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ref: use SmallRng #119

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2 changes: 1 addition & 1 deletion Cargo.toml
Original file line number Diff line number Diff line change
Expand Up @@ -18,7 +18,7 @@ description = "A library for managing temporary files and directories."

[dependencies]
cfg-if = "1"
rand = "0.8"
rand = { version = "0.8", features = ["small_rng", "getrandom"], default_features = false }
remove_dir_all = "0.5"

[target.'cfg(unix)'.dependencies]
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20 changes: 9 additions & 11 deletions src/util.rs
Original file line number Diff line number Diff line change
@@ -1,24 +1,22 @@
use rand::distributions::Alphanumeric;
use rand::{self, Rng};
use rand::{Rng, SeedableRng};
use rand::rngs::SmallRng;
use std::ffi::{OsStr, OsString};
use std::path::{Path, PathBuf};
use std::{io, str};
use std::io;

use crate::error::IoResultExt;

fn tmpname(prefix: &OsStr, suffix: &OsStr, rand_len: usize) -> OsString {
let mut buf = OsString::with_capacity(prefix.len() + suffix.len() + rand_len);
buf.push(prefix);

// Push each character in one-by-one. Unfortunately, this is the only
// safe(ish) simple way to do this without allocating a temporary
// String/Vec.
unsafe {
rand::thread_rng()
.sample_iter(&Alphanumeric)
.take(rand_len)
.for_each(|b| buf.push(str::from_utf8_unchecked(&[b as u8])))
}
let small_rng = SmallRng::from_entropy();
buf.push(small_rng
.sample_iter(&Alphanumeric)
.take(rand_len)
.collect::<String>()
Comment on lines +16 to +18
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Suggested change
.sample_iter(&Alphanumeric)
.take(rand_len)
.collect::<String>()
.sample_iter(Alphanumeric)
.take(rand_len)
.map(char::from)
.collect::<String>()

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That's still going to allocate, unfortunately.

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@taiki-e taiki-e Dec 31, 2020

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I'll look for a way to avoid the allocation.

EDIT: See #119 (comment)

);
Comment on lines +14 to +19
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We can avoid allocation by using char::encode_utf8.
https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=421dfa2eebe631075a03b06e08c6e7cf

Suggested change
let small_rng = SmallRng::from_entropy();
buf.push(small_rng
.sample_iter(&Alphanumeric)
.take(rand_len)
.collect::<String>()
);
let mut tmp = [0; 1];
let small_rng = SmallRng::from_entropy();
small_rng
.sample_iter(Alphanumeric)
.take(rand_len)
.for_each(|c| buf.push((c as char).encode_utf8(&mut tmp)));

Can we probably resolve performance issues in combination with putting a smallrng in a thread-local variable?

buf.push(suffix);
buf
}
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