Implement failover with libraries found in the system

This allows scancode to install and run in systems not supported by
extractcode-libarchive and extractcode-7zip

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

Author: tardyp

src/extractcode/libarchive2.py

@@ -24,6 +24,7 @@
 import mmap
 import os
 
+import ctypes
 from ctypes import c_char_p, c_wchar_p
 from ctypes import c_int, c_longlong
 from ctypes import c_size_t, c_ssize_t
@@ -85,6 +86,10 @@
 # keys for plugin-provided locations
 EXTRACTCODE_LIBARCHIVE_LIBDIR = 'extractcode.libarchive.libdir'
 EXTRACTCODE_LIBARCHIVE_DLL = 'extractcode.libarchive.dll'
+_LIBRARY_NAME = "libarchive"
+
+def load_lib_failover():
+    pass
 
 
 def load_lib():
@@ -96,11 +101,16 @@ def load_lib():
     dll = get_location(EXTRACTCODE_LIBARCHIVE_DLL)
     libdir = get_location(EXTRACTCODE_LIBARCHIVE_LIBDIR)
     if not (dll and libdir) or not os.path.isfile(dll) or not os.path.isdir(libdir):
-        raise Exception(
-            'CRITICAL: libarchive DLL is not installed. '
-            'Unable to continue: you need to install a valid extractcode-libarchive '
-            'plugin with a valid libarchive DLL available.'
-    )
+        filepath = ctypes.util.find_library(_LIBRARY_NAME)
+        libarchive = ctypes.cdll.LoadLibrary(filepath)
+        if libarchive is None:
+            raise ImportError(
+                'CRITICAL: libarchive DLL is not installed. '
+                'Unable to continue: you need to install a valid extractcode-libarchive '
+                'plugin with a valid libarchive DLL available '
+                'or to have "libarchive" library installed in your system.'
+        )
+        return libarchive
     return command.load_shared_library(dll, libdir)
 
 

src/extractcode/sevenzip.py

@@ -24,6 +24,7 @@
 import os
 import pprint
 import re
+from shutil import which
 
 import attr
 
@@ -80,11 +81,15 @@ def get_bin_locations():
     cmd_loc = get_location(EXTRACTCODE_7ZIP_EXE)
     libdir = get_location(EXTRACTCODE_7ZIP_LIBDIR)
     if not (cmd_loc and libdir) or not os.path.isfile(cmd_loc) or not os.path.isdir(libdir):
-        raise Exception(
-            'CRITICAL: 7zip executable is not installed. '
-            'Unable to continue: you need to install a valid extractcode-7z '
-            'plugin with a valid executable available.'
-    )
+        sevenzip = which("7z")
+        if sevenzip is None:
+            raise ImportError(
+                'CRITICAL: 7zip executable is not installed. '
+                'Unable to continue: you need to install a valid extractcode-7z '
+                'plugin with a valid executable available '
+                'or install p7zip in your system, so that a "7z" program is available on your PATH'
+            )
+        return "", sevenzip
 
     return libdir, cmd_loc