fix(server): resolve Content-Length header before rejecting closed bodies

A closed response body whose empty payload is declared only via the
Content-Length header (not the content_length field) was rejected as
invalid reuse, turning master's 200 into a 500. Fall back to parsing
the header when the field is unset.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: quinnj

src/http_server_streams.jl

@@ -452,10 +452,14 @@ function _write_response_body_to_stream!(stream::Stream, body)::Nothing
         return nothing
     end
     if body isa AbstractBody
-        if body_closed(body) &&
-           _server_stream_allows_body(stream) &&
-           stream.response.content_length != 0
-            throw(ArgumentError("body is closed"))
+        if body_closed(body) && _server_stream_allows_body(stream)
+            # The handler may declare an empty payload via the Content-Length
+            # header alone; the content_length field stays -1 until startwrite
+            # resolves it, so consult the header before rejecting the body.
+            declared = stream.response.content_length >= 0 ?
+                stream.response.content_length :
+                _parse_content_length(stream.response.headers)
+            declared == 0 || throw(ArgumentError("body is closed"))
         end
         buf = Vector{UInt8}(undef, 16 * 1024)
         try

test/http_handlers_tests.jl

@@ -220,6 +220,22 @@ end
     end
 end
 
+@testset "HTTP streamhandler ignores closed body with Content-Length: 0 header" begin
+    body = HT.CallbackBody(_ -> 0, () -> nothing)
+    HT.body_close!(body)
+    baked = HT.Response(200, ["Content-Length" => "0"], body)
+    server = HT.listen!(HT.streamhandler(_ -> baked), "127.0.0.1", 0; listenany = true)
+    address = HT.server_addr(server)
+    try
+        resp = HT.get("http://$(address)/"; status_exception = false, retry = false)
+        @test resp.status == 200
+        @test isempty(_read_all_handler_bytes(resp.body))
+    finally
+        HT.forceclose(server)
+        wait(server)
+    end
+end
+
 @testset "HTTP router live request handler server" begin
     router = HT.Router()
     HT.register!(router, "GET", "/hello/{name}", _router_hello_request)