From 7642785881550bd71815623c4f93e0516217429c Mon Sep 17 00:00:00 2001
From: Ayke van Laethem <aykevanlaethem@gmail.com>
Date: Tue, 19 Dec 2017 23:47:07 +0100
Subject: [PATCH] extmod/vfs_fat_file: Implement SEEK_CUR for non-zero offset.

CPython doesn't allow SEEK_CUR with non-zero offset for files in text mode,
and uPy inherited this behaviour for both text and binary files.  It makes
sense to provide full support for SEEK_CUR of binary-mode files in uPy, and
to do this in a minimal way means also allowing to use SEEK_CUR with
non-zero offsets on text-mode files.  That seems to be a fair compromise.
---
 extmod/vfs_fat_file.c               | 6 +-----
 tests/extmod/vfs_fat_fileio1.py     | 6 ++----
 tests/extmod/vfs_fat_fileio1.py.exp | 2 +-
 3 files changed, 4 insertions(+), 10 deletions(-)

diff --git a/extmod/vfs_fat_file.c b/extmod/vfs_fat_file.c
index 1fcbb253dc..6154c8483f 100644
--- a/extmod/vfs_fat_file.c
+++ b/extmod/vfs_fat_file.c
@@ -134,11 +134,7 @@ STATIC mp_uint_t file_obj_ioctl(mp_obj_t o_in, mp_uint_t request, uintptr_t arg,
                 break;
 
             case 1: // SEEK_CUR
-                if (s->offset != 0) {
-                    *errcode = MP_EOPNOTSUPP;
-                    return MP_STREAM_ERROR;
-                }
-                // no-operation
+                f_lseek(&self->fp, f_tell(&self->fp) + s->offset);
                 break;
 
             case 2: // SEEK_END
diff --git a/tests/extmod/vfs_fat_fileio1.py b/tests/extmod/vfs_fat_fileio1.py
index d19df120b5..8b9ff92eb8 100644
--- a/tests/extmod/vfs_fat_fileio1.py
+++ b/tests/extmod/vfs_fat_fileio1.py
@@ -91,10 +91,8 @@ with open("foo_file.txt") as f2:
 
     f2.seek(0, 1) # SEEK_CUR
     print(f2.read(1))
-    try:
-        f2.seek(1, 1) # SEEK_END
-    except OSError as e:
-        print(e.args[0] == uerrno.EOPNOTSUPP)
+    f2.seek(2, 1) # SEEK_CUR
+    print(f2.read(1))
 
     f2.seek(-2, 2) # SEEK_END
     print(f2.read(1))
diff --git a/tests/extmod/vfs_fat_fileio1.py.exp b/tests/extmod/vfs_fat_fileio1.py.exp
index d777585cf7..a66f07605c 100644
--- a/tests/extmod/vfs_fat_fileio1.py.exp
+++ b/tests/extmod/vfs_fat_fileio1.py.exp
@@ -7,7 +7,7 @@ hello!world!
 12
 h
 e
-True
+o
 d
 True
 [('foo_dir', 16384, 0)]