gnu: python-file: Work around "double encoding" bug in file@5.28.

* gnu/packages/patches/python-file-double-encoding-bug.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/python.scm (python-file)[source]: Use it.
(python2-file)[source]: Use the source of FILE directly, without the
patch.

gnu/local.mk

@@ -780,6 +780,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/python-3.5-fix-tests.patch		\
   %D%/packages/patches/python-dendropy-exclude-failing-tests.patch \
   %D%/packages/patches/python-django-fix-testcase.patch		\
+  %D%/packages/patches/python-file-double-encoding-bug.patch	\
   %D%/packages/patches/python-fix-tests.patch			\
   %D%/packages/patches/python-ipython-inputhook-ctype.patch	\
   %D%/packages/patches/python-rarfile-fix-tests.patch		\

gnu/packages/patches/python-file-double-encoding-bug.patch

@@ -0,0 +1,50 @@
+Fix bug that breaks file's Python bindings when using Python 3. This patch
+should not be applied when using Python 2.
+
+Copied from upstream source repository:
+
+https://github.com/file/file/commit/73e043d2a986234b187a00ed0c8d1f7bf83df372
+
+From 73e043d2a986234b187a00ed0c8d1f7bf83df372 Mon Sep 17 00:00:00 2001
+From: Christos Zoulas <christos@zoulas.com>
+Date: Tue, 28 Jun 2016 17:10:22 +0000
+Subject: [PATCH] PR/562: Reiner Herrmann: Avoid double encoding with python3
+
+---
+ python/magic.py | 6 +++---
+ 1 file changed, 3 insertions(+), 3 deletions(-)
+
+diff --git a/python/magic.py b/python/magic.py
+index c48f7d5..b0f7a17 100644
+--- a/python/magic.py
++++ b/python/magic.py
+@@ -134,7 +134,7 @@ class Magic(object):
+         if isinstance(r, str):
+             return r
+         else:
+-            return str(r).encode('utf-8')
++            return str(r, 'utf-8')
+ 
+     def descriptor(self, fd):
+         """
+@@ -152,7 +152,7 @@ class Magic(object):
+         if isinstance(r, str):
+             return r
+         else:
+-            return str(r).encode('utf-8')
++            return str(r, 'utf-8')
+ 
+     def error(self):
+         """
+@@ -163,7 +163,7 @@ class Magic(object):
+         if isinstance(e, str):
+             return e
+         else:
+-            return str(e).encode('utf-8')
++            return str(e, 'utf-8')
+ 
+     def setflags(self, flags):
+         """
+-- 
+2.10.1
+

gnu/packages/python.scm

@@ -6459,6 +6459,10 @@ Python's @code{ctypes} foreign function interface (FFI).")
   (package
     (inherit file)
     (name "python-file")
+    (source (origin
+              (inherit (package-source file))
+              ;; This patch should not be applied to python2-file.
+              (patches (search-patches "python-file-double-encoding-bug.patch"))))
     (build-system python-build-system)
     (arguments
      '(#:tests? #f                                ;no tests
@@ -6487,6 +6491,7 @@ serve the same purpose: provide Python bindings for libmagic.")
   (let ((base (package-with-python2 (strip-python2-variant python-file))))
     (package
       (inherit base)
+      (source (package-source file))
       (native-inputs
        `(("python2-setuptools" ,python2-setuptools)
          ,@(package-native-inputs base))))))