LibWeb: Include empty header values when joining duplicated headers

trflynn89 · trflynn89 · commit cbfae97101ed · 2025-11-26T21:22:35.000-05:00
Fixes a regression from commit: f675cfe It is not sufficient to only check if the builder is empty, as we will then drop empty header values (when the first found value is empty). This is tested in WPT by /cors/origin.htm, but that requires an HTTP server.
diff --git a/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Headers.cpp b/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Headers.cpp
@@ -86,14 +86,17 @@ Optional<ByteString> HeaderList::get(StringView name) const
     // 2. Return the values of all headers in list whose name is a byte-case-insensitive match for name, separated from
     //    each other by 0x2C 0x20, in order.
     StringBuilder builder;
+    bool first = true;
 
     for (auto const& header : *this) {
         if (!header.name.equals_ignoring_ascii_case(name))
             continue;
 
-        if (!builder.is_empty())
+        if (!first)
             builder.append(", "sv);
+
         builder.append(header.value);
+        first = false;
     }
 
     return builder.to_byte_string();
diff --git a/Tests/LibWeb/Text/expected/Fetch/empty-header-value.txt b/Tests/LibWeb/Text/expected/Fetch/empty-header-value.txt
@@ -0,0 +1,4 @@
+X-Foo: ""
+X-Foo: ", a"
+X-Foo: "a"
+X-Foo: "a, "
diff --git a/Tests/LibWeb/Text/input/Fetch/empty-header-value.html b/Tests/LibWeb/Text/input/Fetch/empty-header-value.html
@@ -0,0 +1,20 @@
+<!doctype html>
+<script src="../include.js"></script>
+<script type="text/javascript">
+    test(() => {
+        {
+            let headers = new Headers({ "X-Foo": "" });
+            println(`X-Foo: "${headers.get("X-Foo")}"`);
+
+            headers.append("X-Foo", "a");
+            println(`X-Foo: "${headers.get("X-Foo")}"`);
+        }
+        {
+            let headers = new Headers({ "X-Foo": "a" });
+            println(`X-Foo: "${headers.get("X-Foo")}"`);
+
+            headers.append("X-Foo", "");
+            println(`X-Foo: "${headers.get("X-Foo")}"`);
+        }
+    });
+</script>
