ssl: support session resumption on reconnections with same session

when connections get reset due to max number of requests being reached,
the same TLS session is going to be reused, as long as it's valid.

This change is ported from the same feature in net-http, including [the
tls 1.3
improvements](ddf5c52b5f)

lib/httpx/connection.rb

@@ -42,7 +42,7 @@ module HTTPX
 
     def_delegator :@write_buffer, :empty?
 
-    attr_reader :type, :io, :origin, :origins, :state, :pending, :options
+    attr_reader :type, :io, :origin, :origins, :state, :pending, :options, :ssl_session
 
     attr_writer :timers
 
@@ -138,6 +138,12 @@ module HTTPX
 
     def merge(connection)
       @origins |= connection.instance_variable_get(:@origins)
+      if connection.ssl_session
+        @ssl_session = connection.ssl_session
+        @io.session_new_cb do |sess|
+          @ssl_session = sess
+        end if @io
+      end
       connection.purge_pending do |req|
         send(req)
       end
@@ -594,14 +600,21 @@ module HTTPX
     end
 
     def build_socket(addrs = nil)
-      transport_type = case @type
-                       when "tcp" then TCP
-                       when "ssl" then SSL
-                       when "unix" then UNIX
-                       else
-                         raise Error, "unsupported transport (#{@type})"
+      case @type
+      when "tcp"
+        TCP.new(@origin, addrs, @options)
+      when "ssl"
+        sock = SSL.new(@origin, addrs, @options)
+        sock.ssl_session = @ssl_session
+        sock.session_new_cb do |sess|
+          @ssl_session = sess
+        end
+        sock
+      when "unix"
+        UNIX.new(@origin, addrs, @options)
+      else
+        raise Error, "unsupported transport (#{@type})"
       end
-      transport_type.new(@origin, addrs, @options)
     end
 
     def on_error(error)

lib/httpx/io/ssl.rb

@@ -15,6 +15,8 @@ module HTTPX
       {}.freeze
     end
 
+    attr_writer :ssl_session
+
     def initialize(_, _, options)
       super
 
@@ -39,6 +41,15 @@ module HTTPX
       @verify_hostname = @ctx.verify_hostname
     end
 
+    if OpenSSL::SSL::SSLContext.method_defined?(:session_new_cb=)
+      def session_new_cb(&pr)
+        @ctx.session_new_cb = proc { |_, sess| pr.call(sess) }
+      end
+    else
+      # session_new_cb not implemented under JRuby
+      def session_new_cb; end
+    end
+
     def protocol
       @io.alpn_protocol || super
     rescue StandardError
@@ -81,6 +92,11 @@ module HTTPX
         else
           @io.hostname = @sni_hostname
         end
+        if @ssl_session &&
+           Process.clock_gettime(Process::CLOCK_REALTIME) < (@ssl_session.time.to_f + @ssl_session.timeout)
+          puts "reusing session y'all: #{@ssl_session}"
+          @io.session = @ssl_session
+        end
         @io.sync_close = true
       end
       try_ssl_connect

test/https_test.rb

@@ -93,6 +93,10 @@ class HTTPSTest < Minitest::Test
         connection_count = http.pool.connection_count
         assert connection_count == 2, "expected to have 2 connections, instead have #{connection_count}"
         assert http.connection_exausted, "expected 1 connnection to have exhausted"
+
+        # ssl session ought to be reused
+        conn = http.pool.connections.first
+        assert conn.io.instance_variable_get(:@io).session_reused? unless RUBY_ENGINE == "jruby"
       end
     end