Fix sandbox violation when using subproject as a symlink

Fix "Tried to grab file outside current (sub)project" error when subproject exists within
a source tree but it is used through a symlink. Using subprojects as symlinks is very useful
feature when migrating an existing codebase to meson that all sources do not need to be
immediately moved to subprojects folder.

mesonbuild/interpreter/interpreter.py

@@ -2851,7 +2851,7 @@ Try setting b_lundef to false instead.'''.format(self.coredata.options[OptionKey
             # subproject files, as long as they are scheduled to be installed.
             if validate_installable_file(norm):
                 return
-        norm = Path(srcdir, subdir, fname).resolve()
+        norm = Path(os.path.abspath(Path(srcdir, subdir, fname)))
         if os.path.isdir(norm):
             inputtype = 'directory'
         else:

test cases/unit/106 subproject symlink/main.c

@@ -0,0 +1,6 @@
+extern int foo(void);
+
+int main(void)
+{
+    return foo();
+}

test cases/unit/106 subproject symlink/meson.build

@@ -0,0 +1,8 @@
+project('foo', 'c')
+
+symlinked_subproject = subproject('symlinked_subproject')
+
+executable('foo',
+    sources : 'main.c',
+    dependencies : symlinked_subproject.get_variable('dep')
+)

test cases/unit/106 subproject symlink/symlinked_subproject/meson.build

@@ -0,0 +1,3 @@
+project('symlinked_subproject', 'c', version : '1.0.0')
+
+dep = declare_dependency(sources : 'src.c')

test cases/unit/106 subproject symlink/symlinked_subproject/src.c

@@ -0,0 +1,4 @@
+int foo(void)
+{
+    return 0;
+}

unittests/allplatformstests.py

@@ -4263,3 +4263,16 @@ class AllPlatformTests(BasePlatformTests):
         if self.backend is Backend.ninja:
             self.assertIn('Generating file.txt with a custom command', out)
             self.assertIn('Generating subdir/file.txt with a custom command', out)
+
+    def test_symlinked_subproject(self):
+        testdir = os.path.join(self.unit_test_dir, '106 subproject symlink')
+        subproject_dir = os.path.join(testdir, 'subprojects')
+        subproject = os.path.join(testdir, 'symlinked_subproject')
+        symlinked_subproject = os.path.join(testdir, 'subprojects', 'symlinked_subproject')
+        if not os.path.exists(subproject_dir):
+            os.mkdir(subproject_dir)
+        os.symlink(subproject, symlinked_subproject)
+        self.addCleanup(os.remove, symlinked_subproject)
+
+        self.init(testdir)
+        self.build()