ninjabackend: fix class file path to include build_subdir

Compute the class file path relative to the private dir
using the full name of the .class file, not just the src.fname.
For example, for target subdir "java" and source
"java/icd/Api.java", the class path is "icd/Api.class" rather
than just "Api.class".

Using the wrong class file name caused unnecessary rebuilds
of jar files.

Fixes: #15582

mesonbuild/backend/ninjabackend.py

@@ -1614,7 +1614,9 @@ class NinjaBackend(backends.Backend):
             rel_src = src.rel_to_builddir(self.build_to_src)
             rel_srcs.append(rel_src)
 
-            plain_class_path = src.fname[:-4] + 'class'
+            # Preserve any additional path components on top of the target's subdir
+            plain_class_path = os.path.relpath(src.relative_name(), target.get_subdir())
+            plain_class_path = plain_class_path[:-4] + 'class'
             plain_class_paths.append(plain_class_path)
             rel_obj = os.path.join(self.get_target_private_dir(target), plain_class_path)
             rel_objs.append(rel_obj)

test cases/java/11 build subdir/com-mesonbuild-HelloWorldMainLauncherClass.java

@@ -0,0 +1,7 @@
+package com.mesonbuild;
+
+public class HelloWorldMainLauncherClass {
+    public static void main(String [] args) {
+        System.out.println("hello nanowar of steel");
+    }
+}

test cases/java/11 build subdir/copyfile.py

@@ -0,0 +1,6 @@
+#!/usr/bin/env python3
+
+import sys
+import shutil
+
+shutil.copyfile(sys.argv[1], sys.argv[2])

test cases/java/11 build subdir/meson.build

@@ -0,0 +1,13 @@
+project('build subdir java', 'java')
+
+copyfile = find_program('copyfile.py')
+
+generated = custom_target(
+  command: [copyfile, '@INPUT@', '@OUTPUT@'],
+  input: 'com-mesonbuild-HelloWorldMainLauncherClass.java',
+  output: 'HelloWorldMainLauncherClass.java',
+  build_subdir: 'com/mesonbuild',
+)
+
+jar('hello', generated,
+  main_class: 'com.mesonbuild.HelloWorldMainLauncherClass')

unittests/platformagnostictests.py

@@ -15,7 +15,7 @@ from unittest import skipIf, SkipTest
 from pathlib import Path
 
 from .baseplatformtests import BasePlatformTests
-from .helpers import IS_CI
+from .helpers import *
 from mesonbuild.mesonlib import EnvironmentVariables, ExecutableSerialisation, MesonException, is_linux, python_command, windows_proof_rmtree
 from mesonbuild.mformat import Formatter, match_path
 from mesonbuild.optinterpreter import OptionInterpreter, OptionException
@@ -626,3 +626,11 @@ class PlatformAgnosticTests(BasePlatformTests):
 
         self.init(testdir)
         self.build()
+
+    @skip_if_not_language('java')
+    def test_java_build_subdir_correct_deps(self):
+        """Test that jar() with build_subdir sources does not rebuild unnecessarily."""
+        testdir = os.path.join(self.src_root, 'test cases', 'java', '11 build subdir')
+        self.init(testdir)
+        self.build()
+        self.assertBuildIsNoop()