Optimise buffer usage in zip walk functions

[glynternet]

We can reuse buffers in the Reader struct to reduce the number of allocations when walking a zip.
The pointer passed to the handler will now point to the same memory address but we have already stated that the file should only be used during the duration of the WalkFn callback so this is fine.

The largest improvements are for zips containing many files but we still get a 15% speedup from zips containing as little as 10 files. These optimisations only improve the overhead of scanning a zip file, not processing contents etc.

$ benchstat /tmp/before /tmp/after
name                       old time/op    new time/op    delta
WalkZipReaderAt/10-16        6.03µs ± 1%    5.09µs ± 3%  -15.66%  (p=0.004 n=5+6)
WalkZipReaderAt/100-16       24.2µs ± 1%    14.8µs ± 2%  -39.03%  (p=0.002 n=6+6)
WalkZipReaderAt/1000-16       215µs ± 2%     109µs ± 0%  -49.25%  (p=0.004 n=6+5)
WalkZipReaderAt/10000-16     2.31ms ± 1%    1.11ms ± 1%  -51.76%  (p=0.004 n=6+5)
WalkZipReaderAt/100000-16    21.6ms ± 2%    11.7ms ± 2%  -46.06%  (p=0.002 n=6+6)

name                       old alloc/op   new alloc/op   delta
WalkZipReaderAt/10-16        8.08kB ± 0%    5.70kB ± 0%  -29.50%  (p=0.002 n=6+6)
WalkZipReaderAt/100-16       30.0kB ± 0%     5.9kB ± 0%  -80.45%  (p=0.002 n=6+6)
WalkZipReaderAt/1000-16       252kB ± 0%       9kB ± 0%  -96.51%  (p=0.002 n=6+6)
WalkZipReaderAt/10000-16     2.48MB ± 0%    0.04MB ± 0%  -98.20%  (p=0.000 n=5+6)
WalkZipReaderAt/100000-16    25.0MB ± 0%     0.5MB ± 0%  -97.90%  (p=0.002 n=6+6)

name                       old allocs/op  new allocs/op  delta
WalkZipReaderAt/10-16          39.0 ± 0%       9.0 ± 0%  -76.92%  (p=0.002 n=6+6)
WalkZipReaderAt/100-16          399 ± 0%       100 ± 0%  -74.94%  (p=0.002 n=6+6)
WalkZipReaderAt/1000-16       4.00k ± 0%     1.00k ± 0%  -74.93%  (p=0.002 n=6+6)
WalkZipReaderAt/10000-16      40.0k ± 0%     10.0k ± 0%  -74.99%  (p=0.002 n=6+6)
WalkZipReaderAt/100000-16      400k ± 0%      100k ± 0%  -75.00%  (p=0.002 n=6+6)

[changelog-app]

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Type

• Feature
• Improvement
• Fix
• Break
• Deprecation
• Manual task
• Migration

Description

Some optimisations have been made which should reduce the resource overhead required when inspecting zip files, such as .jars, .wars, .pars, etc.

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[hpryce]

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