coredata: Use our own implementation of commonpath

os.path.commonpath was added in Python 3.5, so just write our own for
now. pathlib was added in Python 3.4, so this should be ok. We need to
use that instead of doing str.split() etc because Windows path handling
has a lot of exceptions and pathlib handles all that for us.

Also adds a unit test for this.

mesonbuild/coredata.py

@@ -13,7 +13,8 @@
 # limitations under the License.
 
 import pickle, os, uuid
-from .mesonlib import MesonException, default_libdir, default_libexecdir, default_prefix
+from .mesonlib import MesonException, commonpath
+from .mesonlib import default_libdir, default_libexecdir, default_prefix
 
 version = '0.38.0.dev1'
 backendlist = ['ninja', 'vs2010', 'vs2015', 'xcode']
@@ -158,7 +159,7 @@ class CoreData:
         if option.endswith('dir') and os.path.isabs(value) and \
            option not in builtin_dir_noprefix_options:
             # Value must be a subdir of the prefix
-            if os.path.commonpath([value, prefix]) != prefix:
+            if commonpath([value, prefix]) != prefix:
                 m = 'The value of the {!r} option is {!r} which must be a ' \
                     'subdir of the prefix {!r}.\nNote that if you pass a ' \
                     'relative path, it is assumed to be a subdir of prefix.'

mesonbuild/mesonlib.py

@@ -496,3 +496,28 @@ def Popen_safe(args, write=None, stderr=subprocess.PIPE, **kwargs):
                          stderr=stderr, **kwargs)
     o, e = p.communicate(write)
     return p, o, e
+
+def commonpath(paths):
+    '''
+    For use on Python 3.4 where os.path.commonpath is not available.
+    We currently use it everywhere so this receives enough testing.
+    '''
+    # XXX: Replace me with os.path.commonpath when we start requiring Python 3.5
+    import pathlib
+    if not paths:
+        raise ValueError('arg is an empty sequence')
+    common = pathlib.PurePath(paths[0])
+    for path in paths[1:]:
+        new = []
+        path = pathlib.PurePath(path)
+        for c, p in zip(common.parts, path.parts):
+            if c != p:
+                break
+            new.append(c)
+        # Don't convert '' into '.'
+        if not new:
+            common = ''
+            break
+        new = os.path.join(*new)
+        common = pathlib.PurePath(new)
+    return str(common)

run_unittests.py

@@ -153,6 +153,23 @@ class InternalTests(unittest.TestCase):
         a = ['-Ldir', '-Lbah'] + a
         self.assertEqual(a, ['-Ibar', '-Ifoo', '-Ibaz', '-I..', '-I.', '-Ldir', '-Lbah', '-Werror', '-O3', '-O2', '-Wall'])
 
+    def test_commonpath(self):
+        from os.path import sep
+        commonpath = mesonbuild.mesonlib.commonpath
+        self.assertRaises(ValueError, commonpath, [])
+        self.assertEqual(commonpath(['/usr', '/usr']), sep + 'usr')
+        self.assertEqual(commonpath(['/usr', '/usr/']), sep + 'usr')
+        self.assertEqual(commonpath(['/usr', '/usr/bin']), sep + 'usr')
+        self.assertEqual(commonpath(['/usr/', '/usr/bin']), sep + 'usr')
+        self.assertEqual(commonpath(['/usr/./', '/usr/bin']), sep + 'usr')
+        self.assertEqual(commonpath(['/usr/bin', '/usr/bin']), sep + 'usr' + sep + 'bin')
+        self.assertEqual(commonpath(['/usr//bin', '/usr/bin']), sep + 'usr' + sep + 'bin')
+        self.assertEqual(commonpath(['/usr/./bin', '/usr/bin']), sep + 'usr' + sep + 'bin')
+        self.assertEqual(commonpath(['/usr/local', '/usr/lib']), sep + 'usr')
+        self.assertEqual(commonpath(['/usr', '/bin']), sep)
+        self.assertEqual(commonpath(['/usr', 'bin']), '')
+        self.assertEqual(commonpath(['blam', 'bin']), '')
+
 
 class LinuxlikeTests(unittest.TestCase):
     def setUp(self):