find_program: Fallback if a wrap file provide the program name

We don't need the legacy variable name system as for dependency()
fallbacks because meson.override_find_program() is largely used already,
so we can just rely on it.

docs/markdown/Wrap-dependency-system-manual.md

@@ -71,6 +71,7 @@ revision = head
 - `directory` - name of the subproject root directory, defaults to the name of the wrap.
 
 Since *0.55.0* those can be used in all wrap types, they were previously reserved to `wrap-file`:
+
 - `patch_url` - download url to retrieve an optional overlay archive
 - `patch_fallback_url` - fallback URL to be used when download from `patch_url` fails *Since: 0.55.0*
 - `patch_filename` - filename of the downloaded overlay archive
@@ -182,6 +183,18 @@ dependency_names = glib-2.0, gobject-2.0, gio-2.0
 With such wrap file, `dependency('glib-2.0')` will automatically fallback to use
 `glib.wrap` and return `glib_dep` variable from the subproject.
 
+Programs can also be provided by wrap files, with the `program_names` key:
+```ini
+[wrap-git]
+...
+
+[provide]
+program_names = myprog, otherprog
+```
+
+With such wrap file, `find_program('myprog')` will automatically fallback to use
+the subproject, assuming it uses `meson.override_find_program('myprog')`.
+
 ## Using wrapped projects
 
 Wraps provide a convenient way of obtaining a project into your subproject directory.

docs/markdown/snippets/implicit_fallback.md

@@ -14,3 +14,8 @@ Wrap files can define the dependencies it provides in the `[provide]` section.
 When a wrap file provides the dependency `foo` any call do `dependency('foo')`
 will automatically fallback to that subproject even if no `fallback` keyword
 argument is given. See [Wrap documentation](Wrap-dependency-system-manual.md#provide_section).
+
+## `find_program()` fallback
+
+When a program cannot be found on the system but a wrap file has its name in the
+`[provide]` section, that subproject will be used as fallback.

mesonbuild/interpreter.py

@@ -3251,7 +3251,7 @@ external dependencies (including libraries) must go to "dependencies".''')
 
         return success
 
-    def program_from_file_for(self, for_machine, prognames, silent):
+    def program_from_file_for(self, for_machine, prognames):
         for p in unholder(prognames):
             if isinstance(p, mesonlib.File):
                 continue # Always points to a local (i.e. self generated) file.
@@ -3290,15 +3290,13 @@ external dependencies (including libraries) must go to "dependencies".''')
             if progobj.found():
                 return progobj
 
-    def program_from_overrides(self, command_names, silent=False):
+    def program_from_overrides(self, command_names, extra_info):
         for name in command_names:
             if not isinstance(name, str):
                 continue
             if name in self.build.find_overrides:
                 exe = self.build.find_overrides[name]
-                if not silent:
-                    mlog.log('Program', mlog.bold(name), 'found:', mlog.green('YES'),
-                             '(overridden: %s)' % exe.description())
+                extra_info.append(mlog.blue('(overriden)'))
                 return ExternalProgramHolder(exe, self.subproject, self.backend)
         return None
 
@@ -3316,40 +3314,75 @@ external dependencies (including libraries) must go to "dependencies".''')
                                        % name)
         self.build.find_overrides[name] = exe
 
+    def notfound_program(self, args):
+        return ExternalProgramHolder(dependencies.NonExistingExternalProgram(' '.join(args)), self.subproject)
+
     # TODO update modules to always pass `for_machine`. It is bad-form to assume
     # the host machine.
     def find_program_impl(self, args, for_machine: MachineChoice = MachineChoice.HOST,
                           required=True, silent=True, wanted='', search_dirs=None):
-        if not isinstance(args, list):
-            args = [args]
+        args = mesonlib.listify(args)
 
-        progobj = self.program_from_overrides(args, silent=silent)
-        if progobj is None:
-            progobj = self.program_from_file_for(for_machine, args, silent=silent)
-        if progobj is None:
-            progobj = self.program_from_system(args, search_dirs, silent=silent)
-        if progobj is None and args[0].endswith('python3'):
-            prog = dependencies.ExternalProgram('python3', mesonlib.python_command, silent=True)
-            progobj = ExternalProgramHolder(prog, self.subproject)
-        if required and (progobj is None or not progobj.found()):
-            raise InvalidArguments('Program(s) {!r} not found or not executable'.format(args))
+        extra_info = []
+        progobj = self.program_lookup(args, for_machine, required, search_dirs, extra_info)
         if progobj is None:
-            return ExternalProgramHolder(dependencies.NonExistingExternalProgram(' '.join(args)), self.subproject)
-        # Only store successful lookups
-        self.store_name_lookups(args)
+            progobj = self.notfound_program(args)
+
+        if not progobj.found():
+            mlog.log('Program', mlog.bold(progobj.get_name()), 'found:', mlog.red('NO'))
+            if required:
+                m = 'Program {!r} not found'
+                raise InterpreterException(m.format(progobj.get_name()))
+            return progobj
+
         if wanted:
             version = progobj.get_version(self)
             is_found, not_found, found = mesonlib.version_compare_many(version, wanted)
             if not is_found:
                 mlog.log('Program', mlog.bold(progobj.get_name()), 'found:', mlog.red('NO'),
-                         'found {!r} but need:'.format(version),
-                         ', '.join(["'{}'".format(e) for e in not_found]))
+                         'found', mlog.normal_cyan(version), 'but need:',
+                         mlog.bold(', '.join(["'{}'".format(e) for e in not_found])))
                 if required:
                     m = 'Invalid version of program, need {!r} {!r} found {!r}.'
-                    raise InvalidArguments(m.format(progobj.get_name(), not_found, version))
-                return ExternalProgramHolder(dependencies.NonExistingExternalProgram(' '.join(args)), self.subproject)
+                    raise InterpreterException(m.format(progobj.get_name(), not_found, version))
+                return self.notfound_program(args)
+            extra_info.insert(0, mlog.normal_cyan(version))
+
+        # Only store successful lookups
+        self.store_name_lookups(args)
+        mlog.log('Program', mlog.bold(progobj.get_name()), 'found:', mlog.green('YES'), *extra_info)
         return progobj
 
