failover to python-magic detection if vendored library not present

This allows scancode to work on platforms not supported by typecode-libmagic
e.g. M1 Macs or windows WSL

Most of this code is from https://github.com/ahupp/python-magic/blob/e0ccc6d/magic/loader.py
by Adam Hupp, MIT License, compatible with typecode's Apache

Signed-off-by: Pierre Tardy <pierre.tardy@renault.com>

Author: tardyp

src/typecode/magic2.py

@@ -43,6 +43,7 @@
 
 import ctypes
 import os
+import warnings
 
 from commoncode import command
 from plugincode.location_provider import get_location
@@ -79,18 +80,59 @@
 TYPECODE_LIBMAGIC_DATABASE = 'typecode.libmagic.db'
 
 
+def load_lib_failover():
+  # loader from python-magic
+  libmagic = None
+  # Let's try to find magic or magic1
+  dll = (ctypes.util.find_library('magic')
+        or ctypes.util.find_library('magic1')
+        or ctypes.util.find_library('cygmagic-1')
+        or ctypes.util.find_library('libmagic-1')
+        or ctypes.util.find_library('msys-magic-1')  # for MSYS2
+  )
+  # necessary because find_library returns None if it doesn't find the library
+  if dll:
+      libmagic = ctypes.CDLL(dll)
+
+  if not libmagic or not libmagic._name:
+      windows_dlls = ['magic1.dll', 'cygmagic-1.dll', 'libmagic-1.dll', 'msys-magic-1.dll']
+      platform_to_lib = {'darwin': ['/opt/local/lib/libmagic.dylib',
+                                    '/usr/local/lib/libmagic.dylib'] +
+                                  # Assumes there will only be one version installed
+                                  glob.glob('/usr/local/Cellar/libmagic/*/lib/libmagic.dylib'),  # flake8:noqa
+                        'win32': windows_dlls,
+                        'cygwin': windows_dlls,
+                        'linux': ['libmagic.so.1'],
+                        # fallback for some Linuxes (e.g. Alpine) where library search does not work # flake8:noqa
+                        }
+      platform = 'linux' if sys.platform.startswith('linux') else sys.platform
+      for dll in platform_to_lib.get(platform, []):
+          try:
+              libmagic = ctypes.CDLL(dll)
+              break
+          except OSError:
+              pass
+
+  if not libmagic or not libmagic._name:
+      return None
+  return libmagic
+
 def load_lib():
     """
     Return the loaded libmagic shared library object from plugin-provided path.
     """
     dll = get_location(TYPECODE_LIBMAGIC_DLL)
     libdir = get_location(TYPECODE_LIBMAGIC_LIBDIR)
     if not (dll and libdir) or not os.path.isfile(dll) or not os.path.isdir(libdir):
-        raise Exception(
-            'CRITICAL: libmagic DLL and is magic database are not installed. '
-            'Unable to continue: you need to install a valid typecode-libmagic '
-            'plugin with a valid and proper libmagic and magic DB available.'
-    )
+        ret = load_lib_failover()
+        if ret is None:
+            raise ImportError(
+                'CRITICAL: libmagic DLL and is magic database are not installed. '
+                'Unable to continue: you need to install a valid typecode-libmagic '
+                'plugin with a valid and proper libmagic and magic DB available.'
+        )
+        warnings.warn("System libmagic is used. Install typecode-libmagic for best consitency")
+        return ret
     return command.load_shared_library(dll, libdir)
 
 
@@ -164,6 +206,7 @@ def __init__(self, flags, magic_db_location=None):
         self.flags = flags
         self.cookie = _magic_open(self.flags)
         if not magic_db_location:
+            # if no plugin, None is returned, and libmagic will load the default db
             magic_db_location = get_location(TYPECODE_LIBMAGIC_DATABASE)
 
         # Note: this location must always be bytes on Python2 and 3, all OSes
@@ -257,3 +300,9 @@ def check_error(result, func, args):  # NOQA
 _magic_load.restype = ctypes.c_int
 _magic_load.argtypes = [ctypes.c_void_p, ctypes.c_char_p]
 _magic_load.errcheck = check_error
+
+_magic_version = libmagic.magic_version
+_magic_version.restype = ctypes.c_int
+_magic_version.argtypes = []
+
+libmagic_version = _magic_version()

tests/test_libmagic_load.py

@@ -0,0 +1,27 @@
+#
+# Copyright (c) nexB Inc. and others.
+# SPDX-License-Identifier: Apache-2.0
+#
+# Visit https://aboutcode.org and https://github.com/nexB/ for support and download.
+# ScanCode is a trademark of nexB Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+import os
+
+from typecode.magic2 import libmagic_version
+
+
+def test_load_lib():
+    assert libmagic_version > 0