Allow meson build file to exit early. (#2808)

docs/markdown/Reference-manual.md

@@ -1129,6 +1129,33 @@ This function has one keyword argument.
    recurse in the subdir if they all return `true` when queried with
    `.found()`
 
+### subdir_done()
+
+``` meson
+    subdir_done()
+```
+
+Stops further interpretation of the meson script file from the point of
+the invocation. All steps executed up to this point are valid and will
+be executed by meson. This means that all targets defined before the call
+of `subdir_done` will be build.
+
+If the current script was called by `subdir` the execution returns to the
+calling directory and continues as if the script had reached the end.
+If the current script is the top level script meson configures the project
+as defined up to this point.
+
+Example:
+```meson
+project('example exit', 'cpp')
+executable('exe1', 'exe1.cpp')
+subdir_done()
+executable('exe2', 'exe2.cpp')
+```
+
+The executable `exe1` will be build, while the executable `exe2` is not
+build.
+
 ### subproject()
 
 ``` meson

docs/markdown/Release-notes-for-0.46.0.md

@@ -14,3 +14,10 @@ whose contents should look like this:
     ## Feature name
 
     A short description explaining the new feature and how it should be used.
+
+## Allow early return from a script
+
+Added the function `subdir_done()`. Its invocation exits the current script at
+the point of invocation. All previously invoked build targets and commands are
+build/executed. All following ones are ignored. If the current script was
+invoked via `subdir()` the parent script continues normally.

mesonbuild/interpreter.py

@@ -26,7 +26,7 @@ from .dependencies import ExternalProgram
 from .dependencies import InternalDependency, Dependency, DependencyException
 from .interpreterbase import InterpreterBase
 from .interpreterbase import check_stringlist, noPosargs, noKwargs, stringArgs, permittedKwargs, permittedMethodKwargs
-from .interpreterbase import InterpreterException, InvalidArguments, InvalidCode
+from .interpreterbase import InterpreterException, InvalidArguments, InvalidCode, SubdirDoneRequest
 from .interpreterbase import InterpreterObject, MutableInterpreterObject, Disabler
 from .modules import ModuleReturnValue
 
@@ -1612,6 +1612,7 @@ class Interpreter(InterpreterBase):
                            'static_library': self.func_static_lib,
                            'test': self.func_test,
                            'vcs_tag': self.func_vcs_tag,
+                           'subdir_done': self.func_subdir_done,
                            })
         if 'MESON_UNIT_TEST' in os.environ:
             self.funcs.update({'exception': self.func_exception})
@@ -2606,6 +2607,14 @@ root and issuing %s.
         kwargs.setdefault('build_always', True)
         return self.func_custom_target(node, [kwargs['output']], kwargs)
 
+    @stringArgs
+    def func_subdir_done(self, node, args, kwargs):
+        if len(kwargs) > 0:
+            raise InterpreterException('exit does not take named arguments')
+        if len(args) > 0:
+            raise InterpreterException('exit does not take any arguments')
+        raise SubdirDoneRequest()
+
     @stringArgs
     @permittedKwargs(permitted_kwargs['custom_target'])
     def func_custom_target(self, node, args, kwargs):

mesonbuild/interpreterbase.py

@@ -105,6 +105,9 @@ class InvalidCode(InterpreterException):
 class InvalidArguments(InterpreterException):
     pass
 
+class SubdirDoneRequest(BaseException):
+    pass
+
 class InterpreterObject:
     def __init__(self):
         self.methods = {}
@@ -203,6 +206,8 @@ class InterpreterBase:
             try:
                 self.current_lineno = cur.lineno
                 self.evaluate_statement(cur)
+            except SubdirDoneRequest:
+                break
             except Exception as e:
                 if not(hasattr(e, 'lineno')):
                     e.lineno = cur.lineno

test cases/common/188 subdir_done/meson.build

@@ -0,0 +1,10 @@
+# Should run, even though main.cpp does not exist and we call error in the last line.
+# subdir_done jumps to end, so both lines are not executed.
+
+project('example exit', 'cpp')
+
+subdir_done()
+
+executable('main', 'main.cpp')
+error('Unreachable')
+

test cases/failing/71 skip only subdir/meson.build

@@ -0,0 +1,8 @@
+# Check that skip_rest only exits subdir, not the whole script.
+# Should create an error because main.cpp does not exists.
+project('example exit', 'cpp')
+
+subdir('subdir')
+
+message('Good')
+executable('main', 'main.cpp')

test cases/failing/71 skip only subdir/subdir/meson.build

@@ -0,0 +1,3 @@
+subdir_done()
+
+error('Unreachable')