always flush when logging messages

craft_cli/printer.py

@@ -301,6 +301,7 @@ def _log(self, message: _MessageInfo) -> None:
         # prepare the text with (maybe) the timestamp
         timestamp_str = message.created_at.isoformat(sep=" ", timespec="milliseconds")
         self.log.write(f"{timestamp_str} {message.text}\n")
+        self.log.flush()
 
     def spin(self, message: _MessageInfo, spintext: str) -> None:
         """Write a line message including a spin text, only to a terminal."""

tests/integration/test_logging.py

@@ -0,0 +1,72 @@
+import logging
+import multiprocessing
+import os
+import sys
+from textwrap import dedent
+
+from craft_cli import emit, EmitterMode
+
+logger = logging.getLogger()
+
+
+def strip_timestamps(text: str) -> str:
+    lines = []
+    for line in text.splitlines():
+        lines.append(line.split(" ", maxsplit=2)[-1])
+    return "\n".join(lines)
+
+
+def callable_func():
+    logger.info("Message 1 from CHILD process")
+    logger.info("Message 2 from CHILD process")
+
+
+def test_logging_in_multiprocess(tmp_path):
+    logger.setLevel(logging.INFO)
+    emitter_log = tmp_path / "emitter_log.txt"
+    filehandler_log = tmp_path / "filehandler_log.txt"
+
+    filehandler = logging.FileHandler(filehandler_log)
+    os.set_inheritable(filehandler.stream.fileno(), True)
+    logger.addHandler(filehandler)
+    emit.init(mode=EmitterMode.QUIET, appname="testapp", greeting="hi", log_filepath=emitter_log)
+
+    logger.info("Message 1 from PARENT process")
+    logger.info("Message 2 from PARENT process")
+
+    child = multiprocessing.Process(target=callable_func)
+    child.start()
+    child.join()
+
+    logger.info("Message 3 from PARENT process")
+
+    emit.ended_ok()
+
+    emitter_logged = strip_timestamps(emitter_log.read_text())
+    filehandler_logged = filehandler_log.read_text()
+
+    if sys.platform == "linux":
+        # Expect two messages from the parent process, then two from the child process,
+        # then a final one from the parent again.
+        expected_text = dedent(
+            """\
+            Message 1 from PARENT process
+            Message 2 from PARENT process
+            Message 1 from CHILD process
+            Message 2 from CHILD process
+            Message 3 from PARENT process
+            """
+        )
+    else:
+        # Messages from the child process are NOT logged in non-Linux platforms. This
+        # is not by design, but a record of the current expected behavior.
+        expected_text = dedent(
+            """\
+            Message 1 from PARENT process
+            Message 2 from PARENT process
+            Message 3 from PARENT process
+            """
+        )
+
+    assert emitter_logged == "hi\n" + expected_text.rstrip()
+    assert filehandler_logged == expected_text