Update documentation on how to close a connection

Reviewed-by: NBernd Edlinger <bernd.edlinger@hotmail.de>
GH: #5823

doc/man3/SSL_get_error.pod

@@ -38,11 +38,10 @@ if and only if B<ret E<gt> 0>.
 
 =item SSL_ERROR_ZERO_RETURN
 
-The TLS/SSL connection has been closed.
-If the protocol version is SSL 3.0 or higher, this result code is returned only
-if a closure alert has occurred in the protocol, i.e. if the connection has been
-closed cleanly.
-Note that in this case B<SSL_ERROR_ZERO_RETURN> does not necessarily
+The TLS/SSL peer has closed the connection for writing by sending the
+"close notify" alert.
+No more data can be read.
+Note that B<SSL_ERROR_ZERO_RETURN> does not necessarily
 indicate that the underlying transport has been closed.
 
 =item SSL_ERROR_WANT_READ, SSL_ERROR_WANT_WRITE

doc/man3/SSL_shutdown.pod

@@ -35,35 +35,52 @@ performed, so that the peers stay synchronized.
 SSL_shutdown() supports both uni- and bidirectional shutdown by its 2 step
 behaviour.
 
-=over 4
+SSL_shutdown() only closes the write direction.
+It is not possible to call SSL_write() after calling SSL_shutdown().
+The read direction is closed by the peer.
+
+=head2 First to close the connection
 
-=item When the application is the first party to send the "close notify"
+When the application is the first party to send the "close notify"
 alert, SSL_shutdown() will only send the alert and then set the
 SSL_SENT_SHUTDOWN flag (so that the session is considered good and will
-be kept in cache). SSL_shutdown() will then return with 0. If a unidirectional
-shutdown is enough (the underlying connection shall be closed anyway), this
-first call to SSL_shutdown() is sufficient. In order to complete the
-bidirectional shutdown handshake, SSL_shutdown() must be called again.
-The second call will make SSL_shutdown() wait for the peer's "close notify"
-shutdown alert. On success, the second call to SSL_shutdown() will return
-with 1.
-
-=item If the peer already sent the "close notify" alert B<and> it was
+be kept in the cache).
+SSL_shutdown() will then return with 0.
+If a unidirectional shutdown is enough (the underlying connection shall be
+closed anyway), this first call to SSL_shutdown() is sufficient.
+
+In order to complete the bidirectional shutdown handshake, the peer needs
+to send back a "close notify" alert.
+The SSL_RECEIVED_SHUTDOWN flag will be set after receiving and processing
+it.
+SSL_shutdown() will return 1 when it has been received.
+
+The peer is still allowed to send data after receiving the "close notify"
+event.
+If the peer did send data it needs to be processed by calling SSL_read()
+before calling SSL_shutdown() a second time.
+SSL_read() will indicate the end of the peer data by returning <= 0
+and SSL_get_error() returning SSL_ERROR_ZERO_RETURN.
+It is recommended to call SSL_read() between SSL_shutdown() calls.
+
+=head2 Peer closes the connection
+
+If the peer already sent the "close notify" alert B<and> it was
 already processed implicitly inside another function
 (L<SSL_read(3)>), the SSL_RECEIVED_SHUTDOWN flag is set.
+SSL_read() will return <= 0 in that case, and SSL_get_error() will return
+SSL_ERROR_ZERO_RETURN.
 SSL_shutdown() will send the "close notify" alert, set the SSL_SENT_SHUTDOWN
 flag and will immediately return with 1.
 Whether SSL_RECEIVED_SHUTDOWN is already set can be checked using the
 SSL_get_shutdown() (see also L<SSL_set_shutdown(3)> call.
 
-=back
+=head1 NOTES
 
-It is therefore recommended, to check the return value of SSL_shutdown()
-and call SSL_shutdown() again, if the bidirectional shutdown is not yet
-complete (return value of the first call is 0).
+It is recommended to do a bidirectional shutdown by checking the return value
+of SSL_shutdown() and call it again until it returns 1 or a fatal error.
 
 The behaviour of SSL_shutdown() additionally depends on the underlying BIO.
-
 If the underlying BIO is B<blocking>, SSL_shutdown() will only return once the
 handshake step has been finished or an error occurred.
 
@@ -92,8 +109,9 @@ The following return values can occur:
 
 =item Z<>0
 
-The shutdown is not yet finished. Call SSL_shutdown() for a second time,
-if a bidirectional shutdown shall be performed.
+The shutdown is not yet finished: the "close notify" was send but the peer
+did not send it back yet.
+Call SSL_shutdown() again to do a bidirectional shutdown.
 The output of L<SSL_get_error(3)> may be misleading, as an
 erroneous SSL_ERROR_SYSCALL may be flagged even though no error occurred.
 
@@ -104,11 +122,12 @@ and the peer's "close notify" alert was received.
 
 =item E<lt>0
 
-The shutdown was not successful because a fatal error occurred either
-at the protocol level or a connection failure occurred. It can also occur if
-action is need to continue the operation for non-blocking BIOs.
-Call L<SSL_get_error(3)> with the return value B<ret>
-to find out the reason.
+The shutdown was not successful.
+Call L<SSL_get_error(3)> with the return value B<ret> to find out the reason.
+It can occur if an action is needed to continue the operation for non-blocking
+BIOs.
+
+It can also occur when not all data was read using SSL_read().
 
 =back