dtlib: allow dangling aliases with DT(..., force=True)

As a first step towards being more forgiving on invalid inputs, allow
string-valued aliases properties that do not point to valid nodes when
the user requests permissiveness.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>

scripts/dts/python-devicetree/src/devicetree/dtlib.py

@@ -1684,8 +1684,15 @@ class DT:
                     _err("/aliases: alias property name '{}' should include "
                          "only characters from [0-9a-z-]".format(prop.name))
 
-                # Property.to_path() already checks that the node exists
+                # Property.to_path() checks that the node exists, has
-                alias2node[prop.name] = prop.to_path()
+                # the right type, etc. Swallow errors for invalid
+                # aliases with self._force.
+                try:
+                    alias2node[prop.name] = prop.to_path()
+                except DTError:
+                    if self._force:
+                        continue
+                    raise
 
         self.alias2node = alias2node
 

scripts/dts/python-devicetree/tests/test_dtlib.py

@@ -24,13 +24,15 @@ from devicetree import dtlib
 #   - to run a particular test function or functions, use
 #     '-k test_function_pattern_goes_here'
 
-def parse(dts, include_path=()):
+def parse(dts, include_path=(), **kwargs):
-    '''Parse a DTS string 'dts', using the given include path.'''
+    '''Parse a DTS string 'dts', using the given include path.
+
+    Any kwargs are passed on to DT().'''
 
     fd, path = tempfile.mkstemp(prefix='pytest-', suffix='.dts')
     try:
         os.write(fd, dts.encode('utf-8'))
-        return dtlib.DT(path, include_path)
+        return dtlib.DT(path, include_path, **kwargs)
     finally:
         os.close(fd)
         os.unlink(path)
@@ -2250,3 +2252,13 @@ l1: l2: &foo {
 / {
 };
 """)
+
+def test_dangling_alias():
+    dt = parse('''
+/dts-v1/;
+
+/ {
+	aliases { foo = "/missing"; };
+};
+''', force=True)
+    assert dt.get_node('/aliases').props['foo'].to_string() == '/missing'