openssl: avoid undefined behavior on empty SSL_write

SSL_write(3ssl) manpage has this in the WARNINGS section:

       When calling SSL_write() with num=0 bytes to be sent the
       behaviour is undefined.

And indeed, the new test case demonstrates failures when
empty strings are used.  So, match the behavior of IO#write,
IO#write_nonblock, and IO#syswrite by returning zero, as the
OpenSSL::SSL::SSLSocket API already closely mimics the IO one.

* ext/openssl/ossl_ssl.c (ossl_ssl_write_internal):
  avoid undefined behavior
* test/openssl/test_pair.rb (test_write_zero): new test
  [ruby-core:76751] [Bug #12660]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55822 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ChangeLog

@@ -1,3 +1,10 @@
+Sun Aug  7 06:48:21 2016  Eric Wong  <e@80x24.org>
+
+	* ext/openssl/ossl_ssl.c (ossl_ssl_write_internal):
+	  avoid undefined behavior
+	* test/openssl/test_pair.rb (test_write_zero): new test
+	  [ruby-core:76751] [Bug #12660]
+
 Sat Aug  6 09:35:30 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* id_table.h (rb_id_table_iterator_result): add dummy sentinel

ext/openssl/ossl_ssl.c

@@ -1744,7 +1744,13 @@ ossl_ssl_write_internal(VALUE self, VALUE str, VALUE opts)
 
     if (ssl_started(ssl)) {
 	for (;;){
-	    nwrite = SSL_write(ssl, RSTRING_PTR(str), RSTRING_LENINT(str));
+	    int num = RSTRING_LENINT(str);
+
+	    /* SSL_write(3ssl) manpage states num == 0 is undefined */
+	    if (num == 0)
+		goto end;
+
+	    nwrite = SSL_write(ssl, RSTRING_PTR(str), num);
 	    switch(ssl_get_error(ssl, nwrite)){
 	    case SSL_ERROR_NONE:
 		goto end;

test/openssl/test_pair.rb

@@ -311,6 +311,17 @@ module OpenSSL::TestPairM
     }
   end
 
+  def test_write_zero
+    ssl_pair {|s1, s2|
+      assert_equal 0, s2.write_nonblock('', exception: false)
+      assert_kind_of Symbol, s1.read_nonblock(1, exception: false)
+      assert_equal 0, s2.syswrite('')
+      assert_kind_of Symbol, s1.read_nonblock(1, exception: false)
+      assert_equal 0, s2.write('')
+      assert_kind_of Symbol, s1.read_nonblock(1, exception: false)
+    }
+  end
+
   def tcp_pair
     host = "127.0.0.1"
     serv = TCPServer.new(host, 0)