test: Move the receive code into a function

There is quite a bit of code to deal with receiving data from the target
so move it into its own receive() function.

Signed-off-by: Simon Glass <sjg@chromium.org>

test/py/u_boot_spawn.py

@@ -137,6 +137,32 @@ class Spawn:
 
         os.write(self.fd, data.encode(errors='replace'))
 
+    def receive(self, num_bytes):
+        """Receive data from the sub-process's stdin.
+
+        Args:
+            num_bytes (int): Maximum number of bytes to read
+
+        Returns:
+            str: The data received
+
+        Raises:
+            ValueError if U-Boot died
+        """
+        try:
+            c = os.read(self.fd, num_bytes).decode(errors='replace')
+        except OSError as err:
+            # With sandbox, try to detect when U-Boot exits when it
+            # shouldn't and explain why. This is much more friendly than
+            # just dying with an I/O error
+            if self.decode_signal and err.errno == 5:  # I/O error
+                alive, _, info = self.checkalive()
+                if alive:
+                    raise err
+                raise ValueError('U-Boot exited with %s' % info)
+            raise
+        return c
+
     def expect(self, patterns):
         """Wait for the sub-process to emit specific data.
 
@@ -193,18 +219,7 @@ class Spawn:
                 events = self.poll.poll(poll_maxwait)
                 if not events:
                     raise Timeout()
-                try:
-                    c = os.read(self.fd, 1024).decode(errors='replace')
-                except OSError as err:
-                    # With sandbox, try to detect when U-Boot exits when it
-                    # shouldn't and explain why. This is much more friendly than
-                    # just dying with an I/O error
-                    if self.decode_signal and err.errno == 5:  # I/O error
-                        alive, _, info = self.checkalive()
-                        if alive:
-                            raise err
-                        raise ValueError('U-Boot exited with %s' % info)
-                    raise
+                c = self.receive(1024)
                 if self.logfile_read:
                     self.logfile_read.write(c)
                 self.buf += c