net/http: make Server respect shutdown state after handler finishes

If the Server's Shutdown (or SetKeepAlivesEnabled) method was called
while a connection was in a Handler, but after the headers had been
written, the connection was not later closed.

Fixes #9478
Updates #17754 (reverts that workaround)

Change-Id: I65324ab8217373fbb38e12e2b8bffd0a91806072
Reviewed-on: https://go-review.googlesource.com/33141Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/http/serve_test.go

@@ -4943,9 +4943,6 @@ func TestServerSetKeepAlivesEnabledClosesConns(t *testing.T) {
 	}) {
 		t.Fatalf("idle count before SetKeepAlivesEnabled called = %v; want 1", idle0)
 	}
-	if !waitCondition(2*time.Second, 10*time.Millisecond, ts.Config.ExportAllConnsIdle) {
-		t.Fatalf("test server has active conns")
-	}
 
 	ts.Config.SetKeepAlivesEnabled(false)
 

src/net/http/server.go

@@ -1827,6 +1827,14 @@ func (c *conn) serve(ctx context.Context) {
 		c.setState(c.rwc, StateIdle)
 		c.curReq.Store((*response)(nil))
 
+		if !w.conn.server.doKeepAlives() {
+			// We're in shutdown mode. We might've replied
+			// to the user without "Connection: close" and
+			// they might think they can send another
+			// request, but such is life with HTTP/1.1.
+			return
+		}
+
 		if d := c.server.idleTimeout(); d != 0 {
 			c.rwc.SetReadDeadline(time.Now().Add(d))
 			if _, err := c.bufr.Peek(4); err != nil {