From 46801ec92610171092de007050fa7aab59d712b9 Mon Sep 17 00:00:00 2001
From: Jack Kleeman <jackkleeman@gmail.com>
Date: Tue, 6 Jan 2026 19:37:50 +0000
Subject: [PATCH] Send http2 window resize frames after half the window is used
 up (#25847)

### What does this PR do?
It is standard practice to send HTTP2 window resize frames once half the
window is used up:
- [nghttp2](https://en.wikipedia.org/wiki/Nghttp2#HTTP/2_implementation)
- [Go](https://github.com/golang/net/blob/master/http2/flow.go#L31)
- [Rust
h2](https://github.com/hyperium/h2/blob/master/src/proto/streams/flow_control.rs#L17)

The current behaviour of Bun is to send a window update once the client
has sent 65535 bytes exactly. This leads to an interruption in
throughput while the window update is received.

This is not just a performance concern however, I think some clients
will not handle this well, as it means that if you stop sending data
even 1 byte before 65535, you have a deadlock. The reason I came across
this issue is that it appears that the Rust `hyper` crate always
reserves an additional 1 byte from the connection for each http2 stream
(https://github.com/hyperium/hyper/blob/master/src/proto/h2/mod.rs#L122).
This means that when you have two concurrent requests, the client treats
it like the window is exhausted when it actually has a byte remaining,
leading to a sequential dependency between the requests that can create
deadlocks if they depend on running concurrently. I accept this is not a
problem with bun, but its a happy accident that we can resolve such
off-by-one issues by increasing the window size once it is 50% utilized

### How did you verify your code works?
Using wireshark, bun debug logging, and client logging I can observe
that the window updates are now sent after 32767 bytes. This also
resolves the h2 crate client issue.
---
 src/bun.js/api/bun/h2_frame_parser.zig | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/src/bun.js/api/bun/h2_frame_parser.zig b/src/bun.js/api/bun/h2_frame_parser.zig
index de5f88a2a9..37c253b40e 100644
--- a/src/bun.js/api/bun/h2_frame_parser.zig
+++ b/src/bun.js/api/bun/h2_frame_parser.zig
@@ -1205,16 +1205,18 @@ pub const H2FrameParser = struct {
         var it = this.streams.valueIterator();
         while (it.next()) |stream| {
             log("incrementWindowSizeIfNeeded stream {} {} {} {}", .{ stream.id, stream.usedWindowSize, stream.windowSize, this.isServer });
-            if (stream.usedWindowSize >= stream.windowSize) {
+            if (stream.usedWindowSize >= stream.windowSize / 2 and stream.usedWindowSize > 0) {
+                const consumed = stream.usedWindowSize;
                 stream.usedWindowSize = 0;
                 log("incrementWindowSizeIfNeeded stream {} {} {}", .{ stream.id, stream.windowSize, this.isServer });
-                this.sendWindowUpdate(stream.id, UInt31WithReserved.init(@truncate(stream.windowSize), false));
+                this.sendWindowUpdate(stream.id, UInt31WithReserved.init(@truncate(consumed), false));
             }
         }
         log("incrementWindowSizeIfNeeded connection {} {} {}", .{ this.usedWindowSize, this.windowSize, this.isServer });
-        if (this.usedWindowSize >= this.windowSize) {
+        if (this.usedWindowSize >= this.windowSize / 2 and this.usedWindowSize > 0) {
+            const consumed = this.usedWindowSize;
             this.usedWindowSize = 0;
-            this.sendWindowUpdate(0, UInt31WithReserved.init(@truncate(this.windowSize), false));
+            this.sendWindowUpdate(0, UInt31WithReserved.init(@truncate(consumed), false));
         }
     }