kunit: tool: print parsed test results fully incrementally

With the parser rework [1] and run_kernel() rework [2], this allows the
parser to print out test results incrementally.

Currently, that's held up by the fact that the LineStream eagerly
pre-fetches the next line when you call pop().
This blocks parse_test_result() from returning until the line *after*
the "ok 1 - test name" line is also printed.

One can see this with the following example:
$ (echo -e 'TAP version 14\n1..3\nok 1 - fake test'; sleep 2; echo -e 'ok 2 - fake test 2'; sleep 3; echo -e 'ok 3 - fake test 3') | ./tools/testing/kunit/kunit.py parse

Before this patch [1]: there's a pause before 'fake test' is printed.
After this patch: 'fake test' is printed out immediately.

This patch also adds
* a unit test to verify LineStream's behavior directly
* a test case to ensure that it's lazily calling the generator
* an explicit exception for when users go beyond EOF

[1] https://lore.kernel.org/linux-kselftest/20211006170049.106852-1-dlatypov@google.com/
[2] https://lore.kernel.org/linux-kselftest/20211005011340.2826268-1-dlatypov@google.com/

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Author: dlatypov

tools/testing/kunit/kunit_parser.py

@@ -168,42 +168,51 @@ def add_status(self, status: TestStatus) -> None:
 class LineStream:
 	"""
 	A class to represent the lines of kernel output.
-	Provides a peek()/pop() interface over an iterator of
+	Provides a lazy peek()/pop() interface over an iterator of
 	(line#, text).
 	"""
 	_lines: Iterator[Tuple[int, str]]
 	_next: Tuple[int, str]
+	_need_next: bool
 	_done: bool
 
 	def __init__(self, lines: Iterator[Tuple[int, str]]):
 		"""Creates a new LineStream that wraps the given iterator."""
 		self._lines = lines
 		self._done = False
+		self._need_next = True
 		self._next = (0, '')
-		self._get_next()
 
 	def _get_next(self) -> None:
-		"""Advances the LineSteam to the next line."""
+		"""Advances the LineSteam to the next line, if necessary."""
+		if not self._need_next:
+			return
 		try:
 			self._next = next(self._lines)
 		except StopIteration:
 			self._done = True
+		finally:
+			self._need_next = False
 
 	def peek(self) -> str:
 		"""Returns the current line, without advancing the LineStream.
 		"""
+		self._get_next()
 		return self._next[1]
 
 	def pop(self) -> str:
 		"""Returns the current line and advances the LineStream to
 		the next line.
 		"""
-		n = self._next
-		self._get_next()
-		return n[1]
+		s = self.peek()
+		if self._done:
+			raise ValueError(f'LineStream: going past EOF, last line was {s}')
+		self._need_next = True
+		return s
 
 	def __bool__(self) -> bool:
 		"""Returns True if stream has more lines."""
+		self._get_next()
 		return not self._done
 
 	# Only used by kunit_tool_test.py.
@@ -216,6 +225,7 @@ def __iter__(self) -> Iterator[str]:
 
 	def line_number(self) -> int:
 		"""Returns the line number of the current line."""
+		self._get_next()
 		return self._next[0]
 
 # Parsing helper methods:

tools/testing/kunit/kunit_tool_test.py

@@ -13,9 +13,10 @@
 
 import itertools
 import json
+import os
 import signal
 import subprocess
-import os
+from typing import Iterable
 
 import kunit_config
 import kunit_parser
@@ -331,6 +332,45 @@ def test_pound_no_prefix(self):
 				result.status)
 			self.assertEqual('kunit-resource-test', result.test.subtests[0].name)
 
+def line_stream_from_strs(strs: Iterable[str]) -> kunit_parser.LineStream:
+	return kunit_parser.LineStream(enumerate(strs, start=1))
+
+class LineStreamTest(unittest.TestCase):
+
+	def test_basic(self):
+		stream = line_stream_from_strs(['hello', 'world'])
+
+		self.assertTrue(stream, msg='Should be more input')
+		self.assertEqual(stream.line_number(), 1)
+		self.assertEqual(stream.peek(), 'hello')
+		self.assertEqual(stream.pop(), 'hello')
+
+		self.assertTrue(stream, msg='Should be more input')
+		self.assertEqual(stream.line_number(), 2)
+		self.assertEqual(stream.peek(), 'world')
+		self.assertEqual(stream.pop(), 'world')
+
+		self.assertFalse(stream, msg='Should be no more input')
+		with self.assertRaisesRegex(ValueError, 'LineStream: going past EOF'):
+			stream.pop()
+
+	def test_is_lazy(self):
+		called_times = 0
+		def generator():
+			nonlocal called_times
+			for i in range(1,5):
+				called_times += 1
+				yield called_times, str(called_times)
+
+		stream = kunit_parser.LineStream(generator())
+		self.assertEqual(called_times, 0)
+
+		self.assertEqual(stream.pop(), '1')
+		self.assertEqual(called_times, 1)
+
+		self.assertEqual(stream.pop(), '2')
+		self.assertEqual(called_times, 2)
+
 class LinuxSourceTreeTest(unittest.TestCase):
 
 	def setUp(self):