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optimize derivation string parsing

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a bunch of derivation strings contain no escape sequences. we can
optimize for this fact by first scanning for the end of a derivation
string and simply returning the contents unmodified if no escape
sequences were found. to make this even more efficient we can also use
BackedStringViews to avoid copies, avoiding heap allocations for
transient data.

before:

Benchmark 1: nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
  Time (mean ± σ):      6.952 s ±  0.015 s    [User: 5.294 s, System: 1.452 s]
  Range (min … max):    6.926 s …  6.974 s    10 runs

after:

Benchmark 1: nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
  Time (mean ± σ):      6.907 s ±  0.012 s    [User: 5.272 s, System: 1.429 s]
  Range (min … max):    6.893 s …  6.926 s    10 runs
This commit is contained in:
pennae 2023-12-26 22:18:42 +01:00
parent 2cfc4ace35
commit 79d3d412ca
2 changed files with 48 additions and 23 deletions
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6
doc/manual/rl-next/drv-string-parse-hang.md Normal file
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---
synopsis: Fix handling of truncated `.drv` files.
prs: 9673
---
Previously a `.drv` that was truncated in the middle of a string would case nix to enter an infinite loop, eventually exhausting all memory and crashing.