Rollback if an optional subproject fails

If a subproject is not required and fails during its configuration, the
parent project continues, but should not include any target or state set
by the failed subproject. This fix ninja still trying to build targets
generated by subprojects before they fail in their configuration.

The 'build' object is now per-interpreter instead of being global. Once
a subproject interpreter succeed, values from its 'build' object are
merged back into its parent 'build' object.

mesonbuild/build.py

@@ -138,6 +138,19 @@ class Build:
         self.find_overrides = {}
         self.searched_programs = set() # The list of all programs that have been searched for.
 
+    def copy(self):
+        other = Build(self.environment)
+        for k, v in self.__dict__.items():
+            if isinstance(v, (list, dict, set, OrderedDict)):
+                other.__dict__[k] = v.copy()
+            else:
+                other.__dict__[k] = v
+        return other
+
+    def merge(self, other):
+        for k, v in other.__dict__.items():
+            self.__dict__[k] = v
+
     def add_compiler(self, compiler):
         if self.static_linker is None and compiler.needs_static_linker():
             self.static_linker = self.environment.detect_static_linker(compiler)

mesonbuild/interpreter.py

@@ -2286,7 +2286,8 @@ external dependencies (including libraries) must go to "dependencies".''')
         with mlog.nested():
             try:
                 mlog.log('\nExecuting subproject', mlog.bold(dirname), '\n')
-                subi = Interpreter(self.build, self.backend, dirname, subdir, self.subproject_dir,
+                new_build = self.build.copy()
+                subi = Interpreter(new_build, self.backend, dirname, subdir, self.subproject_dir,
                                    self.modules, default_options)
                 subi.subprojects = self.subprojects
 
@@ -2312,6 +2313,7 @@ external dependencies (including libraries) must go to "dependencies".''')
         self.subprojects.update(subi.subprojects)
         self.subprojects[dirname] = SubprojectHolder(subi, self.subproject_dir, dirname)
         self.build_def_files += subi.build_def_files
+        self.build.merge(subi.build)
         return self.subprojects[dirname]
 
     def get_option_internal(self, optname):

test cases/common/205 broken subproject/meson.build

@@ -0,0 +1,2 @@
+project('test broken subproject')
+subproject('broken', required : false)

test cases/common/205 broken subproject/subprojects/broken/broken.c

@@ -0,0 +1 @@
+#error This must not compile

test cases/common/205 broken subproject/subprojects/broken/meson.build

@@ -0,0 +1,4 @@
+project('broken', 'c')
+
+executable('app', 'broken.c')
+assert(false, 'This subproject must fail')