Fix command-line parsing regression in erc-cmd-DCC

* lisp/erc/erc-compat.el (erc-compat--28-split-string-shell-command,
erc-compat--split-string-shell-command): Remove unused function and
macro.
* lisp/erc/erc-dcc.el (erc-cmd-DCC): Use own arg-parsing function.
* lisp/erc/erc.el (erc--shell-parse-regexp,
erc--split-string-shell-cmd): New regexp constant and arg-parsing
function based on those in shell.el.
* test/lisp/erc/erc-dcc-tests.el
(erc-dcc-tests--erc-dcc-do-GET-command): Accept new `nuh' argument
representing message source/sender.
(erc-dcc-do-GET-command): Add tests for regression involving pipe
character.
* test/lisp/erc/erc-tests.el (erc--split-string-shell-cmd): New test.
(Bug#62444)

Thanks to Fernando de Morais for reporting this bug.

lisp/erc/erc-compat.el

@@ -445,27 +445,6 @@ If START or END is negative, it counts from the end."
                        existing))))))
 
 
-;;;; Misc 28.1
-
-(defvar comint-file-name-quote-list)
-(defvar shell-file-name-quote-list)
-(declare-function shell--parse-pcomplete-arguments "shell" nil)
-
-(defun erc-compat--28-split-string-shell-command (string)
-  (require 'comint)
-  (require 'shell)
-  (with-temp-buffer
-    (insert string)
-    (let ((comint-file-name-quote-list shell-file-name-quote-list))
-      (car (shell--parse-pcomplete-arguments)))))
-
-(defmacro erc-compat--split-string-shell-command (string)
-  ;; Autoloaded in Emacs 28.
-  (list (if (fboundp 'split-string-shell-command)
-            'split-string-shell-command
-          'erc-compat--28-split-string-shell-command)
-        string))
-
 (provide 'erc-compat)
 
 ;;; erc-compat.el ends here

lisp/erc/erc-dcc.el

@@ -399,7 +399,7 @@ where FOO is one of CLOSE, GET, SEND, LIST, CHAT, etc."
     (if compat-args
         (setq cmd line
               args compat-args)
-      (setq args (delete "" (erc-compat--split-string-shell-command line))
+      (setq args (delete "" (erc--split-string-shell-cmd line))
             cmd (pop args)))
     (let ((fn (intern-soft (concat "erc-dcc-do-" (upcase cmd) "-command"))))
       (if fn

lisp/erc/erc.el

@@ -3213,6 +3213,42 @@ this function from interpreting the line as a command."
           (erc-display-message nil 'error (current-buffer) 'no-target)
           nil)))))
 
+(defconst erc--shell-parse-regexp
+  (rx (or (+ (not (any ?\s ?\t ?\n ?\\ ?\" ?' ?\;)))
+          (: ?' (group (* (not ?'))) (? ?'))
+          (: ?\" (group (* (or (not (any ?\" ?\\)) (: ?\\ nonl)))) (? ?\"))
+          (: ?\\ (group (? (or nonl ?\n)))))))
+
+(defun erc--split-string-shell-cmd (string)
+  "Parse whitespace-separated arguments in STRING."
+  ;; From `shell--parse-pcomplete-arguments' and friends.  Quirk:
+  ;; backslash-escaped characters appearing within spans of double
+  ;; quotes are unescaped.
+  (with-temp-buffer
+    (insert string)
+    (let ((end (point))
+          args)
+      (goto-char (point-min))
+      (while (and (skip-chars-forward " \t") (< (point) end))
+        (let (arg)
+          (while (looking-at erc--shell-parse-regexp)
+            (goto-char (match-end 0))
+            (cond ((match-beginning 3) ; backslash escape
+                   (push (if (= (match-beginning 3) (match-end 3))
+                             "\\"
+                           (match-string 3))
+                         arg))
+                  ((match-beginning 2) ; double quote
+                   (push (replace-regexp-in-string (rx ?\\ (group nonl))
+                                                   "\\1" (match-string 2))
+                         arg))
+                  ((match-beginning 1) ; single quote
+                   (push (match-string 1) arg))
+                  (t (push (match-string 0) arg))))
+          (push (string-join (nreverse arg)) args)))
+      (nreverse args))))
+
+
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;                    Input commands handlers
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;