configure_file: Fix infinite loop in some cases

We were adding built files to the list of source files to check for
regen. We were also not adding sources files to regen when `command:`
was used.

Fixes #1865

mesonbuild/interpreter.py

@@ -2549,12 +2549,10 @@ class Interpreter(InterpreterBase):
             if not isinstance(inputfile, (str, mesonlib.File)):
                 raise InterpreterException('Input must be a string or a file')
             if isinstance(inputfile, str):
-                inputfile = os.path.join(self.subdir, inputfile)
-                ifile_abs = os.path.join(self.environment.source_dir, inputfile)
-            else:
-                ifile_abs = inputfile.absolute_path(self.environment.source_dir,
-                                                    self.environment.build_dir)
-                inputfile = inputfile.relative_name()
+                inputfile = mesonlib.File.from_source_file(self.environment.source_dir,
+                                                           self.subdir, inputfile)
+            ifile_abs = inputfile.absolute_path(self.environment.source_dir,
+                                                self.environment.build_dir)
         elif 'command' in kwargs and '@INPUT@' in kwargs['command']:
             raise InterpreterException('@INPUT@ used as command argument, but no input file specified.')
         # Validate output
@@ -2575,18 +2573,13 @@ class Interpreter(InterpreterBase):
                 raise InterpreterException('Argument "configuration" is not of type configuration_data')
             mlog.log('Configuring', mlog.bold(output), 'using configuration')
             if inputfile is not None:
-                # Normalize the path of the conffile to avoid duplicates
-                # This is especially important to convert '/' to '\' on Windows
-                conffile = os.path.normpath(inputfile)
-                if conffile not in self.build_def_files:
-                    self.build_def_files.append(conffile)
                 os.makedirs(os.path.join(self.environment.build_dir, self.subdir), exist_ok=True)
                 missing_variables = mesonlib.do_conf_file(ifile_abs, ofile_abs,
                                                           conf.held_object)
                 if missing_variables:
                     var_list = ", ".join(map(repr, sorted(missing_variables)))
                     mlog.warning(
-                        "The variable(s) %s in the input file %r are not "
+                        "The variable(s) %s in the input file %s are not "
                         "present in the given configuration data" % (
                             var_list, inputfile))
             else:
@@ -2616,6 +2609,17 @@ class Interpreter(InterpreterBase):
                 mesonlib.replace_if_different(ofile_abs, dst_tmp)
         else:
             raise InterpreterException('Configure_file must have either "configuration" or "command".')
+        # If the input is a source file, add it to the list of files that we
+        # need to reconfigure on when they change. FIXME: Do the same for
+        # files() objects in the command: kwarg.
+        if inputfile and not inputfile.is_built:
+            # Normalize the path of the conffile (relative to the
+            # source root) to avoid duplicates. This is especially
+            # important to convert '/' to '\' on Windows
+            conffile = os.path.normpath(inputfile.relative_name())
+            if conffile not in self.build_def_files:
+                self.build_def_files.append(conffile)
+        # Install file if requested
         idir = kwargs.get('install_dir', None)
         if isinstance(idir, str):
             cfile = mesonlib.File.from_built_file(ofile_path, ofile_fname)