[3.6] bpo-32228: Reset raw_pos after unwinding the raw stream (GH-4858) (#5389)

Ensure that ``truncate()`` preserves the file position (as reported by ``tell()``) after writes longer than the buffer size..
(cherry picked from commit 059f58ce938d9c3f0286412a4efb1b9131339421)

1 files changed, 17 insertions, 0 deletions

diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 42dbbd3..6c30a44 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -1703,6 +1703,23 @@ class BufferedWriterTest(unittest.TestCase, CommonBufferedTests):
         with self.open(support.TESTFN, "rb", buffering=0) as f:
             self.assertEqual(f.read(), b"abc")
 
+    def test_truncate_after_write(self):
+        # Ensure that truncate preserves the file position after
+        # writes longer than the buffer size.
+        # Issue: https://bugs.python.org/issue32228
+        with self.open(support.TESTFN, "wb") as f:
+            # Fill with some buffer
+            f.write(b'\x00' * 10000)
+        buffer_sizes = [8192, 4096, 200]
+        for buffer_size in buffer_sizes:
+            with self.open(support.TESTFN, "r+b", buffering=buffer_size) as f:
+                f.write(b'\x00' * (buffer_size + 1))
+                # After write write_pos and write_end are set to 0
+                f.read(1)
+                # read operation makes sure that pos != raw_pos
+                f.truncate()
+                self.assertEqual(f.tell(), buffer_size + 2)
+
     @unittest.skipUnless(threading, 'Threading required for this test.')
     @support.requires_resource('cpu')
     def test_threads(self):