io_buffered_writer.py: Improve failed flush test.

Try flushing 4 times.

The first two times, the underlying IO object fails.
The third time, the data is actually flushed.
The last time, write is not called as there was no data left.

Signed-off-by: Jeff Epler <jepler@gmail.com>

tests/basics/io_buffered_writer.py

@@ -31,13 +31,24 @@ print(type(hash(buf)))
 
 # Test failing flush()
 class MyIO(io.IOBase):
+    def __init__(self):
+        self.count = 0
+
     def write(self, buf):
-        return None
+        self.count += 1
+        if self.count < 3:
+            return None
+        print("writing", buf)
+        return len(buf)
+
 
 buf = io.BufferedWriter(MyIO(), 8)
+
 buf.write(b"foobar")
-try:
-    buf.flush()
-    print("flushed")
-except OSError:
-    print("OSError")
+
+for _ in range(4):
+    try:
+        buf.flush()
+        print("flushed")
+    except OSError:
+        print("OSError")

tests/basics/io_buffered_writer.py.exp

@@ -5,3 +5,7 @@ b'foobarfoobar'
 b'foo'
 <class 'int'>
 OSError
+OSError
+writing bytearray(b'foobar')
+flushed
+flushed