Acceptance tests: Add interrupt_interactive_console_until_pattern()

We need a function to interrupt interactive consoles.

Example: Interrupt U-Boot to set different environment values.

Tested-by: Niek Linnenbank <nieklinnenbank@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Tested-by: Liam Merwick <liam.merwick@oracle.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20200120235159.18510-4-f4bug@amsat.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>

tests/acceptance/avocado_qemu/__init__.py

@@ -56,13 +56,15 @@ def pick_default_qemu_bin(arch=None):
 
 
 def _console_interaction(test, success_message, failure_message,
-                         send_string):
+                         send_string, keep_sending=False):
+    assert not keep_sending or send_string
     console = test.vm.console_socket.makefile()
     console_logger = logging.getLogger('console')
     while True:
         if send_string:
             test.vm.console_socket.sendall(send_string.encode())
-            send_string = None # send only once
+            if not keep_sending:
+                send_string = None # send only once
         msg = console.readline().strip()
         if not msg:
             continue
@@ -74,6 +76,32 @@ def _console_interaction(test, success_message, failure_message,
             fail = 'Failure message found in console: %s' % failure_message
             test.fail(fail)
 
+def interrupt_interactive_console_until_pattern(test, success_message,
+                                                failure_message=None,
+                                                interrupt_string='\r'):
+    """
+    Keep sending a string to interrupt a console prompt, while logging the
+    console output. Typical use case is to break a boot loader prompt, such:
+
+        Press a key within 5 seconds to interrupt boot process.
+        5
+        4
+        3
+        2
+        1
+        Booting default image...
+
+    :param test: an Avocado test containing a VM that will have its console
+                 read and probed for a success or failure message
+    :type test: :class:`avocado_qemu.Test`
+    :param success_message: if this message appears, test succeeds
+    :param failure_message: if this message appears, test fails
+    :param interrupt_string: a string to send to the console before trying
+                             to read a new line
+    """
+    _console_interaction(test, success_message, failure_message,
+                         interrupt_string, True)
+
 def wait_for_console_pattern(test, success_message, failure_message=None):
     """
     Waits for messages to appear on the console, while logging the content