gh-116622: Mock the passage of time in Android logcat rate limit tests (#124015)

Mock the passage of time in Android logcat rate limit tests

Co-authored-by: Russell Keith-Magee <russell@keith-magee.com>

Lib/_android_support.py

@@ -165,7 +165,10 @@ class Logcat:
             now = time()
             self._bucket_level += (
                 (now - self._prev_write_time) * MAX_BYTES_PER_SECOND)
-            self._bucket_level = min(self._bucket_level, BUCKET_SIZE)
+
+            # If the bucket level is still below zero, the clock must have gone
+            # backwards, so reset it to zero and continue.
+            self._bucket_level = max(0, min(self._bucket_level, BUCKET_SIZE))
             self._prev_write_time = now
 
             self._bucket_level -= PER_MESSAGE_OVERHEAD + len(tag) + len(message)

Lib/test/test_android.py

@@ -10,7 +10,7 @@ from array import array
 from contextlib import ExitStack, contextmanager
 from threading import Thread
 from test.support import LOOPBACK_TIMEOUT
-from time import sleep, time
+from time import time
 from unittest.mock import patch
 
 
@@ -42,7 +42,8 @@ class TestAndroidOutput(unittest.TestCase):
             for line in self.logcat_process.stdout:
                 self.logcat_queue.put(line.rstrip("\n"))
             self.logcat_process.stdout.close()
-        Thread(target=logcat_thread).start()
+        self.logcat_thread = Thread(target=logcat_thread)
+        self.logcat_thread.start()
 
         from ctypes import CDLL, c_char_p, c_int
         android_log_write = getattr(CDLL("liblog.so"), "__android_log_write")
@@ -78,6 +79,7 @@ class TestAndroidOutput(unittest.TestCase):
     def tearDown(self):
         self.logcat_process.terminate()
         self.logcat_process.wait(LOOPBACK_TIMEOUT)
+        self.logcat_thread.join(LOOPBACK_TIMEOUT)
 
     @contextmanager
     def unbuffered(self, stream):
@@ -369,6 +371,8 @@ class TestAndroidOutput(unittest.TestCase):
                         ):
                             stream.write(obj)
 
+
+class TestAndroidRateLimit(unittest.TestCase):
     def test_rate_limit(self):
         # https://cs.android.com/android/platform/superproject/+/android-14.0.0_r1:system/logging/liblog/include/log/log_read.h;l=39
         PER_MESSAGE_OVERHEAD = 28
@@ -387,6 +391,19 @@ class TestAndroidOutput(unittest.TestCase):
             1024 - PER_MESSAGE_OVERHEAD - len(tag) - len(message.format(0))
         ) + "\n"
 
+        # To avoid depending on the performance of the test device, we mock the
+        # passage of time.
+        mock_now = time()
+
+        def mock_time():
+            # Avoid division by zero by simulating a small delay.
+            mock_sleep(0.0001)
+            return mock_now
+
+        def mock_sleep(duration):
+            nonlocal mock_now
+            mock_now += duration
+
         # See _android_support.py. The default values of these parameters work
         # well across a wide range of devices, but we'll use smaller values to
         # ensure a quick and reliable test that doesn't flood the log too much.
@@ -395,21 +412,24 @@ class TestAndroidOutput(unittest.TestCase):
         with (
             patch("_android_support.MAX_BYTES_PER_SECOND", MAX_KB_PER_SECOND * 1024),
             patch("_android_support.BUCKET_SIZE", BUCKET_KB * 1024),
+            patch("_android_support.sleep", mock_sleep),
+            patch("_android_support.time", mock_time),
         ):
             # Make sure the token bucket is full.
-            sleep(BUCKET_KB / MAX_KB_PER_SECOND)
+            stream.write("Initial message to reset _prev_write_time")
+            mock_sleep(BUCKET_KB / MAX_KB_PER_SECOND)
             line_num = 0
 
             # Write BUCKET_KB messages, and return the rate at which they were
             # accepted in KB per second.
             def write_bucketful():
                 nonlocal line_num
-                start = time()
+                start = mock_time()
                 max_line_num = line_num + BUCKET_KB
                 while line_num < max_line_num:
                     stream.write(message.format(line_num))
                     line_num += 1
-                return BUCKET_KB / (time() - start)
+                return BUCKET_KB / (mock_time() - start)
 
             # The first bucketful should be written with minimal delay. The
             # factor of 2 here is not arbitrary: it verifies that the system can
@@ -427,5 +447,5 @@ class TestAndroidOutput(unittest.TestCase):
             )
 
             # Once the token bucket refills, we should go back to full speed.
-            sleep(BUCKET_KB / MAX_KB_PER_SECOND)
+            mock_sleep(BUCKET_KB / MAX_KB_PER_SECOND)
             self.assertGreater(write_bucketful(), MAX_KB_PER_SECOND * 2)