Fix spurious "success" on SSL system errors:

Fix boostorg/beast#807
Fix boostorg/beast#1373

This resolves an issue where an ssl::stream read operation
returns a successful error code but zero bytes transferred,
violating its contract.

* Treat SSL_ERROR_ZERO_RETURN as EOF

* Use the ssl category for sys_error

* On error, if sys_error is 0 (success),
  return ssl::errors::read_sys_error instead.

include/boost/asio/ssl/detail/impl/engine.ipp

@@ -237,17 +237,18 @@ engine::want engine::perform(int (engine::* op)(void*, std::size_t),
   int sys_error = static_cast<int>(::ERR_get_error());
   std::size_t pending_output_after = ::BIO_ctrl_pending(ext_bio_);
 
-  if (ssl_error == SSL_ERROR_SSL)
+  if (ssl_error == SSL_ERROR_SSL || ssl_error == SSL_ERROR_SYSCALL)
   {
-    ec = boost::system::error_code(sys_error,
+    if(sys_error == 0)
+    {
+      // remap success to a code that indicates failure
+      ec = boost::asio::ssl::error::read_sys_error;
+    }
+    else
+    {
+      ec = boost::system::error_code(sys_error,
         boost::asio::error::get_ssl_category());
-    return want_nothing;
-  }
-
-  if (ssl_error == SSL_ERROR_SYSCALL)
-  {
-    ec = boost::system::error_code(sys_error,
-        boost::asio::error::get_system_category());
+    }
     return want_nothing;
   }
 
@@ -274,8 +275,15 @@ engine::want engine::perform(int (engine::* op)(void*, std::size_t),
     ec = boost::asio::error::eof;
     return want_nothing;
   }
+  else if (ssl_error == SSL_ERROR_ZERO_RETURN)
+  {
+    ec = boost::asio::error::eof;
+    return want_nothing;
+  }
   else
   {
+    BOOST_ASIO_ASSERT(result > 0);
+    //if(result <= 0) std::cerr << "result <= 0, ssl_error = " << ssl_error << std::endl;
     ec = boost::system::error_code();
     return want_nothing;
   }

include/boost/asio/ssl/error.hpp

@@ -46,11 +46,15 @@ enum stream_errors
 #if defined(GENERATING_DOCUMENTATION)
   /// The underlying stream closed before the ssl stream gracefully shut down.
   stream_truncated
+
 #elif (OPENSSL_VERSION_NUMBER < 0x10100000L) && !defined(OPENSSL_IS_BORINGSSL)
   stream_truncated = ERR_PACK(ERR_LIB_SSL, 0, SSL_R_SHORT_READ)
 #else
   stream_truncated = 1
 #endif
+
+  /// An unspecified system error occurred during an ssl read operation.
+  ,read_sys_error = stream_truncated + 1
 };
 
 extern BOOST_ASIO_DECL

include/boost/asio/ssl/impl/error.ipp

@@ -77,6 +77,7 @@ public:
     switch (value)
     {
     case stream_truncated: return "stream truncated";
+    case read_sys_error: return "sys_error=0 on read";
     default: return "asio.ssl.stream error";
     }
   }