Protect breakpoint persistence from print-* vars (#792)

* Protect breakpoint persistence from print-* vars

The output of printing functions (including 'prin1' and, by
extension, 'format' with a '%s' sequence) is affected by a set of
output variables that both end-users and libraries can tweak.

This is fine in the more common use of these functions to format
messages, but the problem is that the same functions are also the
most convenient way to serialize/persist data.

Serialization should be robust to non-default values of
'print-length' and 'print-level' in particular, as setting these to
small numbers can lead to accidental data loss (and in some cases
unreadable output).

Starting in Emacs 29, 'prin1' and co. gained a new optional argument
precisely to protect against this problem.  In the meantime, this
patch ensures that 'print-length' and 'print-level' are set to their
default values during serialization.

* Minor printing simplifications

'dap--persist': 'with-temp-file' creates a fresh buffer that is
already empty, so there is no need to erase it again.  Print
data-to-persist directly to output buffer, avoiding roundtrip via
temporary string.

'dap--print-to-output-buffer': modes are enabled with a 'nil' or
nonnegative numeric argument rather than with 't'; using
'turn-on-font-lock' avoids both this pitfall, as well as redundantly
reenabling the mode.  Use an 'inhibit-read-only' binding for
temporary read-only overrides, as modifications to
'buffer-read-only' will not be undone in case of a non-local exit.

'dap-debug-edit-template': Avoid temporary string roundtrip when
checking for an empty buffer.  Use 'insert-char' for repetitions of
the same character.  Remove redundant 'prin1-to-string' in arguments
to 'format'; it already uses 'prin1' for the '%s' sequence.

dap-mode.el

@@ -455,8 +455,11 @@ This is in contrast to merely setting it to 0."
       "Failed to persist file: %S"
     (make-directory (file-name-directory file) t)
     (with-temp-file file
-      (erase-buffer)
-      (insert (prin1-to-string to-persist)))))
+      ;; Starting with Emacs 29, `prin1' takes an optional third argument
+      ;; which removes the need for these default bindings.
+      (let ((print-length nil)
+            (print-level nil))
+        (prin1 to-persist (current-buffer))))))
 
 (defun dap--set-sessions (debug-sessions)
   "Update list of debug sessions for WORKSPACE to DEBUG-SESSIONS."
@@ -917,14 +920,13 @@ an OUTPUT-BODY."
 (defun dap--print-to-output-buffer (debug-session str)
   "Insert content from STR into the output buffer associated with DEBUG-SESSION."
   (with-current-buffer (get-buffer-create (dap--debug-session-output-buffer debug-session))
-    (font-lock-mode t)
-    (setq-local buffer-read-only nil)
-    (if (and (eq (current-buffer) (window-buffer (selected-window)))
-             (not (= (point) (point-max))))
-        (save-excursion
-          (dap--insert-at-point-max str))
-      (dap--insert-at-point-max str))
-    (setq-local buffer-read-only t))
+    (turn-on-font-lock)
+    (let ((inhibit-read-only t))
+      (if (and (eq (current-buffer) (window-buffer))
+               (not (eobp)))
+          (save-excursion
+            (dap--insert-at-point-max str))
+        (dap--insert-at-point-max str))))
   (when (and dap-auto-show-output
              (not (dap--debug-session-output-displayed debug-session)))
     (setf (dap--debug-session-output-displayed debug-session) t)
@@ -1913,7 +1915,7 @@ the new template can be used normally with `dap-debug'"
                              (emacs-lisp-mode)
                              (current-buffer)))
     (goto-char (point-max))
-    (when (s-blank? (buffer-string))
+    (when (bobp) ;; Empty (accessible portion of) buffer.
       (insert ";; Eval Buffer with `M-x eval-buffer' to register the newly created template."))
     (insert
      (format "\n\n(dap-register-debug-template\n  \"%s%s\"\n"
@@ -1922,13 +1924,12 @@ the new template can be used normally with `dap-debug'"
     (insert "  (list ")
     (-let ((column (current-column))
            ((fst snd . rst) debug-args))
-      (insert (format "%s %s" fst (prin1-to-string snd)))
+      (insert (format "%s %s" fst snd))
       (cl-loop for (k v) on rst by #'cddr
-               do (if (not (equal k :program-to-start))
-                      (progn
-                        (insert "\n")
-                        (--dotimes column (insert " "))
-                        (insert (format "%s %s" k (prin1-to-string v)))))))
+               do (unless (eq k :program-to-start)
+                    (insert "\n")
+                    (insert-char ?\s column)
+                    (insert (format "%s %s" k v)))))
     (insert "))"))
   (pop-to-buffer "*DAP Templates*")
   (goto-char (point-max)))