ssl: let SSLServer accept frozen SSLContext

Do not raise FrozenError in SSLServer.new when
SSLContext#session_id_context cannot be updated. session_id_context is
only necessary for session resumption, so its absence is not critical.

Fixes #742

Author: rhenium

lib/openssl/ssl.rb

@@ -486,7 +486,7 @@ class SSLServer
       def initialize(svr, ctx)
         @svr = svr
         @ctx = ctx
-        unless ctx.session_id_context
+        if !ctx.frozen? && !ctx.session_id_context
           # see #6137 - session id may not exceed 32 bytes
           prng = ::Random.new($0.hash)
           session_id = prng.bytes(16).unpack1('H*')

test/openssl/test_ssl_server.rb

@@ -0,0 +1,67 @@
+# frozen_string_literal: true
+require_relative "utils"
+
+return unless defined?(OpenSSL::SSL)
+
+class OpenSSL::TestSSLServer < OpenSSL::SSLTestCase
+  def test_tcpserver
+    tcps = TCPServer.new("127.0.0.1", 0)
+    sctx = OpenSSL::SSL::SSLContext.new
+    sctx.add_certificate(@svr_cert, @svr_key)
+    server = OpenSSL::SSL::SSLServer.new(tcps, sctx)
+    assert_same(tcps, server.to_io)
+    assert_kind_of(String, sctx.session_id_context)
+    th = Thread.start do
+      sssl = server.accept
+      sssl.puts(sssl.gets)
+    ensure
+      sssl&.close
+    end
+    server_connect(tcps.local_address.ip_port) do |ssl|
+      assert_equal(@svr_cert.to_der, ssl.peer_cert.to_der)
+      ssl.puts("abc")
+      assert_equal("abc\n", ssl.gets)
+    end
+    th.join
+    server.close
+    assert_predicate(tcps, :closed?)
+  end
+
+  def test_ctx_frozen
+    tcps = TCPServer.new("127.0.0.1", 0)
+    sctx = OpenSSL::SSL::SSLContext.new
+    sctx.add_certificate(@svr_cert, @svr_key)
+    sctx.setup
+    server = OpenSSL::SSL::SSLServer.new(tcps, sctx)
+    assert_nil(sctx.session_id_context)
+    th = Thread.start do
+      sssl = server.accept
+      sssl.puts(sssl.gets)
+    ensure
+      sssl&.close
+    end
+    server_connect(tcps.local_address.ip_port) do |ssl|
+      assert_equal(@svr_cert.to_der, ssl.peer_cert.to_der)
+      ssl.puts("abc")
+      assert_equal("abc\n", ssl.gets)
+    end
+    th.join
+    server.close
+  end
+
+  private
+
+  def server_connect(port, ctx = nil)
+    sock = TCPSocket.new("127.0.0.1", port)
+    ssl = ctx ? OpenSSL::SSL::SSLSocket.new(sock, ctx) : OpenSSL::SSL::SSLSocket.new(sock)
+    ssl.sync_close = true
+    ssl.connect
+    yield ssl if block_given?
+  ensure
+    if ssl
+      ssl.close
+    elsif sock
+      sock.close
+    end
+  end
+end