Fine-tune the error message when trying to build outside the project …

…root

We try to backtrack through the filesystem to find the correct directory
to build in, and suggest this as a possible diagnostic. However, our
current heuristic relies on parsing the raw file with string matching to
see if it starts with `project(`, and this may or may not actually work.

Instead, do a bit of recursion and parse each candidate with mparser,
then check if the first node of *that* file is a project() function.

This makes us resilient to a common case: where the root meson.build is
entirely valid, but, the first line is a comment containing e.g. SPDX
license headers and a simple string comparison simply does not cut it.

Fixes the bad error message from mesonbuild#12441, which was supposed to provide
more guidance but did not.

mesonbuild/interpreterbase/interpreterbase.py

@@ -132,20 +132,30 @@ def parse_project(self) -> None:
         self.evaluate_codeblock(self.ast, end=1)
 
     def sanity_check_ast(self) -> None:
-        if not isinstance(self.ast, mparser.CodeBlockNode):
-            raise InvalidCode('AST is of invalid type. Possibly a bug in the parser.')
-        if not self.ast.lines:
-            raise InvalidCode('No statements in code.')
-        first = self.ast.lines[0]
-        if not isinstance(first, mparser.FunctionNode) or first.func_name.value != 'project':
+        def _is_project(ast: mparser.CodeBlockNode) -> object:
+            if not isinstance(ast, mparser.CodeBlockNode):
+                raise InvalidCode('AST is of invalid type. Possibly a bug in the parser.')
+            if not ast.lines:
+                raise InvalidCode('No statements in code.')
+            first = ast.lines[0]
+            return isinstance(first, mparser.FunctionNode) and first.func_name.value == 'project'
+
+        if not _is_project(self.ast):
             p = pathlib.Path(self.source_root).resolve()
             found = p
             for parent in p.parents:
                 if (parent / 'meson.build').is_file():
                     with open(parent / 'meson.build', encoding='utf-8') as f:
-                        if f.readline().startswith('project('):
-                            found = parent
-                            break
+                        code = f.read()
+
+                    try:
+                        ast = mparser.Parser(code, 'empty').parse()
+                    except mparser.ParseException:
+                        continue
+
+                    if _is_project(ast):
+                        found = parent
+                        break
                 else:
                     break
 

unittests/platformagnostictests.py

@@ -278,3 +278,11 @@ def test_cmake_openssl_not_found_bug(self):
         self.meson_native_files.append(os.path.join(testdir, 'nativefile.ini'))
         out = self.init(testdir, allow_fail=True)
         self.assertNotIn('Unhandled python exception', out)
+
+    def test_error_configuring_subdir(self):
+        testdir = os.path.join(self.common_test_dir, '152 index customtarget')
+        out = self.init(os.path.join(testdir, 'subdir'), allow_fail=True)
+
+        self.assertIn('first statement must be a call to project()', out)
+        # provide guidance diagnostics by finding a file whose first AST statement is project()
+        self.assertIn(f'Did you mean to run meson from the directory: "{testdir}"?', out)