Add SSL_get_pending_cipher()

The existing function SSL_get_current_cipher() queries the
current session for the ciphersuite in use, but there is no way
for application code to determine what ciphersuite has been
negotiated and will be used in the future, prior to ChangeCipherState
(or the TLS 1.3 equivalent) causing the new cipher to take effect and
become visible in the session information.  Expose this information
to appropriate application callbacks to use during the handshake.

The name SSL_get_pending_cipher() was chosen for compatibility with
BoringSSL's routine of that name.

Improve the note on macro implementations in SSL_get_current_cipher.pod
while here.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4070)

1 files changed, 18 insertions, 2 deletions

diff --git a/doc/man3/SSL_get_current_cipher.pod b/doc/man3/SSL_get_current_cipher.pod
index cb7e9994a7..e5b249fd1e 100644
--- a/doc/man3/SSL_get_current_cipher.pod
+++ b/doc/man3/SSL_get_current_cipher.pod
@@ -3,13 +3,15 @@
 =head1 NAME
 
 SSL_get_current_cipher, SSL_get_cipher_name, SSL_get_cipher,
-SSL_get_cipher_bits, SSL_get_cipher_version - get SSL_CIPHER of a connection
+SSL_get_cipher_bits, SSL_get_cipher_version,
+SSL_get_pending_cipher - get SSL_CIPHER of a connection
 
 =head1 SYNOPSIS
 
  #include <openssl/ssl.h>
 
  SSL_CIPHER *SSL_get_current_cipher(const SSL *ssl);
+ SSL_CIPHER *SSL_get_pending_cipher(const SSL *ssl);
 
  const char *SSL_get_cipher_name(const SSL *s);
  const char *SSL_get_cipher(const SSL *s);
@@ -30,14 +32,28 @@ SSL_get_cipher_bits() is a
 macro to obtain the number of secret/algorithm bits used and
 SSL_get_cipher_version() returns the protocol name.
 
+SSL_get_pending_cipher() returns a pointer to an SSL_CIPHER object containing
+the description of the cipher (if any) that has been negotiated for future use
+on the connection established with the B<ssl> object, but is not yet in use.
+This may be the case during handshake processing, when control flow can be
+returned to the application via any of several callback methods.  The internal
+sequencing of handshake processing and callback invocation is not guaranteed
+to be stable from release to release, and at present only the callback set
+by SSL_CTX_set_alpn_select_cb() is guaranteed to have a non-NULL return value.
+Other callbacks may be added to this list over time.
+
 =head1 RETURN VALUES
 
 SSL_get_current_cipher() returns the cipher actually used, or NULL if
 no session has been established.
 
+SSL_get_pending_cipher() returns the cipher to be used at the next change
+of cipher suite, or NULL if no such cipher is known.
+
 =head1 NOTES
 
-These are implemented as macros.
+SSL_get_cipher, SSL_get_cipher_bits, SSL_get_cipher_version, and
+SSL_get_cipher_name are implemented as macros.
 
 =head1 SEE ALSO