Add test.support.busy_retry() (#93770)

Add busy_retry() and sleeping_retry() functions to test.support.

Lib/test/support/__init__.py

@@ -2250,3 +2250,79 @@ def late_deletion(obj):
             pass
         atfork_func.reference = ref_cycle
         os.register_at_fork(before=atfork_func)
+
+
+def busy_retry(timeout, err_msg=None, /, *, error=True):
+    """
+    Run the loop body until "break" stops the loop.
+
+    After *timeout* seconds, raise an AssertionError if *error* is true,
+    or just stop if *error is false.
+
+    Example:
+
+        for _ in support.busy_retry(support.SHORT_TIMEOUT):
+            if check():
+                break
+
+    Example of error=False usage:
+
+        for _ in support.busy_retry(support.SHORT_TIMEOUT, error=False):
+            if check():
+                break
+        else:
+            raise RuntimeError('my custom error')
+
+    """
+    if timeout <= 0:
+        raise ValueError("timeout must be greater than zero")
+
+    start_time = time.monotonic()
+    deadline = start_time + timeout
+
+    while True:
+        yield
+
+        if time.monotonic() >= deadline:
+            break
+
+    if error:
+        dt = time.monotonic() - start_time
+        msg = f"timeout ({dt:.1f} seconds)"
+        if err_msg:
+            msg = f"{msg}: {err_msg}"
+        raise AssertionError(msg)
+
+
+def sleeping_retry(timeout, err_msg=None, /,
+                     *, init_delay=0.010, max_delay=1.0, error=True):
+    """
+    Wait strategy that applies exponential backoff.
+
+    Run the loop body until "break" stops the loop. Sleep at each loop
+    iteration, but not at the first iteration. The sleep delay is doubled at
+    each iteration (up to *max_delay* seconds).
+
+    See busy_retry() documentation for the parameters usage.
+
+    Example raising an exception after SHORT_TIMEOUT seconds:
+
+        for _ in support.sleeping_retry(support.SHORT_TIMEOUT):
+            if check():
+                break
+
+    Example of error=False usage:
+
+        for _ in support.sleeping_retry(support.SHORT_TIMEOUT, error=False):
+            if check():
+                break
+        else:
+            raise RuntimeError('my custom error')
+    """
+
+    delay = init_delay
+    for _ in busy_retry(timeout, err_msg, error=error):
+        yield
+
+        time.sleep(delay)
+        delay = min(delay * 2, max_delay)