Add mbedtls_x509_dn_get_next function

Allow iteration through relative DNs when X509 name contains multi-
value RDNs.

Signed-off-by: Werner Lewis <werner.lewis@arm.com>
diff --git a/tests/suites/test_suite_x509parse.function b/tests/suites/test_suite_x509parse.function
index 1d06fe3..2495270 100644
--- a/tests/suites/test_suite_x509parse.function
+++ b/tests/suites/test_suite_x509parse.function
@@ -785,6 +785,69 @@
 }
 /* END_CASE */
 
+/* BEGIN_CASE depends_on:MBEDTLS_X509_CREATE_C:MBEDTLS_X509_USE_C:MBEDTLS_X509_CRT_PARSE_C:!MBEDTLS_X509_REMOVE_INFO */
+void mbedtls_x509_dn_get_next( char * name_str, int next_merged, char * expected_oids, int exp_count, char * exp_dn_gets )
+{
+    int ret = 0, i;
+    size_t len = 0;
+    mbedtls_asn1_named_data *names = NULL;
+    mbedtls_x509_name parsed, *parsed_cur, *parsed_prv;
+    unsigned char buf[1024], out[1024], *c;
+    const char *short_name;
+
+    memset( &parsed, 0, sizeof( parsed ) );
+    memset( out, 0, sizeof( out ) );
+    memset( buf, 0, sizeof( buf ) );
+    c = buf + sizeof( buf );
+
+    TEST_ASSERT( mbedtls_x509_string_to_names( &names, name_str ) == 0 );
+
+    ret = mbedtls_x509_write_names( &c, buf, names );
+    TEST_ASSERT( ret > 0 );
+
+    TEST_ASSERT( mbedtls_asn1_get_tag( &c, buf + sizeof( buf ), &len,
+                       MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) == 0 );
+    TEST_ASSERT( mbedtls_x509_get_name( &c, buf + sizeof( buf ), &parsed ) == 0 );
+
+    // Iterate over names and set next_merged nodes
+    parsed_cur = &parsed;
+    for( ; next_merged != 0 && parsed_cur != NULL; next_merged = next_merged >> 1 )
+    {
+        parsed_cur->next_merged = next_merged & 0b1;
+        parsed_cur = parsed_cur->next;
+    }
+
+    // Iterate over RDN nodes and print OID of first element to buffer
+    parsed_cur = &parsed;
+    len = 0;
+    for( i = 0; parsed_cur != NULL; i++ )
+    {
+        TEST_ASSERT( mbedtls_oid_get_attr_short_name( &parsed_cur->oid,
+                                                      &short_name ) == 0 );
+        len += mbedtls_snprintf( (char*) out + len, 1024 - len, "%s ", short_name );
+        parsed_cur = mbedtls_x509_dn_get_next( parsed_cur );
+    }
+    out[len-1] = 0;
+
+    TEST_ASSERT( exp_count == i );
+    TEST_ASSERT( strcmp( (char *) out, expected_oids ) == 0 );
+
+    TEST_ASSERT( mbedtls_x509_dn_gets( (char *) out, sizeof( out ), &parsed ) >= 0 );
+    TEST_ASSERT( strcmp( (char *) out, exp_dn_gets ) == 0 );
+exit:
+    mbedtls_asn1_free_named_data_list( &names );
+
+    parsed_cur = parsed.next;
+    while( parsed_cur != 0 )
+    {
+        parsed_prv = parsed_cur;
+        parsed_cur = parsed_cur->next;
+        mbedtls_free( parsed_prv );
+    }
+}
+
+/* END_CASE */
+
 /* BEGIN_CASE depends_on:MBEDTLS_FS_IO:MBEDTLS_X509_CRT_PARSE_C */
 void mbedtls_x509_time_is_past( char * crt_file, char * entity, int result )
 {