A password check holds its turn until it finishes (#324)

check_password took a permit from CHECKS in the async function and
ran Argon2 in spawn_blocking. A client that hung up dropped the
future and with it the permit, while the blocking check ran on, so a
flood of sign-ins that disconnected at once ran unbounded Argon2 work,
the very thing CHECKS was added for (#137).

The permit is now owned (CHECKS is an Arc) and moves into the blocking
closure, so it is freed when the check ends, whoever stopped waiting.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/ipx/CHANGELOG.md

@@ -39,6 +39,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - An open page stops getting live updates within half a minute of its account being removed. They went on until the page was closed (#335).
 - Only someone subscribed to a feed, or an admin, may mark it private. Anyone signed in could keep anyone's feed, or a whole OPML's, out of the directory, and only an admin could undo it (#333).
 - An OPML upload may add up to 2,000 feeds; one with more is refused and adds none. Importing tens of thousands held up every change to the feed list for minutes and left them all in it (#338).
+- Password checks stay a few at a time when the person signing in gives up before the answer. Each sign-in that hung up let another start, so a flood of them used every core (#324).
 
 ## [1.4.0] - 2026-10-19
 

crates/ipx/src/auth.rs

@@ -31,8 +31,10 @@ pub fn verify_password(password: &str, stored: &str) -> bool {
 /// How many password checks run at once: each is tens of milliseconds of CPU, and forty wrong
 /// passwords at a time, run on the async workers, made every other request wait 4s (#137).
 /// Half the cores, so a flood of sign-ins waits on itself and the rest of the server has the rest.
-static CHECKS: std::sync::LazyLock<tokio::sync::Semaphore> = std::sync::LazyLock::new(|| {
-    tokio::sync::Semaphore::new(std::thread::available_parallelism().map_or(1, |n| (n.get() / 2).max(1)))
+static CHECKS: std::sync::LazyLock<std::sync::Arc<tokio::sync::Semaphore>> = std::sync::LazyLock::new(|| {
+    std::sync::Arc::new(tokio::sync::Semaphore::new(
+        std::thread::available_parallelism().map_or(1, |n| (n.get() / 2).max(1)),
+    ))
 });
 
 /// What a name that is not an account is checked against, so that refusing it takes as long as
@@ -45,13 +47,19 @@ static DECOY: std::sync::LazyLock<String> =
 /// or it has no password: false either way, in the same time. Off the async workers, and a few at
 /// a time; see `CHECKS`.
 pub async fn check_password(password: String, stored: Option<String>) -> bool {
-    let Ok(_turn) = CHECKS.acquire().await else { return false };
-    tokio::task::spawn_blocking(move || match stored {
+    let Ok(turn) = CHECKS.clone().acquire_owned().await else { return false };
+    // The turn goes with the check, not with this future: a client that hung up dropped the
+    // future, freeing the turn while the check ran on, so disconnecting sign-ins ran any number
+    // at once (#324).
+    tokio::task::spawn_blocking(move || {
+        let _turn = turn;
+        match stored {
             Some(h) => verify_password(&password, &h),
             None => {
                 verify_password(&password, &DECOY);
                 false
             }
+        }
     })
     .await
     .unwrap_or(false)