bitbake: parse/ast: add support for 'built-in' fragments

When reviewing proposed fragments to add settings for DISTRO and MACHINE,
RP noted that such fragments only add clutter and overhead, and there's
no need to maintain them as separate files.

Rather when bitbake sees 'fragmentvar/fragmentvalue' it can expand that into
FRAGMENTVAR = "fragmentvalue".

To achieve that, 'addfragments' directive is extended with a parameter
that sets the name of the variable that holds definitions of such
built-in fragments, for example like this:

"machine:MACHINE distro:DISTRO"

Then each enabled fragment name is matched against these definitions and the
respective variable is set, e.g. 'machine/qemuarm' would match
'machine:MACHINE' and result in MACHINE set to 'qemuarm'.

This happens before any fragment files are looked up on disk,
and no such lookup happens if there was a match, which should prevent
possible misuse of the feature. So the builtin fragment definition
is also an allowlist for them.

Please also see the patches for oe-core that show an application of the feature.

(Bitbake rev: 3b9d7bea915dc7f10e845854f1dae325743f9456)

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/doc/bitbake-user-manual/bitbake-user-manual-metadata.rst

@@ -998,9 +998,9 @@ This directive allows fine-tuning local configurations with configuration
 snippets contained in layers in a structured, controlled way. Typically it would
 go into ``bitbake.conf``, for example::
 
-   addfragments conf/fragments OE_FRAGMENTS OE_FRAGMENTS_METADATA_VARS
+   addfragments conf/fragments OE_FRAGMENTS OE_FRAGMENTS_METADATA_VARS OE_BUILTIN_FRAGMENTS
 
-``addfragments`` takes three parameters:
+``addfragments`` takes four parameters:
 
 -  path prefix for fragment files inside the layer file tree that bitbake
    uses to construct full paths to the fragment files
@@ -1011,6 +1011,8 @@ go into ``bitbake.conf``, for example::
 -  name of variable that contains a list of variable names containing
    fragment-specific metadata (such as descriptions)
 
+-  name of variable that contains definitions for built-in fragments
+
 This allows listing enabled configuration fragments in ``OE_FRAGMENTS``
 variable like this::
 
@@ -1035,6 +1037,19 @@ The implementation will add a flag containing the fragment name to each of those
 when parsing fragments, so that the variables are namespaced by fragment name, and do not override
 each other when several fragments are enabled.
 
+The variable containing a built-in fragment definitions could look like this::
+
+   OE_BUILTIN_FRAGMENTS = "someprefix:SOMEVARIABLE anotherprefix:ANOTHERVARIABLE"
+
+and then if 'someprefix/somevalue' is added to the variable that holds the list
+of enabled fragments:
+
+  OE_FRAGMENTS = "... someprefix/somevalue"
+
+bitbake will treat that as direct value assignment in its configuration::
+
+  SOMEVARIABLE = "somevalue"
+
 Functions
 =========
 

bitbake/lib/bb/parse/ast.py

@@ -343,11 +343,12 @@ class InheritDeferredNode(AstNode):
         bb.parse.BBHandler.inherit_defer(*self.inherit, data)
 
 class AddFragmentsNode(AstNode):
-    def __init__(self, filename, lineno, fragments_path_prefix, fragments_variable, flagged_variables_list_variable):
+    def __init__(self, filename, lineno, fragments_path_prefix, fragments_variable, flagged_variables_list_variable, builtin_fragments_variable):
         AstNode.__init__(self, filename, lineno)
         self.fragments_path_prefix = fragments_path_prefix
         self.fragments_variable = fragments_variable
         self.flagged_variables_list_variable = flagged_variables_list_variable
+        self.builtin_fragments_variable = builtin_fragments_variable
 
     def eval(self, data):
         # No need to use mark_dependency since we would only match a fragment
@@ -360,13 +361,23 @@ class AddFragmentsNode(AstNode):
                    return candidate_fragment_path
            return None
 
+        def check_and_set_builtin_fragment(fragment, data, builtin_fragments):
+            prefix, value = fragment.split('/', 1)
+            if prefix in builtin_fragments.keys():
+                data.setVar(builtin_fragments[prefix], value)
+                return True
+            return False
+
         fragments = data.getVar(self.fragments_variable)
         layers = data.getVar('BBLAYERS')
         flagged_variables = data.getVar(self.flagged_variables_list_variable).split()
+        builtin_fragments = {f[0]:f[1] for f in [f.split(':') for f in data.getVar(self.builtin_fragments_variable).split()] }
 
         if not fragments:
             return
         for f in fragments.split():
+            if check_and_set_builtin_fragment(f, data, builtin_fragments):
+                continue
             layerid, fragment_name = f.split('/', 1)
             full_fragment_name = data.expand("{}/{}.conf".format(self.fragments_path_prefix, fragment_name))
             fragment_path = find_fragment(layers, layerid, full_fragment_name)
@@ -430,7 +441,8 @@ def handleAddFragments(statements, filename, lineno, m):
     fragments_path_prefix = m.group(1)
     fragments_variable = m.group(2)
     flagged_variables_list_variable = m.group(3)
-    statements.append(AddFragmentsNode(filename, lineno, fragments_path_prefix, fragments_variable, flagged_variables_list_variable))
+    builtin_fragments_variable = m.group(4)
+    statements.append(AddFragmentsNode(filename, lineno, fragments_path_prefix, fragments_variable, flagged_variables_list_variable, builtin_fragments_variable))
 
 def runAnonFuncs(d):
     code = []

bitbake/lib/bb/parse/parse_py/ConfHandler.py

@@ -48,7 +48,7 @@ __export_regexp__ = re.compile( r"export\s+([a-zA-Z0-9\-_+.${}/~]+)$" )
 __unset_regexp__ = re.compile( r"unset\s+([a-zA-Z0-9\-_+.${}/~]+)$" )
 __unset_flag_regexp__ = re.compile( r"unset\s+([a-zA-Z0-9\-_+.${}/~]+)\[([a-zA-Z0-9\-_+.][a-zA-Z0-9\-_+.@]+)\]$" )
 __addpylib_regexp__      = re.compile(r"addpylib\s+(.+)\s+(.+)" )
-__addfragments_regexp__  = re.compile(r"addfragments\s+(.+)\s+(.+)\s+(.+)" )
+__addfragments_regexp__  = re.compile(r"addfragments\s+(.+)\s+(.+)\s+(.+)\s+(.+)" )
 
 def init(data):
     return