app: config: Add support for appending to the config string

In some cases, and specifically in the manifest.group-filter and
manifest.project-filter options, it is sometimes useful to be able to
append to a value instead of replacing it completely.

For example, assuming one wants to add to an existing group filter,
without this patch the user needs to do:

(assuming the group filter is currently +unstable,-optional, and the
user wants to add +extras).
> west config manifest.group-filter
> west config manifest.group-filter +unstable,-optional,+extras

With this patch instead:

> west config -a manifest.group-filter ,+extras

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

src/west/app/config.py

@@ -48,6 +48,15 @@
 To set a value for <name>, type:
     west config <name> <value>
 
+To append to a value for <name>, type:
+    west config -a <name> <value>
+A value must exist in the selected configuration file in order to be able
+to append to it. The existing value can be empty.
+Examples:
+    west config -a build.cmake-args -- "-DCMAKE_EXPORT_COMPILE_COMMANDS=ON \
+            -DCMAKE_VERBOSE_MAKEFILE=ON"
+    west config -a manifest.group-filter ,+optional
+
 To list all options and their values:
     west config -l
 
@@ -64,7 +73,7 @@
 
 CONFIG_EPILOG = '''\
 If the configuration file to use is not set, reads use all three in
-precedence order, and writes use the local file.'''
+precedence order, and writes (including appends) use the local file.'''
 
 ALL = ConfigFile.ALL
 SYSTEM = ConfigFile.SYSTEM
@@ -92,13 +101,14 @@ def do_add_parser(self, parser_adder):
             "action to perform (give at most one)"
         ).add_mutually_exclusive_group()
 
-
         group.add_argument('-l', '--list', action='store_true',
-                            help='list all options and their values')
+                           help='list all options and their values')
         group.add_argument('-d', '--delete', action='store_true',
-                            help='delete an option in one config file')
+                           help='delete an option in one config file')
         group.add_argument('-D', '--delete-all', action='store_true',
-                            help="delete an option everywhere it's set")
+                           help="delete an option everywhere it's set")
+        group.add_argument('-a', '--append', action='store_true',
+                           help='append to an existing value')
 
         group = parser.add_argument_group(
             "configuration file to use (give at most one)"
@@ -129,13 +139,17 @@ def do_run(self, args, user_args):
         elif not args.name:
             self.parser.error('missing argument name '
                               '(to list all options and values, use -l)')
+        elif args.value is None and args.append:
+            self.parser.error('-a requires both name and value')
 
         if args.list:
             self.list(args)
         elif delete:
             self.delete(args)
         elif args.value is None:
             self.read(args)
+        elif args.append:
+            self.append(args)
         else:
             self.write(args)
 
@@ -180,6 +194,16 @@ def read(self, args):
             self.dbg(f'{args.name} is unset')
             raise CommandError(returncode=1)
 
+    def append(self, args):
+        self.check_config(args.name)
+        where = args.configfile or LOCAL
+        value = self.config.get(args.name, configfile=where)
+        if value is None:
+            self.die(f'option {args.name} not found in the {where.name.lower()} '
+                     'configuration file')
+        args.value = value + args.value
+        self.write(args)
+
     def write(self, args):
         self.check_config(args.name)
         what = args.configfile or LOCAL

tests/test_config.py

@@ -229,6 +229,10 @@ def test_local_creation_with_topdir():
     assert 'pytest' not in cfg(f=GLOBAL)
     assert cfg(f=LOCAL, topdir=str(topdir))['pytest']['key'] == 'val'
 
+def test_append_basic():
+    pass
+
+
 def test_delete_basic():
     # Basic deletion test: write local, verify global and system deletions
     # don't work, then delete local does work.