+    def program_lookup(self, args, for_machine, required, search_dirs, extra_info):
+        progobj = self.program_from_overrides(args, extra_info)
+        if progobj:
+            return progobj
+
+        fallback = None
+        wrap_mode = self.coredata.get_builtin_option('wrap_mode')
+        if wrap_mode != WrapMode.nofallback:
+            fallback = self.environment.wrap_resolver.find_program_provider(args)
+        if fallback and wrap_mode == WrapMode.forcefallback:
+            return self.find_program_fallback(fallback, args, required, extra_info)
+
+        progobj = self.program_from_file_for(for_machine, args)
+        if progobj is None:
+            progobj = self.program_from_system(args, search_dirs, silent=True)
+        if progobj is None and args[0].endswith('python3'):
+            prog = dependencies.ExternalProgram('python3', mesonlib.python_command, silent=True)
+            progobj = ExternalProgramHolder(prog, self.subproject) if prog.found() else None
+        if progobj is None and fallback and required:
+            progobj = self.find_program_fallback(fallback, args, required, extra_info)
+
+        return progobj
+
+    def find_program_fallback(self, fallback, args, required, extra_info):
+        mlog.log('Fallback to subproject', mlog.bold(fallback), 'which provides program',
+                 mlog.bold(' '.join(args)))
+        sp_kwargs = { 'required': required }
+        self.do_subproject(fallback, 'meson', sp_kwargs)
+        return self.program_from_overrides(args, extra_info)
+
     @FeatureNewKwargs('find_program', '0.53.0', ['dirs'])
     @FeatureNewKwargs('find_program', '0.52.0', ['version'])
     @FeatureNewKwargs('find_program', '0.49.0', ['disabler'])
@@ -3362,7 +3395,7 @@ external dependencies (including libraries) must go to "dependencies".''')
         disabled, required, feature = extract_required_kwarg(kwargs, self.subproject)
         if disabled:
             mlog.log('Program', mlog.bold(' '.join(args)), 'skipped: feature', mlog.bold(feature), 'disabled')
-            return ExternalProgramHolder(dependencies.NonExistingExternalProgram(' '.join(args)), self.subproject)
+            return self.notfound_program(args)
 
         search_dirs = extract_search_dirs(kwargs)
         wanted = mesonlib.stringlistify(kwargs.get('version', []))

mesonbuild/wrap/wrap.py

@@ -126,6 +126,7 @@ class PackageDefinition:
 
     def parse_provide_section(self):
         self.provide = {self.name: None}
+        self.provide_programs = []
         if self.config.has_section('provide'):
             for k, v in self.config['provide'].items():
                 if k == 'dependency_names':
@@ -134,6 +135,11 @@ class PackageDefinition:
                     names = {n.strip(): None for n in v.split(',')}
                     self.provide.update(names)
                     continue
+                if k == 'program_names':
+                    # A coma separated list of program names
+                    names = {n.strip(): None for n in v.split(',')}
+                    self.provide_programs += names
+                    continue
                 if not v:
                     m = ('Empty dependency variable name for {!r} in {}. '
                          'If the subproject uses meson.override_dependency() '
@@ -199,6 +205,14 @@ class Resolver:
             return wrap.name
         return directory
 
+    def find_program_provider(self, names: T.List[str]):
+        wraps = [i[0] for i in self.wraps.values()]
+        for name in names:
+            for wrap in wraps:
+                if wrap and name in wrap.provide_programs:
+                    return wrap.name
+        return None
+
     def resolve(self, packagename: str, method: str, current_subproject: str = '') -> str:
         self.current_subproject = current_subproject
         self.packagename = packagename

test cases/common/187 find override/meson.build

@@ -10,3 +10,6 @@ if not gencodegen.found()
 endif
 
 subdir('otherdir')
+
+tool = find_program('sometool')
+assert(tool.found())

test cases/common/187 find override/subprojects/sub.wrap

@@ -0,0 +1,5 @@
+[wrap-file]
+directory = sub
+
+[provide]
+program_names = sometool

test cases/common/187 find override/subprojects/sub/meson.build

@@ -0,0 +1,4 @@
+project('tools')
+
+exe = find_program('gencodegen')
+meson.override_find_program('sometool', exe)