From c09ff210125b0ebecbe9d74bba2ae401322e32ef Mon Sep 17 00:00:00 2001
From: Sebastian Leisinger <leisinger@anapaya.net>
Date: Wed, 3 Dec 2025 10:30:46 +0100
Subject: [PATCH] Fix dropping oversized unblocks datagram senders

When dropping oversized datagrams after an MTU change, notify
callers of `send_datagram_wait` that are waiting for
space in the send buffer.

Tracking issue: https://github.com/quinn-rs/quinn/issues/2456
---
 quinn-proto/src/connection/datagrams.rs |  7 ++-
 quinn-proto/src/connection/mod.rs       |  7 ++-
 quinn-proto/src/tests/mod.rs            | 81 +++++++++++++++++++++++++
 3 files changed, 93 insertions(+), 2 deletions(-)

diff --git a/quinn-proto/src/connection/datagrams.rs b/quinn-proto/src/connection/datagrams.rs
index ccb0cb3ba..3b5e027f9 100644
--- a/quinn-proto/src/connection/datagrams.rs
+++ b/quinn-proto/src/connection/datagrams.rs
@@ -142,9 +142,12 @@ impl DatagramState {
 
     /// Discard outgoing datagrams with a payload larger than `max_payload` bytes
     ///
+    /// Returns whether any datagrams were dropped.
+    ///
     /// Used to ensure that reductions in MTU don't get us stuck in a state where we have a datagram
     /// queued but can't send it.
-    pub(super) fn drop_oversized(&mut self, max_payload: usize) {
+    pub(super) fn drop_oversized(&mut self, max_payload: usize) -> bool {
+        let mut dropped_any = false;
         self.outgoing.retain(|datagram| {
             let result = datagram.data.len() < max_payload;
             if !result {
@@ -154,9 +157,11 @@ impl DatagramState {
                     max_payload
                 );
                 self.outgoing_total -= datagram.data.len();
+                dropped_any = true;
             }
             result
         });
+        dropped_any
     }
 
     /// Attempt to write a datagram frame into `buf`, consuming it from `self.outgoing`
diff --git a/quinn-proto/src/connection/mod.rs b/quinn-proto/src/connection/mod.rs
index 235a63c0a..eff3db542 100644
--- a/quinn-proto/src/connection/mod.rs
+++ b/quinn-proto/src/connection/mod.rs
@@ -1821,7 +1821,12 @@ impl Connection {
                     .congestion
                     .on_mtu_update(self.path.mtud.current_mtu());
                 if let Some(max_datagram_size) = self.datagrams().max_size() {
-                    self.datagrams.drop_oversized(max_datagram_size);
+                    if self.datagrams.drop_oversized(max_datagram_size)
+                        && self.datagrams.send_blocked
+                    {
+                        self.datagrams.send_blocked = false;
+                        self.events.push_back(Event::DatagramsUnblocked);
+                    }
                 }
             }
 
diff --git a/quinn-proto/src/tests/mod.rs b/quinn-proto/src/tests/mod.rs
index 566957c06..74a64b38e 100644
--- a/quinn-proto/src/tests/mod.rs
+++ b/quinn-proto/src/tests/mod.rs
@@ -3379,6 +3379,87 @@ fn voluntary_ack_with_large_datagrams() {
     );
 }
 
+/// Verify that dropping oversized datagrams will trigger a DatagramsUnblocked event.
+#[test]
+fn oversized_datagrams_trigger_unblock() {
+    let _guard = subscribe();
+    let mut pair = Pair::default();
+    // Start the connection with a large MTU.
+    const INITIAL_MTU: usize = 1300;
+    pair.mtu = INITIAL_MTU;
+
+    let mut client_config = client_config();
+    let mut transport_config = TransportConfig::default();
+    let send_buffer_size = transport_config.datagram_send_buffer_size;
+    transport_config.initial_mtu(INITIAL_MTU as u16);
+    client_config.transport_config(transport_config.into());
+
+    let (client_ch, _) = pair.connect_with(client_config);
+
+    // Send datagrams until the send buffer is full.
+    let max_size = pair.client_datagrams(client_ch).max_size().unwrap();
+    let data = vec![0; max_size];
+    loop {
+        match pair
+            .client_datagrams(client_ch)
+            .send(data.clone().into(), false)
+        {
+            Ok(_) => {}
+            Err(SendDatagramError::Blocked(_)) => {
+                break;
+            }
+            Err(e) => panic!("unexpected error: {e}"),
+        }
+    }
+    // Set the MTU to a smaller value so the queued datagrams cannot be sent.
+    pair.mtu = 1200;
+
+    // Drive the pair until black hole detection kicks in and the path MTU is adjusted.
+    while pair.step() {
+        let err = loop {
+            if let Err(e) = pair
+                .client_datagrams(client_ch)
+                .send(data.clone().into(), false)
+            {
+                break e;
+            }
+        };
+        match err {
+            SendDatagramError::Blocked(_) => {
+                // continue with the next step but drain the DatagramsUnblocked events
+                // emitted datagrams were sent out.
+                while let Some(event) = pair.client_conn_mut(client_ch).poll() {
+                    tracing::info!("ignoring connection event: {event:?}");
+                }
+            }
+            SendDatagramError::TooLarge => {
+                // mtu adjusted, break the loop
+                break;
+            }
+            _ => panic!("unexpected error: {err}"),
+        }
+    }
+
+    assert_eq!(
+        pair.client_conn_mut(client_ch)
+            .stats()
+            .path
+            .black_holes_detected,
+        1,
+        "expected a black hole to have been detected",
+    );
+
+    assert_eq!(
+        pair.client_datagrams(client_ch).send_buffer_space(),
+        send_buffer_size,
+        "expected the send buffer to be empty after too large datagrams were dropped",
+    );
+    match pair.client_conn_mut(client_ch).poll() {
+        Some(Event::DatagramsUnblocked) => {}
+        _ => panic!("expected DatagramsUnblocked event"),
+    }
+}
+
 #[test]
 fn reject_short_idcid() {
     let _guard = subscribe();