test: add TestNode.wait_for_debug_log

Author: jamesob

test/functional/test_framework/test_node.py

@@ -422,6 +422,42 @@ def assert_debug_log(self, expected_msgs, unexpected_msgs=None, timeout=2):
             time.sleep(0.05)
         self._raise_assertion_error('Expected messages "{}" does not partially match log:\n\n{}\n\n'.format(str(expected_msgs), print_log))
 
+    def wait_for_debug_log(self, expected_msgs, timeout=10, ignore_case=False) -> int:
+        """
+        Block until we see a particular debug log message fragment or until we exceed the timeout.
+        Return:
+            the number of log lines we encountered when matching
+        """
+        time_end = time.time() + timeout * self.timeout_factor
+        prev_size = self.debug_log_bytes()
+        re_flags = re.MULTILINE | (re.IGNORECASE if ignore_case else 0)
+
+        while True:
+            found = True
+            with open(self.debug_log_path, encoding='utf-8') as dl:
+                dl.seek(prev_size)
+                log = dl.read()
+
+            for expected_msg in expected_msgs:
+                if re.search(re.escape(expected_msg), log, flags=re_flags) is None:
+                    found = False
+
+            if found:
+                num_logs = len(log.splitlines())
+                return num_logs
+
+            if time.time() >= time_end:
+                print_log = " - " + "\n - ".join(log.splitlines())
+                break
+
+            # No sleep here because we want to detect the message fragment as fast as
+            # possible.
+
+        self._raise_assertion_error(
+            'Expected messages "{}" does not partially match log:\n\n{}\n\n'.format(
+                str(expected_msgs), print_log))
+        return -1  # useless return to satisfy linter
+
     @contextlib.contextmanager
     def profile_with_perf(self, profile_name: str):
         """