Fixed #31692 -- Prevented unneeded .po file compilation.

Thanks Nick Pope and Simon Charette for the reviews.

django/core/management/commands/compilemessages.py

@@ -122,10 +122,20 @@ class Command(BaseCommand):
         with concurrent.futures.ThreadPoolExecutor() as executor:
             futures = []
             for i, (dirpath, f) in enumerate(locations):
-                if self.verbosity > 0:
-                    self.stdout.write('processing file %s in %s' % (f, dirpath))
                 po_path = Path(dirpath) / f
                 mo_path = po_path.with_suffix('.mo')
+                try:
+                    if mo_path.stat().st_mtime >= po_path.stat().st_mtime:
+                        if self.verbosity > 0:
+                            self.stdout.write(
+                                'File “%s” is already compiled and up to date.'
+                                % po_path
+                            )
+                        continue
+                except FileNotFoundError:
+                    pass
+                if self.verbosity > 0:
+                    self.stdout.write('processing file %s in %s' % (f, dirpath))
 
                 if has_bom(po_path):
                     self.stderr.write(

tests/i18n/test_compilation.py

@@ -49,6 +49,8 @@ class PoFileTests(MessageCompilationTests):
         # Put file in read-only mode.
         old_mode = mo_file_en.stat().st_mode
         mo_file_en.chmod(stat.S_IREAD)
+        # Ensure .po file is more recent than .mo file.
+        mo_file_en.with_suffix('.po').touch()
         try:
             with self.assertRaisesMessage(CommandError, 'compilemessages generated one or more errors.'):
                 call_command('compilemessages', locale=['en'], stderr=err_buffer, verbosity=0)
@@ -56,6 +58,14 @@ class PoFileTests(MessageCompilationTests):
         finally:
             mo_file_en.chmod(old_mode)
 
+    def test_no_compile_when_unneeded(self):
+        mo_file_en = Path(self.MO_FILE_EN)
+        mo_file_en.touch()
+        stdout = StringIO()
+        call_command('compilemessages', locale=['en'], stdout=stdout, verbosity=1)
+        msg = '%s” is already compiled and up to date.' % mo_file_en.with_suffix('.po')
+        self.assertIn(msg, stdout.getvalue())
+
 
 class PoFileContentsTests(MessageCompilationTests):
     # Ticket #11240