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perf: optimize CSP nonce generation #14340

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34e0f59
perf: optimize CSP nonce generation
Tunglies Oct 21, 2025
1a01a36
fix: adjust CSP nonce buffer size based on target pointer width
Tunglies Oct 21, 2025
8f6d625
Optimize CSP nonce generation
Tunglies Oct 21, 2025
8c42c83
Update .changes/change-pr-14340.md
Tunglies Oct 21, 2025
b46856b
fix: correct patch type for CSP nonce optimization
Tunglies Oct 22, 2025
282ed06
Merge branch 'dev' into perf-csp
Tunglies Oct 22, 2025
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5 changes: 5 additions & 0 deletions .changes/change-pr-14340.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
---
"tauri": "patch:perf"
---

perf: optimize CSP nonce generation
25 changes: 16 additions & 9 deletions crates/tauri/src/manager/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@ use std::{
borrow::Cow,
collections::HashMap,
fmt,
fmt::Write,
sync::{atomic::AtomicBool, Arc, Mutex, MutexGuard},
};

Expand Down Expand Up @@ -127,7 +128,7 @@ fn replace_csp_nonce(
directive: &str,
hashes: Vec<String>,
) {
let mut nonces = Vec::new();
let mut nonces = Vec::with_capacity(asset.matches(token).count());
*asset = replace_with_callback(asset, token, || {
#[cfg(target_pointer_width = "64")]
let mut raw = [0u8; 8];
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@lucasfernog do you remember why we used usize here at the first place (tracing back to cf54dcf)? It doesn't quite make sense to me

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@vrmiguel vrmiguel Oct 29, 2025 •
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Maybe I don't get the question, but the code is still using a usize there, just that it's using its underlying byte representation instead, right?

In any case, seems like this code can be simplified down to:

let mut raw = [0u8; std::mem::size_of::<usize>()];
getrandom::fill(&mut raw).expect("failed to get random bytes");
let nonce = usize::from_ne_bytes(raw);

Or even:

let mut raw = 0_usize.to_ne_bytes();
getrandom::fill(&mut raw).expect("failed to get random bytes");
let nonce = usize::from_ne_bytes(raw);

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I meant it doesn't make sense to make nonce length depends on target pointer size

Expand All @@ -141,17 +142,23 @@ fn replace_csp_nonce(
nonce.to_string()
});

if !(nonces.is_empty() && hashes.is_empty()) {
let nonce_sources = nonces
.into_iter()
.map(|n| format!("'nonce-{n}'"))
.collect::<Vec<String>>();
if !nonces.is_empty() || !hashes.is_empty() {
let sources = csp.entry(directive.into()).or_default();
let self_source = "'self'".to_string();
if !sources.contains(&self_source) {
let self_source = "'self'";
if !sources.contains(self_source) {
sources.push(self_source);
}
sources.extend(nonce_sources);
#[cfg(target_pointer_width = "64")]
let mut buf = String::with_capacity(28);
#[cfg(target_pointer_width = "32")]
let mut buf = String::with_capacity(20);
#[cfg(target_pointer_width = "16")]
let mut buf = String::with_capacity(14);
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Are these capacities meant to be how many bytes to alloc to store 'nonce-{0..usize::MAX}'? If so, isn't it a bit oversized for 32 and 16 ptr widths? Feels like it should be 18 and 13 respectively, unless I'm missing something

If we want to be a bit fancy-pants (😛) we could use this helper function:

const fn nonce_source_capacity() -> usize {
    // 'nonce-' prefix + digits of usize::max + closing quote
    8 + usize::MAX.ilog10() as usize + 1
}

(You can test that by replacing usize for u64, u32 and u16 respectively)

for nonce in nonces {
buf.clear();
write!(&mut buf, "'nonce-{}'", nonce).unwrap();
sources.push(buf.clone());
}
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Reusing buf doesn't seem helpful in this scenario, if we'll clone it anyway. It means that the code will allocate nonces.len()+1 times

for nonce in nonces {
      let mut buf = String::with_capacity(nonce_capacity());
      write!(&mut buf, "'nonce-{}'", nonce).unwrap();
      sources.push(buf);
}

Something like that ^ would allocate n times rather than n+1 times

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I honestly doubt these optimizations will actually make any differences, format! Itself is quite optimized to estimate the buffer size already

sources.extend(hashes);
}
}
Expand Down
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