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Avoid rewriting files with identical contents #368

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merged 1 commit into from Sep 2, 2019

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@kelnos
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commented Aug 14, 2019

I think sbt handles this reasonably (by hashing file contents), but it appears that maven pays more attention to file modification times. It slows down things enough already that guardrail runs each time a build is run, but having to recompile files that haven't changed just adds insult to injury.

  1. Running guardrail for the first time should have roughly the same performance (an extra stat() per file, which will fail since the file doesn't exist).
  2. Running guardrail when files need to be updated will be either roughly the same or a bit slower, depending: a) if the file sizes are different, it's just a stat() plus integer comparison extra, so roughly the same; b) if the file sizes are the same, it has to read the existing file off disk and compare, so things will be a bit slower.
  3. Running guardrail when files exist already and don't need to be updated should be slightly faster (an extra stat() and integer comparison, as well as reading the existing file off disk to compare, but no write of the file).

Of course, guardrail still has to do all the work to render the new files' contents to memory no matter what; in general that should be the dominating time factor. There's only one case that ends up being worst (where "worst" is probably a few hundreds of nanoseconds per file extra), which should be the very, very, very uncommon case (as it requires the files to be different, but also the same size).

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@kelnos kelnos requested a review from blast-hardcheese Aug 14, 2019

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Looks good

@blast-hardcheese blast-hardcheese merged commit 1840d47 into twilio:master Sep 2, 2019

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