[spack/spec.py] raise a query failure error if a property query returns None (#7277)

* [SPACK/spec.py] When a query through ForwardQueryToPackage returns
'None', treat that as query failure and raise RuntimeError with
suitable message. This overrides the current behavior to raise an
AttributeError which is now triggered only when no suitable query
property is found and there is no default handler.

* [spack/spec.py] Fix style.

* [SPACK/spec.py] In case of query failure, i.e. property returning
'None', raise AttributeError instead of RuntimeError in order to
pass the unit test. Also, small update in the logic distinguishing
query failure and lack of relevant property/attribute handling.

lib/spack/spack/spec.py

@@ -770,10 +770,6 @@ def __init__(self, attribute_name, default_handler=None):
                 instance
         """
         self.attribute_name = attribute_name
-        # Turn the default handler into a function with the right
-        # signature that always returns None
-        if default_handler is None:
-            default_handler = lambda descriptor, spec, cls: None
         self.default = default_handler
 
     def __get__(self, instance, cls):
@@ -792,8 +788,11 @@ def __get__(self, instance, cls):
 
         The first call that produces a value will stop the chain.
 
-        If no call can handle the request or a None value is produced,
-        then AttributeError is raised.
+        If no call can handle the request then AttributeError is raised with a
+        message indicating that no relevant attribute exists.
+        If a call returns None, an AttributeError is raised with a message
+        indicating a query failure, e.g. that library files were not found in a
+        'libs' query.
         """
         pkg = instance.package
         try:
@@ -814,34 +813,53 @@ def __get__(self, instance, cls):
         # Try to get the generic method from Package
         callbacks_chain.append(lambda: getattr(pkg, self.attribute_name))
         # Final resort : default callback
-        callbacks_chain.append(lambda: self.default(self, instance, cls))
+        if self.default is not None:
+            callbacks_chain.append(lambda: self.default(self, instance, cls))
 
         # Trigger the callbacks in order, the first one producing a
         # value wins
         value = None
+        message = None
         for f in callbacks_chain:
             try:
                 value = f()
+                # A callback can return None to trigger an error indicating
+                # that the query failed.
+                if value is None:
+                    msg  = "Query of package '{name}' for '{attrib}' failed\n"
+                    msg += "\tprefix : {spec.prefix}\n"
+                    msg += "\tspec : {spec}\n"
+                    msg += "\tqueried as : {query.name}\n"
+                    msg += "\textra parameters : {query.extra_parameters}"
+                    message = msg.format(
+                        name=pkg.name, attrib=self.attribute_name,
+                        spec=instance, query=instance.last_query)
+                else:
+                    return value
                 break
             except AttributeError:
                 pass
-        # 'None' value raises AttributeError : this permits to 'disable'
-        # the call in a particular package by returning None from the
-        # queried attribute, or will trigger an exception if  things
-        # searched for were not found
-        if value is None:
-            fmt = '\'{name}\' package has no relevant attribute \'{query}\'\n'  # NOQA: ignore=E501
-            fmt += '\tspec : \'{spec}\'\n'
-            fmt += '\tqueried as : \'{spec.last_query.name}\'\n'
-            fmt += '\textra parameters : \'{spec.last_query.extra_parameters}\'\n'  # NOQA: ignore=E501
-            message = fmt.format(
-                name=pkg.name,
-                query=self.attribute_name,
-                spec=instance
-            )
+        # value is 'None'
+        if message is not None:
+            # Here we can use another type of exception. If we do that, the
+            # unit test 'test_getitem_exceptional_paths' in the file
+            # lib/spack/spack/test/spec_dag.py will need to be updated to match
+            # the type.
             raise AttributeError(message)
-
-        return value
+        # 'None' value at this point means that there are no appropriate
+        # properties defined and no default handler, or that all callbacks
+        # raised AttributeError. In this case, we raise AttributeError with an
+        # appropriate message.
+        fmt = '\'{name}\' package has no relevant attribute \'{query}\'\n'  # NOQA: ignore=E501
+        fmt += '\tspec : \'{spec}\'\n'
+        fmt += '\tqueried as : \'{spec.last_query.name}\'\n'
+        fmt += '\textra parameters : \'{spec.last_query.extra_parameters}\'\n'  # NOQA: ignore=E501
+        message = fmt.format(
+            name=pkg.name,
+            query=self.attribute_name,
+            spec=instance
+        )
+        raise AttributeError(message)
 
     def __set__(self, instance, value):
         cls_name = type(instance).__name__