server: set all four http.Server socket timeouts (#118)

The http.Server was built with only ReadHeaderTimeout set; the other three
timeouts were zero, which in net/http means no limit, so a peer could hold a
connection open past the header phase, responses had no write deadline, and
keep-alive connections were never reaped. ReadTimeout 15s, WriteTimeout 75s,
and IdleTimeout 120s now join ReadHeaderTimeout 10s as named constants.
WriteTimeout must stay above the 60s chimw.Timeout handler budget, because
net/http arms the write deadline once request headers are read; a test fails
the build if either number moves alone. The server literal moved into
newHTTPServer so the configuration can be asserted without binding a socket
(closes #99)

Model: opus-5

internal/server/server.go

@@ -33,8 +33,52 @@ type Params struct {
 // shutdownTimeout is how long to wait for graceful shutdown.
 const shutdownTimeout = 30 * time.Second
 
-// readHeaderTimeout is the max duration for reading request headers.
-const readHeaderTimeout = 10 * time.Second
+// Socket-level timeouts for the HTTP server.
+//
+// These bound time spent on the connection itself and are a distinct
+// control from the per-request handler budget enforced by
+// chimw.Timeout(requestTimeout) in routes.go: that one cancels the
+// request context after requestTimeout but never touches the socket,
+// so without the values below a peer can hold a connection open
+// forever (slowloris, unreaped keep-alives).
+//
+// The one hard constraint between the two controls is
+// writeTimeout > requestTimeout. net/http arms the write deadline
+// once the request headers have been read, so on a plaintext
+// connection it covers handler execution AND the response flush. If
+// writeTimeout were <= requestTimeout the server would sever the
+// connection before a handler that legitimately consumed its full
+// budget could emit anything, making the 60s budget unreachable in
+// practice. The margin between them is the response-flush allowance.
+//
+// The only clients of this service are browsers loading the dashboard
+// and a Prometheus scraper; the values are sized for those.
+const (
+	// readHeaderTimeout is the max duration for reading request
+	// headers.
+	readHeaderTimeout = 10 * time.Second
+
+	// readTimeout bounds reading the entire request, headers plus
+	// body. Every route here is a GET with no body, so this only
+	// ever needs to cover headers; the extra 5s over
+	// readHeaderTimeout is slack, not a real allowance, and keeps a
+	// body dribbled one byte at a time from holding the read side
+	// open indefinitely.
+	readTimeout = 15 * time.Second
+
+	// writeTimeout must exceed the requestTimeout handler budget
+	// (60s) per the note above. The 15s difference is the allowance
+	// for flushing a completed response to a slow client.
+	writeTimeout = 75 * time.Second
+
+	// idleTimeout reaps keep-alive connections between requests. It
+	// is deliberately longer than the common Prometheus scrape
+	// intervals (15s/30s/60s) so the scraper reuses its connection
+	// rather than reconnecting every cycle, while a browser tab
+	// left open on the dashboard stops occupying a connection
+	// within two minutes of going quiet.
+	idleTimeout = 120 * time.Second
+)
 
 // Server is the HTTP server.
 type Server struct {
@@ -76,16 +120,29 @@ func New(
 	return srv, nil
 }
 
+// newHTTPServer builds the listening http.Server with every
+// socket-level timeout set. All four are set deliberately: a zero
+// value in net/http means "no limit", not "some default".
+func newHTTPServer(
+	listenAddr string,
+	handler http.Handler,
+) *http.Server {
+	return &http.Server{
+		Addr:              listenAddr,
+		Handler:           handler,
+		ReadTimeout:       readTimeout,
+		ReadHeaderTimeout: readHeaderTimeout,
+		WriteTimeout:      writeTimeout,
+		IdleTimeout:       idleTimeout,
+	}
+}
+
 // Run starts the HTTP server.
 func (s *Server) Run() {
 	s.SetupRoutes()
 
 	listenAddr := fmt.Sprintf(":%d", s.port)
-	s.httpServer = &http.Server{
-		Addr:              listenAddr,
-		Handler:           s,
-		ReadHeaderTimeout: readHeaderTimeout,
-	}
+	s.httpServer = newHTTPServer(listenAddr, s)
 
 	s.log.Info("http server starting", "addr", listenAddr)