From bc95264c432d952e83d316cf1935c352eefea6b7 Mon Sep 17 00:00:00 2001
From: Will Wang <willwang1@meta.com>
Date: Sat, 3 Oct 2026 05:49:21 -0700
Subject: [PATCH] Requeue a split body event without recounting its chain
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Summary:
Avoid rebuilding a `BufQueue` and walks the whole chain to count its bytes again. Calling the `.move()`

 Old code:
  1. `bodyBuf.move()` hands back a bare unique_ptr<IOBuf> and the calculated `chainLength_` is recalculated walking the chain again (this is unnecessary)

New code:
  1. std::move copies nothing on its own. It only marks bodyBuf as an rvalue BufQueue which then use the move constructor

Cowboy edge task T289680342: this walk is ≈ 96 % of the measured cost (≈ $60 K a year on edge hosts).

Reviewed By: hanidamlaj

Differential Revision: D122754751

fbshipit-source-id: fe5ee78f318332de6f1b842c786e8c30e4b80c3e
---
 proxygen/lib/http/coro/HTTPStreamSource.cpp   |  2 +-
 .../lib/http/coro/test/HTTPSourceTests.cpp    | 22 +++++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/proxygen/lib/http/coro/HTTPStreamSource.cpp b/proxygen/lib/http/coro/HTTPStreamSource.cpp
index 75baab326..a8681b343 100644
--- a/proxygen/lib/http/coro/HTTPStreamSource.cpp
+++ b/proxygen/lib/http/coro/HTTPStreamSource.cpp
@@ -267,7 +267,7 @@ folly::coro::Task<HTTPBodyEvent> HTTPStreamSource::readBodyEvent(uint32_t max) {
       // the queue. If this happens, EOM must be false.
       BufQueue bodyBuf(std::move(res.event.body));
       res.event.body = bodyBuf.splitAtMost(max);
-      bodyQueue_.emplace_front(bodyBuf.move(), res.eom);
+      bodyQueue_.emplace_front(std::move(bodyBuf), res.eom);
       res.eom = false;
       length = max;
     }
diff --git a/proxygen/lib/http/coro/test/HTTPSourceTests.cpp b/proxygen/lib/http/coro/test/HTTPSourceTests.cpp
index 33d744508..7166ce238 100644
--- a/proxygen/lib/http/coro/test/HTTPSourceTests.cpp
+++ b/proxygen/lib/http/coro/test/HTTPSourceTests.cpp
@@ -224,6 +224,28 @@ TEST_F(HTTPStreamSourceTest, Body) {
   EXPECT_EQ(bodyEvents_[4].eom, true);
 }
 
+// A body event larger than the read limit is split, and the rest goes back to
+// the front of the queue with its length.
+TEST_F(HTTPStreamSourceTest, SplitBodyKeepsTheRemainderLength) {
+  auto resp = std::make_unique<HTTPMessage>();
+  resp->setStatusCode(200);
+  stream_.headers(std::move(resp));
+  stream_.body(BufQueue(folly::IOBuf::copyBuffer("abcdef")), 0);
+  stream_.eom();
+
+  co_withExecutor(&evb_, drainSource(&stream_, 4)).start();
+  evb_.loopOnce();
+
+  EXPECT_FALSE(error_.hasValue());
+  ASSERT_GE(bodyEvents_.size(), 2);
+  EXPECT_EQ(bodyEvents_[0].event.body.chainLength(), 4);
+  EXPECT_FALSE(bodyEvents_[0].eom);
+  EXPECT_EQ(bodyEvents_[1].event.body.chainLength(), 2);
+  auto rest = bodyEvents_[1].event.body.move();
+  rest->coalesce();
+  EXPECT_EQ(rest->moveToFbString(), std::string("ef"));
+}
+
 TEST_F(HTTPStreamSourceTest, PushPromiseBeforeResponseHeaders) {
   auto promiseSource = HTTPFixedSource::makeFixedResponse(
       200, folly::IOBuf::copyBuffer("push body"));