bpo-32186: Release the GIL during lseek and fstat (GH-4652) (#4661)

In _io_FileIO_readall_impl(), lseek() and _Py_fstat_noraise() were called
without releasing the GIL. This can cause all threads to hang for
unlimited time when calling FileIO.read() and the NFS server is not
accessible.
(cherry picked from commit 6a89481)

Author: miss-islington

Misc/NEWS.d/next/Library/2017-11-30-20-38-16.bpo-32186.O42bVe.rst

@@ -0,0 +1,3 @@
+io.FileIO.readall() and io.FileIO.read() now release the GIL when
+getting the file size. Fixed hang of all threads with inaccessible NFS
+server.  Patch by Nir Soffer.

Modules/_io/fileio.c

@@ -691,19 +691,23 @@ _io_FileIO_readall_impl(fileio *self)
     Py_ssize_t bytes_read = 0;
     Py_ssize_t n;
     size_t bufsize;
+    int fstat_result;
 
     if (self->fd < 0)
         return err_closed();
 
+    Py_BEGIN_ALLOW_THREADS
     _Py_BEGIN_SUPPRESS_IPH
 #ifdef MS_WINDOWS
     pos = _lseeki64(self->fd, 0L, SEEK_CUR);
 #else
     pos = lseek(self->fd, 0L, SEEK_CUR);
 #endif
     _Py_END_SUPPRESS_IPH
+    fstat_result = _Py_fstat_noraise(self->fd, &status);
+    Py_END_ALLOW_THREADS
 
-    if (_Py_fstat_noraise(self->fd, &status) == 0)
+    if (fstat_result == 0)
         end = status.st_size;
     else
         end = (Py_off_t)-1;