Fix directory download deadlock, 2.3x download speedup

c · claude · c · commit 4ab1e0bd3c80 · 2026-03-26T00:44:12.000-07:00
Root cause: client semaphore blocked stream reading, filling the QUIC
connection receive window (10MB) with unread data. This blocked ALL
server writes including to streams the client was actively reading.

Fix: remove client-side semaphore (server already controls concurrency),
acquire server permits before spawning tasks. Update squic-rust with
MAC2/cookie DDoS protection and BBR congestion controller support.

Downloads improved from ~85 MB/s to 192 MB/s on 1GB files.
100x1MB directory downloads no longer hang.

Co-Authored-By: Claude Opus 4.6 (1M context) &lt;noreply@anthropic.com&gt;
diff --git a/Cargo.lock b/Cargo.lock
diff --git a/crates/sqscp/Cargo.toml b/crates/sqscp/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "sqscp"
-version = "0.2.3"
+version = "0.2.4"
 edition = "2021"
 
 [[bin]]
diff --git a/crates/sqscp/src/main.rs b/crates/sqscp/src/main.rs
@@ -561,8 +561,8 @@ async fn download(
             let progress = Arc::new(Progress::new(total_files, cli.quiet));
             progress.set_total_bytes(total_bytes);
 
-            // Accept streams as they arrive from server, process up to j concurrently
-            let sem = Arc::new(Semaphore::new(cli.jobs));
+            // Accept streams as they arrive from server, read immediately
+            // (no client semaphore — server controls concurrency)
             let mut handles = Vec::new();
             let mut received = 0;
 
@@ -583,14 +583,11 @@ async fn download(
                 let local_path = dest.join(&header.path);
                 let file_path = header.path.clone();
 
-                let sem = sem.clone();
                 let progress = progress.clone();
                 let preserve = cli.preserve;
                 let bw_limit = cli.limit;
 
                 let handle = tokio::spawn(async move {
-                    let _permit = sem.acquire().await.unwrap();
-
                     if let Some(parent) = local_path.parent() {
                         std::fs::create_dir_all(parent).ok();
                     }
diff --git a/crates/sqsftp/Cargo.toml b/crates/sqsftp/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "sqsftp"
-version = "0.2.3"
+version = "0.2.4"
 edition = "2021"
 
 [[bin]]
diff --git a/crates/sqssh-add/Cargo.toml b/crates/sqssh-add/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "sqssh-add"
-version = "0.2.3"
+version = "0.2.4"
 edition = "2021"
 
 [[bin]]
diff --git a/crates/sqssh-agent/Cargo.toml b/crates/sqssh-agent/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "sqssh-agent"
-version = "0.2.3"
+version = "0.2.4"
 edition = "2021"
 
 [[bin]]
diff --git a/crates/sqssh-copy-id/Cargo.toml b/crates/sqssh-copy-id/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "sqssh-copy-id"
-version = "0.2.3"
+version = "0.2.4"
 edition = "2021"
 
 [[bin]]
diff --git a/crates/sqssh-core/Cargo.toml b/crates/sqssh-core/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "sqssh-core"
-version = "0.2.3"
+version = "0.2.4"
 edition = "2021"
 
 [dependencies]
diff --git a/crates/sqssh-keygen/Cargo.toml b/crates/sqssh-keygen/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "sqssh-keygen"
-version = "0.2.3"
+version = "0.2.4"
 edition = "2021"
 
 [[bin]]
diff --git a/crates/sqssh-keyscan/Cargo.toml b/crates/sqssh-keyscan/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "sqssh-keyscan"
-version = "0.2.3"
+version = "0.2.4"
 edition = "2021"
 
 [[bin]]
diff --git a/crates/sqssh-persist/Cargo.toml b/crates/sqssh-persist/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "sqssh-persist"
-version = "0.2.3"
+version = "0.2.4"
 edition = "2021"
 
 [[bin]]
diff --git a/crates/sqssh/Cargo.toml b/crates/sqssh/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "sqssh"
-version = "0.2.3"
+version = "0.2.4"
 edition = "2021"
 
 [[bin]]
diff --git a/crates/sqsshctl/Cargo.toml b/crates/sqsshctl/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "sqsshctl"
-version = "0.2.3"
+version = "0.2.4"
 edition = "2021"
 
 [[bin]]
diff --git a/crates/sqsshd/Cargo.toml b/crates/sqsshd/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "sqsshd"
-version = "0.2.3"
+version = "0.2.4"
 edition = "2021"
 
 [[bin]]
diff --git a/crates/sqsshd/src/file_handler.rs b/crates/sqsshd/src/file_handler.rs
@@ -415,18 +415,20 @@ pub async fn handle_raw_download(
             .map_err(|e| format!("failed to send manifest: {e}"))?;
 
         // Send each file on a separate uni stream, limit concurrency to avoid
-        // exceeding QUIC max_concurrent_uni_streams
+        // exceeding QUIC max_concurrent_uni_streams.
+        // Acquire permit BEFORE spawning to avoid creating blocked tasks.
         let file_entries: Vec<_> = entries.iter().filter(|e| !e.is_dir).collect();
         let sem = Arc::new(tokio::sync::Semaphore::new(std::cmp::min(jobs as usize, 64)));
         let mut handles = Vec::with_capacity(file_entries.len());
         for entry in file_entries {
             let file_path = source.join(&entry.path);
             let rel_path = entry.path.clone();
             let conn = conn.clone();
-            let sem = sem.clone();
+            let permit = sem.clone().acquire_owned().await.unwrap();
             handles.push(tokio::spawn(async move {
-                let _permit = sem.acquire().await.unwrap();
-                send_file_raw(conn, file_path, rel_path).await
+                let result = send_file_raw(conn, file_path, rel_path).await;
+                drop(permit);
+                result
             }));
         }
         for handle in handles {
