gitks/rate_limit.rs

//! Repository-level rate limiting via per-repo semaphores.
//!
//! Prevents any single repository from consuming all server resources.
//! Uses `tokio::sync::Semaphore` with configurable max concurrent operations.
//!
//! Integration pattern:
//!   let _guard = rate_limit::acquire(svc, header).await?;
//!   // ... handle request ...
//!   // guard is dropped here → permit released

use dashmap::DashMap;
use std::sync::{Arc, OnceLock, RwLock};
use tokio::sync::Semaphore;

/// Default max concurrent operations per repository.
const DEFAULT_MAX_CONCURRENT: usize = 5;

/// Global rate limiter state.
struct RateLimiter {
    /// Per-repository semaphores. Key = repository relative_path.
    semaphores: DashMap<String, Arc<Semaphore>>,
    /// Max concurrent operations per repository (protected by RwLock for runtime updates).
    max_concurrent: RwLock<usize>,
}

static RATE_LIMITER: OnceLock<RateLimiter> = OnceLock::new();

fn limiter() -> &'static RateLimiter {
    RATE_LIMITER.get_or_init(|| {
        let max = std::env::var("GITKS_RATE_LIMIT_MAX_CONCURRENT")
            .ok()
            .and_then(|v| v.parse().ok())
            .unwrap_or(DEFAULT_MAX_CONCURRENT);

        tracing::info!(
            max_concurrent = max,
            "repository-level rate limiter initialized"
        );

        RateLimiter {
            semaphores: DashMap::new(),
            max_concurrent: RwLock::new(max),
        }
    })
}

/// Get the current max_concurrent value.
fn get_max_concurrent() -> usize {
    *limiter()
        .max_concurrent
        .read()
        .unwrap_or_else(|e| e.into_inner())
}

/// A guard that holds a rate-limit permit. The permit is released on drop.
pub struct RateLimitGuard {
    /// The semaphore permit. Dropping this releases the permit.
    _permit: tokio::sync::OwnedSemaphorePermit,
}

/// Acquire a rate-limit permit for the given repository.
///
/// If the repository has `max_concurrent` operations already in flight,
/// this will wait asynchronously until a permit becomes available.
///
/// Returns `None` if no repository header is provided (e.g., global health checks).
pub async fn acquire(repo_relative_path: Option<&str>) -> Option<RateLimitGuard> {
    let repo = repo_relative_path?;
    if repo.is_empty() {
        return None;
    }
    let max_concurrent = get_max_concurrent();
    if max_concurrent == 0 {
        // Unlimited
        return None;
    }

    let sem = limiter()
        .semaphores
        .entry(repo.to_string())
        .or_insert_with(|| Arc::new(Semaphore::new(max_concurrent)))
        .value()
        .clone();

    // Release DashMap reference before awaiting
    let _ = repo;

    match tokio::time::timeout(
        std::time::Duration::from_secs(30),
        sem.clone().acquire_owned(),
    )
    .await
    {
        Ok(Ok(permit)) => {
            tracing::debug!(
                repo = %repo_relative_path.unwrap_or(""),
                available = sem.available_permits(),
                "rate limit permit acquired"
            );
            Some(RateLimitGuard { _permit: permit })
        }
        Ok(Err(_closed)) => {
            // Semaphore was closed — recreate it
            tracing::warn!(
                repo = %repo_relative_path.unwrap_or(""),
                "rate limit semaphore closed, recreating"
            );
            let new_sem = Arc::new(Semaphore::new(get_max_concurrent()));
            let permit = new_sem
                .clone()
                .acquire_owned()
                .await
                .expect("newly created semaphore should have permits");
            limiter()
                .semaphores
                .insert(repo_relative_path.unwrap_or("").to_string(), new_sem);
            Some(RateLimitGuard { _permit: permit })
        }
        Err(_elapsed) => {
            tracing::warn!(
                repo = %repo_relative_path.unwrap_or(""),
                max_concurrent = get_max_concurrent(),
                "rate limit timeout waiting for permit"
            );
            None
        }
    }
}

/// Acquire a rate-limit permit, returning a tonic error on timeout / overload.
pub async fn acquire_or_reject(
    repo_relative_path: Option<&str>,
) -> Result<Option<RateLimitGuard>, tonic::Status> {
    let repo = repo_relative_path.unwrap_or("");
    if repo.is_empty() {
        return Ok(None);
    }
    match acquire(Some(repo)).await {
        Some(guard) => Ok(Some(guard)),
        None => {
            if get_max_concurrent() == 0 {
                return Ok(None);
            }
            // Timeout — reject with resource exhausted
            Err(tonic::Status::resource_exhausted(format!(
                "rate limit exceeded for repository '{repo}': max {max} concurrent operations",
                max = get_max_concurrent()
            )))
        }
    }
}

/// Remove the semaphore for a repository (called on repo deletion).
pub fn remove_repository(repo_relative_path: &str) {
    limiter().semaphores.remove(repo_relative_path);
    tracing::debug!(repo = %repo_relative_path, "rate limit semaphore removed");
}

/// Update the max concurrent limit at runtime.
///
/// Only replaces semaphores that have no active permits (idle repos).
/// Semaphores with in-flight operations are left untouched to avoid
/// a race where active permits are held against a now-replaced semaphore
/// while the new one grants a full set of permits — leading to over-admission.
/// Those repos will pick up the new limit once all permits are returned.
pub fn set_max_concurrent(max: usize) {
    let l = limiter();

    let old_max = get_max_concurrent();

    match l.max_concurrent.write() {
        Ok(mut guard) => {
            *guard = max;
        }
        Err(e) => {
            let mut guard = e.into_inner();
            *guard = max;
        }
    }

    let keys: Vec<String> = l
        .semaphores
        .iter()
        .filter_map(|entry| {
            let sem = entry.value();
            if sem.available_permits() == old_max {
                Some(entry.key().clone())
            } else {
                None
            }
        })
        .collect();

    for key in keys {
        l.semaphores
            .insert(key, Arc::new(Semaphore::new(max)));
    }

    tracing::info!(max_concurrent = max, "rate limit max_concurrent updated");
}