...]
Final File size: 208.858 GiB
Total time = 63537.734 seconds. CPU (152.580%) Thu Oct 21 18:52:28 2021
Renamed final file from "/diska/tmp/plot-k33-2021-10-21-01-13-IDSTR.plot.2.tmp" to "/diska/chia/plots/k33/plot-k33-2021-10-21-01-13-IDSTR.plot"

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Anything else needed here? Who can provide feedback or approval?

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Requesting review - @hoffmang9 @wjblanke

[rostislav]

Looks good to me. I've also verified that this works on Windows.

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Can someone merge this? Is anything else needed?

[hoffmang9]

This is likely to take a bit broader testing and that's the only hesitation. Right this moment there are other things at a higher priority. Should it test well on BSD/MacOS both flavors, three FSs etc.

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This is likely to take a bit broader testing and that's the only hesitation. Right this moment there are other things at a higher priority. Should it test well on BSD/MacOS both flavors, three FSs etc.

Okay. It's very old libc calls, so I'm sure all UNIX/Linux variants and Mac OS (>9) will work the same, but I do not criticize at all your wanting to test it fully. Let me know if I can help.

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If I could get this in before the next chia-blockchain release, so its in there, that would be ideal. Is there anything I can do to help testing this? Just checking since it's been a few weeks here, happy to help if I can.

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Ping. Sorry to be a noodge, but just wanted to make sure there's a plan to get the required testing done and get this merged. Again, let me know if I can help.

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@rostislav @hoffmang9 Any status on this PR?