Add support to free/allocate SSL buffers

OpenSSL already has the feature of SSL_MODE_RELEASE_BUFFERS that can
be set to release the read or write buffers when data has finished
reading or writing. OpenSSL will automatically re-allocate the buffers
as needed. This can be quite aggressive in terms of memory allocation.

This provides a manual mechanism. SSL_free_buffers() will free
the data buffers if there's no pending data. SSL_alloc_buffers()
will realloc them; but this function is not strictly necessary, as it's
still done automatically in the state machine.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2240)

1 files changed, 67 insertions, 0 deletions

diff --git a/doc/man3/SSL_alloc_buffers.pod b/doc/man3/SSL_alloc_buffers.pod
new file mode 100644
index 0000000000..de33efb937
--- /dev/null
+++ b/doc/man3/SSL_alloc_buffers.pod
@@ -0,0 +1,67 @@
+=pod
+
+=head1 NAME
+
+SSL_free_buffers, SSL_alloc_buffers - manage SSL structure buffers
+
+=head1 SYNOPSIS
+
+ #include <openssl/ssl.h>
+
+ int SSL_free_buffers(SSL *ssl);
+ int SSL_alloc_buffers(SSL *ssl);
+
+=head1 DESCRIPTION
+
+SSL_free_buffers() frees the read and write buffers of the given B<ssl>.
+SSL_alloc_buffers() allocates the read and write buffers of the given B<ssl>.
+
+The B<SSL_MODE_RELEASE_BUFFERS> mode releases read or write buffers whenever
+the buffers have been drained. These functions allow applications to manually
+control when buffers are freed and allocated.
+
+After freeing the buffers, the buffers are automatically reallocted upon a
+new read or write. The SSL_alloc_buffers() does not need to be called, but
+can be used to make sure the buffers are pre-allocated. This can be used to
+avoid allocation during data processing or with CRYPTO_set_mem_functions()
+to control where and how buffers are allocated.
+
+=head1 RETURN VALUES
+
+The following return values can occur:
+
+=over 4
+
+=item 0 (Failure)
+
+The SSL_free_buffers() function returns 0 when there is pending data to be
+read or written. The SSL_alloc_buffers() function returns 0 when there is
+an allocation failure.
+
+=item 1 (Success)
+
+The SSL_free_buffers() function returns 1 if the buffers have been freed. This
+value is also returned if the buffers had been freed before calling
+SSL_free_buffers().
+The SSL_alloc_buffers() function returns 1 if the buffers have been allocated.
+This valus is also returned if the buffers had been allocated before calling
+SSL_alloc_buffers().
+
+=back
+
+=head1 SEE ALSO
+
+L<SSL_free(3)>, L<SSL_clear(3)>,
+L<SSL_new(3)>, L<SSL_CTX_set_mode(3)>,
+L<CRYPTO_set_mem_functions>
+
+=head1 COPYRIGHT
+
+Copyright 2017 The OpenSSL Project Authors. All Rights Reserved.
+
+Licensed under the OpenSSL license (the "License").  You may not use
+this file except in compliance with the License.  You can obtain a copy
+in the file LICENSE in the source distribution or at
+L<https://www.openssl.org/source/license.html>.
+
+=cut