closes bpo-27805: Ignore ESPIPE in initializing seek of append-mode files. (GH-17112)

This change, which follows the behavior of C stdio's fdopen and Python 2's file object, allows pipes to be opened in append mode.

Lib/_pyio.py

@@ -1577,7 +1577,11 @@ class FileIO(RawIOBase):
                 # For consistent behaviour, we explicitly seek to the
                 # end of file (otherwise, it might be done only on the
                 # first write()).
-                os.lseek(fd, 0, SEEK_END)
+                try:
+                    os.lseek(fd, 0, SEEK_END)
+                except OSError as e:
+                    if e.errno != errno.ESPIPE:
+                        raise
         except:
             if owned_fd is not None:
                 os.close(owned_fd)

Lib/test/test_io.py

@@ -3906,6 +3906,17 @@ class MiscIOTest(unittest.TestCase):
                 self.open(support.TESTFN, mode)
             self.assertIn('invalid mode', str(cm.exception))
 
+    def test_open_pipe_with_append(self):
+        # bpo-27805: Ignore ESPIPE from lseek() in open().
+        r, w = os.pipe()
+        self.addCleanup(os.close, r)
+        f = self.open(w, 'a')
+        self.addCleanup(f.close)
+        # Check that the file is marked non-seekable. On Windows, however, lseek
+        # somehow succeeds on pipes.
+        if sys.platform != 'win32':
+            self.assertFalse(f.seekable())
+
     def test_io_after_close(self):
         for kwargs in [
                 {"mode": "w"},