Always process handshake records in full.

This removes the last place where non-app-data hooks leave anything
uncomsumed in rrec. (There is still a place where non-app-data hooks see
a non-empty rrec an entrance. read_app_data calls into read_handshake.
That'll be fixed in a later patch in this series.)

This should not change behavior, though some error codes may change due
to some processing happening in a slightly different order.

Since we do this in a few places, this adds a BUF_MEM_append with tests.

Change-Id: I9fe1fc0103e47f90e3c9f4acfe638927aecdeff6
Reviewed-on: https://boringssl-review.googlesource.com/21345
Commit-Queue: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: Steven Valdez <svaldez@google.com>

ssl/internal.h

diff --git a/ssl/internal.h b/ssl/internal.h
index 4c1f9ea..f141923 100644
--- a/ssl/internal.h
+++ b/ssl/internal.h
@@ -1000,9 +1000,13 @@
 // dtls_clear_incoming_messages releases all buffered incoming messages.
 void dtls_clear_incoming_messages(SSL *ssl);
 
-// dtls_has_incoming_messages returns one if there are buffered incoming
-// messages ahead of the current message and zero otherwise.
-int dtls_has_incoming_messages(const SSL *ssl);
+// tls_has_unprocessed_handshake_data returns whether there is buffered
+// handshake data that has not been consumed by |get_message|.
+bool tls_has_unprocessed_handshake_data(const SSL *ssl);
+
+// dtls_has_unprocessed_handshake_data behaves like
+// |tls_has_unprocessed_handshake_data| for DTLS.
+bool dtls_has_unprocessed_handshake_data(const SSL *ssl);
 
 struct DTLS_OUTGOING_MESSAGE {
   uint8_t *data;
@@ -2687,7 +2691,10 @@
                        int peek);
 int ssl3_read_change_cipher_spec(SSL *ssl);
 void ssl3_read_close_notify(SSL *ssl);
-int ssl3_read_handshake_bytes(SSL *ssl, uint8_t *buf, int len);
+// ssl3_get_record reads a new input record. On success, it places it in
+// |ssl->s3->rrec| and returns one. Otherwise it returns <= 0 on error or if
+// more data is needed.
+int ssl3_get_record(SSL *ssl);
 int ssl3_write_app_data(SSL *ssl, bool *out_needs_handshake, const uint8_t *buf,
                         int len);