From 74d28c030d9a37e37e68985db53d68ab2ce56fd0 Mon Sep 17 00:00:00 2001
From: Mohammad Nejati <ashtumashtum@gmail.com>
Date: Thu, 6 Jun 2024 09:57:07 +0000
Subject: [PATCH] Partial writes don't cause bytes_transferred underflow

Fixes #2880
---
 include/boost/beast/websocket/impl/write.hpp |  9 ++++--
 test/beast/websocket/write.cpp               | 29 ++++++++++++++++++++
 2 files changed, 35 insertions(+), 3 deletions(-)

diff --git a/include/boost/beast/websocket/impl/write.hpp b/include/boost/beast/websocket/impl/write.hpp
index 6e75be79..493572e4 100644
--- a/include/boost/beast/websocket/impl/write.hpp
+++ b/include/boost/beast/websocket/impl/write.hpp
@@ -346,8 +346,8 @@ operator()(
                         beast::detail::bind_continuation(std::move(*this)));
             }
             // VFALCO What about consuming the buffer on error?
-            bytes_transferred_ +=
-                bytes_transferred - impl.wr_fb.size();
+            if(bytes_transferred > impl.wr_fb.size())
+                bytes_transferred_ += bytes_transferred - impl.wr_fb.size();
             if(impl.check_stop_now(ec))
                 goto upcall;
             while(remain_ > 0)
@@ -431,7 +431,10 @@ operator()(
                             ),
                             beast::detail::bind_continuation(std::move(*this)));
                 }
-                n = bytes_transferred - impl.wr_fb.size();
+                if(bytes_transferred > impl.wr_fb.size())
+                    n = bytes_transferred - impl.wr_fb.size();
+                else
+                    n = 0;
                 bytes_transferred_ += n;
                 if(impl.check_stop_now(ec))
                     goto upcall;
diff --git a/test/beast/websocket/write.cpp b/test/beast/websocket/write.cpp
index ebbc9191..84978969 100644
--- a/test/beast/websocket/write.cpp
+++ b/test/beast/websocket/write.cpp
@@ -806,6 +806,34 @@ public:
         BEAST_EXPECT(n1 < n0 + s.size());
     }
 
+    void
+    testIssue2880()
+    {
+        for(auto fragment : {false, true})
+        {
+            net::io_context ioc;
+            stream<test::stream> wsc{ioc};
+            stream<test::stream> wss{ioc};
+            wsc.next_layer().connect(wss.next_layer());
+            wsc.async_handshake(
+                "localhost", "/", [](error_code){});
+            wss.async_accept([](error_code){});
+            ioc.run();
+            ioc.restart();
+            // limit the write size to be less than the header buffer
+            wsc.next_layer().write_size(3);
+            if(fragment)
+                wsc.write_buffer_bytes(8);
+            wsc.async_write(sbuf("*********"),
+                [&](error_code ec, std::size_t n)
+                {
+                    BEAST_EXPECTS(ec, ec.message());
+                    BEAST_EXPECT(n == 0);
+                });
+            wsc.next_layer().close();
+            ioc.run();
+        }
+    }
 
 #if BOOST_ASIO_HAS_CO_AWAIT
     void testAwaitableCompiles(
@@ -835,6 +863,7 @@ public:
         testIssue227();
         testIssue300();
         testIssue1666();
+        testIssue2880();
 #if BOOST_ASIO_HAS_CO_AWAIT
         boost::ignore_unused(&write_test::testAwaitableCompiles);
 #endif