py/modio: Fix the case where write fails in BufferedWriter.flush.

Previously, there was no test coverage of the "write failed" path.  In
fact, the assertion would fire instead of gracefully raising a Python
exception.

Slightly re-organize the code to place the assertion later.  Add a test
case which exercises all paths, and update the expected output.

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

tests/basics/io_buffered_writer.py

@@ -28,3 +28,27 @@ print(bts.getvalue())
 
 # hashing a BufferedWriter
 print(type(hash(buf)))
+
+# Test failing flush()
+class MyIO(io.IOBase):
+    def __init__(self):
+        self.count = 0
+
+    def write(self, buf):
+        self.count += 1
+        if self.count < 3:
+            return None
+        print("writing", buf)
+        return len(buf)
+
+
+buf = io.BufferedWriter(MyIO(), 8)
+
+buf.write(b"foobar")
+
+for _ in range(4):
+    try:
+        buf.flush()
+        print("flushed")
+    except OSError:
+        print("OSError")

tests/basics/io_buffered_writer.py.exp

@@ -4,3 +4,8 @@ b'foobarfoobar'
 b'foobarfoobar'
 b'foo'
 <class 'int'>
+OSError
+OSError
+writing bytearray(b'foobar')
+flushed
+flushed