Cast OSError raised by os.lseek into IOError so we're consistent with Python 2.x stdlib

src/gevent/_fileobjectposix.py

 from __future__ import absolute_import
 import os
+import sys
 import io
 from io import BufferedReader
 from io import BufferedWriter
@@ -8,6 +9,7 @@ from io import DEFAULT_BUFFER_SIZE
 from io import RawIOBase
 from io import UnsupportedOperation
 
+from gevent._compat import PY3, reraise
 from gevent._fileobjectcommon import cancel_wait_ex
 from gevent._fileobjectcommon import FileObjectBase
 from gevent.hub import get_hub
@@ -140,7 +142,16 @@ class GreenFileDescriptorIO(RawIOBase):
             self.hub.wait(self._write_event)
 
     def seek(self, offset, whence=0):
-        return os.lseek(self._fileno, offset, whence)
+        try:
+            return os.lseek(self._fileno, offset, whence)
+        except OSError as ex:
+            if not PY3:
+                # Python 2.x
+                # make sure on Python 2.x we raise an IOError
+                exc_info = sys.exc_info()
+                reraise(IOError, IOError(*ex.args), tb=exc_info[2])
+            # otherwise just re-raise the original exception
+            raise
 
 class FlushingBufferedWriter(BufferedWriter):
 

src/gevent/tests/test__fileobject.py

@@ -13,6 +13,7 @@ from gevent.testing.sysinfo import PY3
 from gevent.testing.flaky import reraiseFlakyTestRaceConditionLibuv
 from gevent.testing.skipping import skipOnLibuvOnCIOnPyPy
 from gevent.testing.skipping import skipOnWindows
+from gevent.testing.skipping import skipOnPy37
 
 try:
     ResourceWarning
@@ -144,6 +145,25 @@ class Test(greentest.TestCase):
         self.assertEqual(native_data, s)
         self.assertEqual(native_data, fileobj_data)
 
+    @skipOnPy37
+    @skipOnWindows("FileObject not used on Win32")
+    def test_seek_raises_ioerror(self):
+        # Issue #1323
+        fd = sys.stdout
+        with self.assertRaises(OSError) as ctx:
+            os.lseek(fd.fileno(), 0, 0)
+        os_ex = ctx.exception
+
+        with self.assertRaises(IOError) as ctx:
+            f = FileObject(fd, 'r')
+            f.seek(0)
+        io_ex = ctx.exception
+
+        self.assertEqual(io_ex.args, os_ex.args)
+        self.assertEqual(io_ex.errno, os_ex.errno)
+        self.assertEqual(io_ex.strerror, os_ex.strerror)
+        self.assertEqual(str(io_ex), str(os_ex))
+
     def test_close_pipe(self):
         # Issue #190, 203
         r, w = os.pipe()