Move post-handshake message handling out of read_app_data.

This finishes getting rid of ssl_read_bytes! Now we have separate
entry-points for the various cases. For now, I've kept TLS handshake
consuming records partially. When we do the BIO-less API, I expect that
will need to change, since we won't have the record buffer available.

(Instead, the ssl3_read_handshake_bytes and extend_handshake_buffer pair
will look more like the DTLS side or Go and pull the entire record into
init_buf.)

This change opts to make read_app_data drive the message to completion
in anticipation of DTLS 1.3. That hasn't been specified, but
NewSessionTicket certainly will exist. Knowing that DTLS necessarily has
interleave seems something better suited for the SSL_PROTOCOL_METHOD
internals to drive.

It needs refining, but SSL_PROTOCOL_METHOD is now actually a half-decent
abstraction boundary between the higher-level protocol logic and
DTLS/TLS-specific record-layer and message dispatchy bits.

BUG=83

Change-Id: I9b4626bb8a29d9cb30174d9e6912bb420ed45aff
Reviewed-on: https://boringssl-review.googlesource.com/9001
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

ssl/internal.h

diff --git a/ssl/internal.h b/ssl/internal.h
index 1b3f28a..7a4a958 100644
--- a/ssl/internal.h
+++ b/ssl/internal.h
@@ -1052,7 +1052,13 @@
   /* release_current_message is called to release the current handshake message.
    * If |free_buffer| is one, buffers will also be released. */
   void (*release_current_message)(SSL *ssl, int free_buffer);
-  int (*read_app_data)(SSL *ssl, uint8_t *buf, int len, int peek);
+  /* read_app_data reads up to |len| bytes of application data into |buf|. On
+   * success, it returns the number of bytes read. Otherwise, it returns <= 0
+   * and sets |*out_got_handshake| to whether the failure was due to a
+   * post-handshake handshake message. If so, it fills in the current message as
+   * in |ssl_get_message|. */
+  int (*read_app_data)(SSL *ssl, int *out_got_handshake,  uint8_t *buf, int len,
+                       int peek);
   int (*read_change_cipher_spec)(SSL *ssl);
   void (*read_close_notify)(SSL *ssl);
   int (*write_app_data)(SSL *ssl, const void *buf_, int len);
@@ -1255,10 +1261,11 @@
 int ssl3_send_finished(SSL *ssl, int a, int b);
 int ssl3_supports_cipher(const SSL_CIPHER *cipher);
 int ssl3_dispatch_alert(SSL *ssl);
-int ssl3_read_app_data(SSL *ssl, uint8_t *buf, int len, int peek);
+int ssl3_read_app_data(SSL *ssl, int *out_got_handshake, uint8_t *buf, int len,
+                       int peek);
 int ssl3_read_change_cipher_spec(SSL *ssl);
 void ssl3_read_close_notify(SSL *ssl);
-int ssl3_read_bytes(SSL *ssl, int type, uint8_t *buf, int len, int peek);
+int ssl3_read_handshake_bytes(SSL *ssl, uint8_t *buf, int len);
 int ssl3_write_app_data(SSL *ssl, const void *buf, int len);
 int ssl3_write_bytes(SSL *ssl, int type, const void *buf, int len);
 int ssl3_output_cert_chain(SSL *ssl);
@@ -1287,7 +1294,8 @@
  * more data is needed. */
 int dtls1_get_record(SSL *ssl);
 
-int dtls1_read_app_data(SSL *ssl, uint8_t *buf, int len, int peek);
+int dtls1_read_app_data(SSL *ssl, int *out_got_handshake, uint8_t *buf, int len,
+                        int peek);
 int dtls1_read_change_cipher_spec(SSL *ssl);
 void dtls1_read_close_notify(SSL *ssl);