Add 'disabler' argument to functions returning not-found objects

When dependency(), find_library(), find_program(), or
python.find_installation() return a not-found object and disabler is
true, they return a Disabler object instead.

docs/markdown/Python-module.md

@@ -48,6 +48,9 @@ Keyword arguments are the following:
   whether it was found or not. Since *0.48.0*  the value of a
   [`feature`](Build-options.md#features) option can also be passed to the
   `required` keyword argument.
+- `disabler`: if `true` and no python installation can be found, return a
+  [disabler object](#disabler-object) instead of a not-found object.
+  *Since 0.49.0*
 
 **Returns**: a [python installation][`python_installation` object]
 

docs/markdown/Reference-manual.md

@@ -408,6 +408,9 @@ otherwise. This function supports the following keyword arguments:
 [library-specific](Dependencies.md#dependencies-with-custom-lookup-functionality)
 keywords may also be accepted (e.g. `modules` specifies submodules to use for
 dependencies such as Qt5 or Boost. )
+- `disabler` if `true` and the dependency couldn't be found, return a
+  [disabler object](#disabler-object) instead of a not-found dependency.
+  *Since 0.49.0*
 
 If dependency_name is `''`, the dependency is always not found.  So with
 `required: false`, this always returns a dependency object for which the
@@ -600,6 +603,10 @@ Keyword arguments are the following:
   defined there, then from the system. If set to `true`, the cross
   file is ignored and the program is only searched from the system.
 
+- `disabler` if `true` and the program couldn't be found, return a
+  [disabler object](#disabler-object) instead of a not-found object.
+  *Since 0.49.0*
+
 Meson will also autodetect scripts with a shebang line and run them
 with the executable/interpreter specified in it both on Windows
 (because the command invocator will reject the command otherwise) and
@@ -1666,6 +1673,9 @@ the following methods:
   argument, which can be either a string or a list of strings.  Since
   *0.47.0* the value of a [`feature`](Build-options.md#features)
   option can also be passed to the `required` keyword argument.
+  *Since 0.49.0* if the keyword argument `disabler` is `true` and the
+  dependency couldn't be found, return a [disabler object](#disabler-object)
+  instead of a not-found dependency.
 
 - `first_supported_argument(list_of_strings)`, given a list of
   strings, returns the first argument that passes the `has_argument`

docs/markdown/snippets/disabler.md

@@ -0,0 +1,6 @@
+## Return `Disabler()` instead of not-found object
+
+Functions such as `dependency()`, `find_library()`, `find_program()`, and
+`python.find_installation()` have a new keyword argument: `disabler`. When set
+to `true` those functions return `Disabler()` objects instead of not-found
+objects.

mesonbuild/interpreter.py

@@ -27,7 +27,7 @@ from .dependencies import InternalDependency, Dependency, NotFoundDependency, De
 from .interpreterbase import InterpreterBase
 from .interpreterbase import check_stringlist, flatten, noPosargs, noKwargs, stringArgs, permittedKwargs, noArgsFlattening
 from .interpreterbase import InterpreterException, InvalidArguments, InvalidCode, SubdirDoneRequest
-from .interpreterbase import InterpreterObject, MutableInterpreterObject, Disabler
+from .interpreterbase import InterpreterObject, MutableInterpreterObject, Disabler, disablerIfNotFound
 from .interpreterbase import FeatureNew, FeatureDeprecated, FeatureNewKwargs
 from .interpreterbase import ObjectHolder
 from .modules import ModuleReturnValue
@@ -1381,6 +1381,8 @@ class CompilerHolder(InterpreterObject):
         mlog.log('Header <{0}> has symbol "{1}":'.format(hname, symbol), h)
         return haz
 
+    @FeatureNewKwargs('compiler.find_library', '0.49.0', ['disabler'])
+    @disablerIfNotFound
     @permittedKwargs({
         'required',
         'dirs',
@@ -2776,6 +2778,8 @@ external dependencies (including libraries) must go to "dependencies".''')
         self.store_name_lookups(args)
         return progobj
 
+    @FeatureNewKwargs('find_program', '0.49.0', ['disabler'])
+    @disablerIfNotFound
     @permittedKwargs(permitted_kwargs['find_program'])
     def func_find_program(self, node, args, kwargs):
         if not args:
@@ -2902,8 +2906,10 @@ external dependencies (including libraries) must go to "dependencies".''')
         elif name == 'openmp':
             FeatureNew('OpenMP Dependency', '0.46.0').use(self.subproject)
 
+    @FeatureNewKwargs('dependency', '0.49.0', ['disabler'])
     @FeatureNewKwargs('dependency', '0.40.0', ['method'])
     @FeatureNewKwargs('dependency', '0.38.0', ['default_options'])
+    @disablerIfNotFound
     @permittedKwargs(permitted_kwargs['dependency'])
     def func_dependency(self, node, args, kwargs):
         self.validate_arguments(args, 1, [str])

mesonbuild/interpreterbase.py

@@ -145,6 +145,17 @@ def noArgsFlattening(f):
     setattr(f, 'no-args-flattening', True)
     return f
 
+def disablerIfNotFound(f):
+    @wraps(f)
+    def wrapped(*wrapped_args, **wrapped_kwargs):
+        kwargs = _get_callee_args(wrapped_args)[3]
+        disabler = kwargs.pop('disabler', False)
+        ret = f(*wrapped_args, **wrapped_kwargs)
+        if disabler and not ret.held_object.found():
+            return Disabler()
+        return ret
+    return wrapped
+
 class permittedKwargs:
 
     def __init__(self, permitted):

mesonbuild/modules/python.py

@@ -23,7 +23,7 @@ from mesonbuild.modules import ModuleReturnValue
 from ..interpreterbase import (
     noPosargs, noKwargs, permittedKwargs,
     InterpreterObject, InvalidArguments,
-    FeatureNew
+    FeatureNew, FeatureNewKwargs, disablerIfNotFound
 )
 from ..interpreter import ExternalProgramHolder, extract_required_kwarg
 from ..interpreterbase import flatten
@@ -467,6 +467,8 @@ class PythonModule(ExtensionModule):
             return mesonlib.version_compare(version, '>= 3.0')
         return True
 
+    @FeatureNewKwargs('python.find_installation', '0.49.0', ['disabler'])
+    @disablerIfNotFound
     @permittedKwargs(['required'])
     def find_installation(self, interpreter, state, args, kwargs):
         feature_check = FeatureNew('Passing "feature" option to find_installation', '0.48.0')

test cases/common/164 disabler/meson.build

@@ -31,4 +31,12 @@ endif
 
 assert(number == 2, 'If found handled incorrectly, value should be 2 but is @0@'.format(number))
 
+dep = dependency('notfounddep', required : false, disabler : true)
+app = executable('myapp', 'notfound.c', dependencies : [dep])
 
+cc = meson.get_compiler('c')
+dep = cc.find_library('notfounddep', required : false, disabler : true)
+app = executable('myapp', 'notfound.c', dependencies : [dep])
+
+dep = find_program('donotfindme', required : false, disabler : true)
+app = executable('myapp', 'notfound.c', dependencies : [dep])