Issue #17835: Fix test_io when the default OS pipe buffer size is larger than one million bytes.

Lib/test/test_io.py

@@ -2880,7 +2880,7 @@ class SignalsTest(unittest.TestCase):
             # The buffered IO layer must check for pending signal
             # handlers, which in this case will invoke alarm_interrupt().
             self.assertRaises(ZeroDivisionError,
-                              wio.write, item * (3 * 1000 * 1000))
+                        wio.write, item * (support.PIPE_MAX_SIZE // len(item) + 1))
             t.join()
             # We got one byte, get another one and check that it isn't a
             # repeat of the first one.
@@ -2978,7 +2978,7 @@ class SignalsTest(unittest.TestCase):
         select = support.import_module("select")
         # A quantity that exceeds the buffer size of an anonymous pipe's
         # write end.
-        N = 1024 * 1024
+        N = support.PIPE_MAX_SIZE
         r, w = os.pipe()
         fdopen_kwargs["closefd"] = False
         # We need a separate thread to read from the pipe and allow the

Lib/test/test_support.py

@@ -400,6 +400,15 @@ def fcmp(x, y): # fuzzy comparison function
         return (len(x) > len(y)) - (len(x) < len(y))
     return (x > y) - (x < y)
 
+
+# A constant likely larger than the underlying OS pipe buffer size, to
+# make writes blocking.
+# Windows limit seems to be around 512 B, and many Unix kernels have a
+# 64 KiB pipe buffer size or 16 * PAGE_SIZE: take a few megs to be sure.
+# (see issue #17835 for a discussion of this number).
+PIPE_MAX_SIZE = 4 *1024 * 1024 + 1
+
+
 try:
     unicode
     have_unicode = True

Misc/NEWS

@@ -131,6 +131,9 @@ IDLE
 Tests
 -----
 
+- Issue #17835: Fix test_io when the default OS pipe buffer size is larger
+  than one million bytes.
+
 - Issue #17531: Fix tests that thought group and user ids were always the int
   type. Also, always allow -1 as a valid group and user id.