Only return separate encoding if `program_supports_compression`

Why:

* To prevent archives such as archive.rar.gz from breaking patool.

This change addresses the need by:

* move program_supports_compression above the `import util` line
    * If we don't do this, we have a circular import.
* Import program_supports_compression into util.
* Only return separate mime and encoding if program_support_compression
  returns True.
* Otherwise return the mime of the encoding and no encoding.
* Add fixes so that current tests still pass.
    * In program_support_compression:
        - Test tar separately and check all supported tar wrappers.
    * In test_tar:
        - Add file requirement for compress in the compress check.
        - Otherwise tar tries and failes when compress isn't installed.

patoolib/__init__.py

@@ -25,7 +25,7 @@ import importlib
 from .configuration import App, Version as __version__
 __all__ = ['list_formats', 'list_archive', 'extract_archive', 'test_archive',
     'create_archive', 'diff_archives', 'search_archive', 'repack_archive',
-    'recompress_archive']
+    'recompress_archive', 'program_supports_compression']
 
 
 # Supported archive commands
@@ -284,6 +284,18 @@ ProgramModules = {
 }
 
 
+def program_supports_compression (program, compression):
+    """Decide if the given program supports the compression natively.
+    @return: True iff the program supports the given compression format
+      natively, else False.
+    """
+    if program in ('tar', ):
+        return compression in ('gzip', 'bzip2', 'xz', 'lzip', 'compress', 'lzma') + py_lzma
+    elif program in ('star', 'bsdtar', 'py_tarfile'):
+        return compression in ('gzip', 'bzip2') + py_lzma
+    return False
+
+
 from . import util
 
 def get_archive_format (filename):
@@ -336,16 +348,6 @@ def find_archive_program (format, command, program=None):
     raise util.PatoolError("could not find an executable program to %s format %s; candidates are (%s)," % (command, format, ",".join(programs)))
 
 
-def program_supports_compression (program, compression):
-    """Decide if the given program supports the compression natively.
-    @return: True iff the program supports the given compression format
-      natively, else False.
-    """
-    if program in ('tar', 'star', 'bsdtar', 'py_tarfile'):
-        return compression in ('gzip', 'bzip2') + py_lzma
-    return False
-
-
 def list_formats ():
     """Print information about available archive formats to stdout."""
     print("Archive programs of", App)

patoolib/util.py

@@ -24,7 +24,7 @@ import tempfile
 import time
 import traceback
 import locale
-from . import configuration, ArchiveMimetypes, ArchiveCompressions
+from . import configuration, ArchiveMimetypes, ArchiveCompressions, program_supports_compression
 try:
     from shutil import which
 except ImportError:
@@ -314,7 +314,12 @@ def guess_mime_file (filename):
         elif mime2 in ArchiveMimetypes:
             mime = mime2
             encoding = get_file_mime_encoding(outparts)
+    # Only return mime and encoding if the given mime can natively support the encoding.
+    if program_supports_compression(ArchiveMimetypes.get(mime), encoding):
         return mime, encoding
+    else:
+        # If encoding is None, default back to `mime`.
+        return Encoding2Mime.get(encoding, mime), None
 
 
 def guess_mime_file_mime (file_prog, filename):

tests/archives/test_tar.py

@@ -69,6 +69,7 @@ class TestTar (ArchiveTest):
         self.archive_commands('t.tgz.foo', skip_create=True)
 
     @needs_program('file')
+    @needs_program('compress')
     @needs_codec(program, 'compress')
     def test_tar_z_file (self):
         self.archive_commands('t.tar.Z.foo', skip_create=True)

tests/test_mime.py

@@ -138,6 +138,7 @@ class TestMime (unittest.TestCase):
         self.mime_test_file("t.zpaq", "application/zpaq")
         self.mime_test_file("t.zpaq.foo", "application/zpaq")
 
+
     @needs_program('file')
     @needs_program('lzip')
     def test_mime_file_lzip (self):