Pcomplete: Better obey completion-at-point-functions

Functions on `completion-at-point-functions` should not modify
the buffer.  Pcomplete itself mostly abides by this but Eshell's
use of it doesn't.  Try and catch those cases.  Also fix one of
those cases.

* lisp/pcomplete.el (pcomplete-allow-modifications): New var.
(pcomplete-completions-at-point): Enforce it.
(pcomplete, pcomplete-expand-and-complete, pcomplete-expand):
Rebind it since these commands expect the extra side effects.

* lisp/eshell/em-cmpl.el (eshell--pcomplete-insert-tab): New function,
extracted from `eshell-complete-parse-arguments`.
(eshell-complete-parse-arguments): Use it and obey
`pcomplete-allow-modifications`.

lisp/eshell/em-cmpl.el

@@ -311,18 +311,24 @@ to writing a completion function."
       (describe-prefix-bindings)
     (call-interactively 'pcomplete-help)))
 
+(defun eshell--pcomplete-insert-tab ()
+  (if (not pcomplete-allow-modifications)
+      (throw 'pcompleted nil)
+    (insert-and-inherit "\t")
+    (throw 'pcompleted t)))
+
 (defun eshell-complete-parse-arguments ()
   "Parse the command line arguments for `pcomplete-argument'."
   (when (and eshell-no-completion-during-jobs
 	     (eshell-interactive-process-p))
-    (insert-and-inherit "\t")
-    (throw 'pcompleted t))
+    (eshell--pcomplete-insert-tab))
   (let ((end (point-marker))
 	(begin (save-excursion (eshell-bol) (point)))
 	(posns (list t))
 	args delim)
-    (when (memq this-command '(pcomplete-expand
-			       pcomplete-expand-and-complete))
+    (when (and pcomplete-allow-modifications
+	       (memq this-command '(pcomplete-expand
+			            pcomplete-expand-and-complete)))
       (run-hook-with-args 'eshell-expand-input-functions begin end)
       (if (= begin end)
 	  (end-of-line))
@@ -335,14 +341,11 @@ to writing a completion function."
 	       (setq begin (1+ (cadr delim))
 		     args (eshell-parse-arguments begin end)))
 	      ((eq (car delim) ?\()
-	       (eshell-complete-lisp-symbol)
-	       (throw 'pcompleted t))
+	       (throw 'pcompleted (elisp-completion-at-point)))
 	      (t
-	       (insert-and-inherit "\t")
-	       (throw 'pcompleted t))))
+	       (eshell--pcomplete-insert-tab))))
     (when (get-text-property (1- end) 'comment)
-      (insert-and-inherit "\t")
-      (throw 'pcompleted t))
+      (eshell--pcomplete-insert-tab))
     (let ((pos begin))
       (while (< pos end)
 	(if (get-text-property pos 'arg-begin)

lisp/pcomplete.el

@@ -189,6 +189,16 @@ and how is entirely up to the behavior of the
 `pcomplete-parse-arguments-function'."
   :type 'boolean)
 
+(defvar pcomplete-allow-modifications nil
+  "If non-nil, allow effects in `pcomplete-parse-arguments-function'.
+For the `pcomplete' command, it was common for functions in
+`pcomplete-parse-arguments-function' to make modifications to the
+buffer, like expanding variables are such.
+For `completion-at-point-functions', this is not an option any more, so
+this variable is used to tell `pcomplete-parse-arguments-function'
+whether it can do the modifications like it used to, or whether
+it should refrain from doing so.")
+
 (defcustom pcomplete-parse-arguments-function
   #'pcomplete-parse-buffer-arguments
   "A function to call to parse the current line's arguments.
@@ -392,6 +402,9 @@ Same as `pcomplete' but using the standard completion UI."
   ;; imposing the pcomplete UI over the standard UI.
   (catch 'pcompleted
     (let* ((pcomplete-stub)
+           (buffer-read-only
+            ;; Make sure the function obeys `pcomplete-allow-modifications'.
+            (if pcomplete-allow-modifications buffer-read-only t))
            pcomplete-seen pcomplete-norm-func
            pcomplete-args pcomplete-last pcomplete-index
            (pcomplete-autolist pcomplete-autolist)
@@ -526,6 +539,7 @@ completion functions list (it should occur fairly early in the list)."
 	  pcomplete-last-completion-raw nil)
     (catch 'pcompleted
       (let* ((pcomplete-stub)
+	     (pcomplete-allow-modifications t)
 	     pcomplete-seen pcomplete-norm-func
 	     pcomplete-args pcomplete-last pcomplete-index
 	     (pcomplete-autolist pcomplete-autolist)
@@ -551,7 +565,8 @@ completion functions list (it should occur fairly early in the list)."
   "Expand the textual value of the current argument.
 This will modify the current buffer."
   (interactive)
-  (let ((pcomplete-expand-before-complete t))
+  (let ((pcomplete-expand-before-complete t)
+	(pcomplete-allow-modifications t))
     (with-suppressed-warnings ((obsolete pcomplete))
       (pcomplete))))
 
@@ -569,6 +584,7 @@ This will modify the current buffer."
 This will modify the current buffer."
   (interactive)
   (let ((pcomplete-expand-before-complete t)
+	(pcomplete-allow-modifications t)
 	(pcomplete-expand-only-p t))
     (with-suppressed-warnings ((obsolete pcomplete))
       (pcomplete))