change --reload-extra-file option to get multiple files at once

Co-authored-by: Randall Leeds <randall@bleeds.info>

gunicorn/config.py

@@ -9,6 +9,7 @@
 import copy
 import grp
 import inspect
+import itertools
 import os
 import pwd
 import re
@@ -940,15 +941,22 @@ class ReloadExtraFiles(Setting):
     section = "Debugging"
     cli = ["--reload-extra-file"]
     meta = "FILES"
-    validator = validate_list_of_existing_files
     default = []
+    nargs = '+'
     desc = """\
         Extends :ref:`reload` option to also watch and reload on additional files
         (e.g., templates, configurations, specifications, etc.).
 
         .. versionadded:: 19.8
+        .. versionchanged:: 22.0
+           It is now possible to pass multiple arguments to each instance of the flag.
         """
 
+    # Once Python 3.8 is required, the `extend` makes this overload unnecessary
+    def validator(val):
+        val = list(itertools.chain.from_iterable(val))
+        return validate_list_of_existing_files(val)
+
 
 class Spew(Setting):
     name = "spew"

tests/test_config.py

@@ -318,6 +318,36 @@ def nworkers_changed_3(server, new_value, old_value):
     assert c.nworkers_changed(1, 2, 3) == 3
 
 
+def test_reload_extra_files(tmp_path):
+    try:
+        tmp_path.mkdir()
+    except FileExistsError:
+        pass
+
+    foo_path = tmp_path / 'foo'
+    bar_path = tmp_path / 'bar'
+    baz_path = tmp_path / 'baz'
+    foo_path.touch()
+    bar_path.touch()
+    baz_path.touch()
+
+    args = ["prog_name", "--config", cfg_file()]
+    with AltArgs(args):
+        app = NoConfigApp()
+    assert app.cfg.reload_extra_files == []
+
+    args.extend(['--reload-extra-file', str(foo_path)])
+    with AltArgs(args):
+        app = NoConfigApp()
+    print(app.cfg.reload_extra_files)
+    assert app.cfg.reload_extra_files == [str(foo_path)]
+
+    args.extend(['--reload-extra-file', str(bar_path), str(baz_path)])
+    with AltArgs(args):
+        app = NoConfigApp()
+    assert app.cfg.reload_extra_files == [str(foo_path), str(bar_path), str(baz_path)]
+
+
 def test_statsd_host():
     c = config.Config()
     assert c.statsd_host is None