Recognize shebang lines that pass '-S/--split-string' to 'env'

* etc/NEWS: announce the change.

* lisp/files.el (auto-mode-interpreter-regexp): Add optional '-S'
switch to the ignored group capturing the env invocation.
Allow multiple spaces between #!, interpreter and first argument:
empirically, Linux's 'execve' accepts that.  (Bug#66902)

* test/lisp/files-tests.el (files-tests--check-shebang): New helper to
generate a temporary file with a given interpreter line, and assert
that the mode picked by 'set-auto-mode' is derived from an expected
mode.  Write the 'should' form so that failure reports include useful
context; for example:

    (ert-test-failed
     ((should
       (equal (list shebang actual-mode) (list shebang expected-mode)))
      :form
      (equal ("#!/usr/bin/env -S make -f" fundamental-mode)
	     ("#!/usr/bin/env -S make -f" makefile-mode))
      :value nil :explanation
      (list-elt 1 (different-atoms fundamental-mode makefile-mode))))

* test/lisp/files-tests.el (files-tests-auto-mode-interpreter): New
test; exercise some aspects of 'interpreter-mode-alist'.

etc/NEWS

@@ -233,6 +233,12 @@ to enter the file you want to modify.
 It can be used to customize the look of the appointment notification
 displayed on the mode line when 'appt-display-mode-line' is non-nil.
 
+---
+*** Emacs now recognizes shebang lines that pass -S/--split-string to env.
+When visiting a script that invokes 'env -S INTERPRETER ARGS...' in
+its shebang line, Emacs will now skip over 'env -S' and deduce the
+major mode based on the interpreter.
+
 ** Emacs Server and Client
 
 ---

lisp/files.el

@@ -3245,8 +3245,16 @@ and `inhibit-local-variables-suffixes'.  If
     temp))
 
 (defvar auto-mode-interpreter-regexp
-  (purecopy "#![ \t]?\\([^ \t\n]*\
-/bin/env[ \t]\\)?\\([^ \t\n]+\\)")
+  (purecopy
+   (concat
+    "#![ \t]*"
+    ;; Optional group 1: env(1) invocation.
+    "\\("
+    "[^ \t\n]*/bin/env[ \t]*"
+    "\\(?:-S[ \t]*\\|--split-string\\(?:=\\|[ \t]*\\)\\)?"
+    "\\)?"
+    ;; Group 2: interpreter.
+    "\\([^ \t\n]+\\)"))
   "Regexp matching interpreters, for file mode determination.
 This regular expression is matched against the first line of a file
 to determine the file's mode in `set-auto-mode'.  If it matches, the file

test/lisp/files-tests.el

@@ -1656,6 +1656,31 @@ The door of all subtleties!
   (should (equal (file-name-base "foo") "foo"))
   (should (equal (file-name-base "foo/bar") "bar")))
 
+(defun files-tests--check-shebang (shebang expected-mode)
+  "Assert that mode for SHEBANG derives from EXPECTED-MODE."
+  (let ((actual-mode
+         (ert-with-temp-file script-file
+           :text shebang
+           (find-file script-file)
+           (if (derived-mode-p expected-mode)
+               expected-mode
+             major-mode))))
+    ;; Tuck all the information we need in the `should' form: input
+    ;; shebang, expected mode vs actual.
+    (should
+     (equal (list shebang actual-mode)
+            (list shebang expected-mode)))))
+
+(ert-deftest files-tests-auto-mode-interpreter ()
+  "Test that `set-auto-mode' deduces correct modes from shebangs."
+  (files-tests--check-shebang "#!/bin/bash" 'sh-mode)
+  (files-tests--check-shebang "#!/usr/bin/env bash" 'sh-mode)
+  (files-tests--check-shebang "#!/usr/bin/env python" 'python-base-mode)
+  (files-tests--check-shebang "#!/usr/bin/env python3" 'python-base-mode)
+  (files-tests--check-shebang "#!/usr/bin/env -S awk -v FS=\"\\t\" -v OFS=\"\\t\" -f" 'awk-mode)
+  (files-tests--check-shebang "#!/usr/bin/env -S make -f" 'makefile-mode)
+  (files-tests--check-shebang "#!/usr/bin/make -f" 'makefile-mode))
+
 (ert-deftest files-test-dir-locals-auto-mode-alist ()
   "Test an `auto-mode-alist' entry in `.dir-locals.el'"
   (find-file (ert-resource-file "whatever.quux"))