From 67bde14d037987a5bafb787b46765b86a6ac284c Mon Sep 17 00:00:00 2001
From: freedom-winds <rodriguezedward6mbmq@gmail.com>
Date: Mon, 17 Aug 2026 00:21:08 +0800
Subject: [PATCH] fix(ffi): preserve header order when replacing values

---
 src/ext/mod.rs        | 19 +++++++++-----
 src/ffi/http_types.rs | 61 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 73 insertions(+), 7 deletions(-)

diff --git a/src/ext/mod.rs b/src/ext/mod.rs
index 0fccb5ffec..f7b328f80b 100644
--- a/src/ext/mod.rs
+++ b/src/ext/mod.rs
@@ -223,14 +223,19 @@ impl OriginalHeaderOrder {
     }
 
     pub(crate) fn insert(&mut self, name: HeaderName) {
-        if !self.num_entries.contains_key(&name) {
-            let idx = 0;
-            self.num_entries.insert(name.clone(), 1);
-            self.entry_order.push((name, idx));
+        self.num_entries.insert(name.clone(), 1);
+
+        if let Some(first_position) = self
+            .entry_order
+            .iter()
+            .position(|(entry_name, _)| entry_name == &name)
+        {
+            self.entry_order
+                .retain(|(entry_name, _)| entry_name != &name);
+            self.entry_order.insert(first_position, (name, 0));
+        } else {
+            self.entry_order.push((name, 0));
         }
-        // Replacing an already existing element does not
-        // change ordering, so we only care if its the first
-        // header name encountered
     }
 
     pub(crate) fn append<N>(&mut self, name: N)
diff --git a/src/ffi/http_types.rs b/src/ffi/http_types.rs
index a9c6cf6b0f..2044e037bd 100644
--- a/src/ffi/http_types.rs
+++ b/src/ffi/http_types.rs
@@ -704,4 +704,65 @@ mod tests {
             HYPER_ITER_CONTINUE
         }
     }
+    #[cfg(all(feature = "http1", feature = "ffi"))]
+    #[test]
+    fn test_headers_set_replaces_multi_value_header_in_original_order() {
+        let mut headers = hyper_headers::default();
+
+        add_header(&mut headers, b"Set-CookiE", b"a=b");
+        add_header(&mut headers, b"Content-Encoding", b"gzip");
+        add_header(&mut headers, b"SET-COOKIE", b"c=d");
+        add_header(&mut headers, b"X-After", b"present");
+
+        let name = b"Set-Cookie";
+        let value = b"replacement";
+        assert!(matches!(
+            hyper_headers_set(
+                &mut headers,
+                name.as_ptr(),
+                name.len(),
+                value.as_ptr(),
+                value.len(),
+            ),
+            hyper_code::HYPERE_OK
+        ));
+
+        let mut output = Vec::<u8>::new();
+        hyper_headers_foreach(&headers, concat, &mut output as *mut _ as *mut c_void);
+
+        assert_eq!(
+            output,
+            b"Set-Cookie: replacement\r\nContent-Encoding: gzip\r\nX-After: present\r\n"
+        );
+    }
+
+    fn add_header(headers: &mut hyper_headers, name: &[u8], value: &[u8]) {
+        assert!(matches!(
+            hyper_headers_add(
+                headers,
+                name.as_ptr(),
+                name.len(),
+                value.as_ptr(),
+                value.len(),
+            ),
+            hyper_code::HYPERE_OK
+        ));
+    }
+
+    extern "C" fn concat(
+        output: *mut c_void,
+        name: *const u8,
+        name_len: usize,
+        value: *const u8,
+        value_len: usize,
+    ) -> c_int {
+        unsafe {
+            let output = &mut *(output as *mut Vec<u8>);
+            output.extend(std::slice::from_raw_parts(name, name_len));
+            output.extend(b": ");
+            output.extend(std::slice::from_raw_parts(value, value_len));
+            output.extend(b"\r\n");
+        }
+        HYPER_ITER_CONTINUE
+    }
 }