[Fix clojure-emacs#2527] Enable configuring REPL auto-clear

Enables setting a limit to the buffer size of the REPL buffer (off by
default), which when exceeded after printing out a result or other
output, will clear old REPL output from the beginning to ensure it stays
within the limit. When clearing it also ensures partial input or output
is erased.

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### Changes
 
+* [#2527](https://github.com/clojure-emacs/cider/issues/2527): Enable auto-clear of REPL buffer by setting a limit to the max buffer size.
 * [#2852](https://github.com/clojure-emacs/cider/issues/2852): Convert 1-based column numbers in response map to Emacs' 0-based system.
 
 ## 0.25.0 (2020-06-04)

cider-repl.el

@@ -666,6 +666,28 @@ namespaces.  STRING is REPL's output."
 Each functions takes a string and must return a modified string.  Also see
 `cider-run-chained-hook'.")
 
+(defcustom cider-repl-buffer-size-limit nil
+  "The max size of the REPL buffer.
+Setting this to nil removes the limit."
+  :group 'cider
+  :type 'boolean
+  :package-version '(cider . "0.25.0"))
+
+(defun cider-clear-old-repl-output-if-limit-exceeded ()
+  "Clears printed output to meet `cider-repl-buffer-size-limit'.
+If limit exceeded, clears any partial input or results until under the limit.
+Clears partial input or results that remain."
+  (let ((size (buffer-size)))
+    (when (> size cider-repl-buffer-size-limit)
+      (let* ((over-limit (- size cider-repl-buffer-size-limit))
+             (next-prompt-or-input (next-single-char-property-change over-limit 'field))
+             (next-prompt-after-limit (if (eq (get-char-property next-prompt-or-input 'field) 'cider-repl-prompt)
+                                          next-prompt-or-input
+                                        (next-single-char-property-change next-prompt-or-input 'field)))
+             (inhibit-read-only t))
+        (cider-repl--clear-region (point-min) next-prompt-after-limit)
+        (cider-repl--clear-region cider-repl-output-start cider-repl-output-end)))))
+
 (defun cider-repl--emit-output (buffer string face)
   "Using BUFFER, emit STRING as output font-locked using FACE.
 Before inserting, run `cider-repl-preoutput-hook' on STRING."
@@ -682,7 +704,9 @@ Before inserting, run `cider-repl-preoutput-hook' on STRING."
       (when (and (= (point) cider-repl-prompt-start-mark)
                  (not (bolp)))
         (insert-before-markers "\n")
-        (set-marker cider-repl-output-end (1- (point))))))
+        (set-marker cider-repl-output-end (1- (point))))
+      (when cider-repl-buffer-size-limit
+        (cider-clear-old-repl-output-if-limit-exceeded))))
   (when-let* ((window (get-buffer-window buffer t)))
     ;; If the prompt is on the first line of the window, then scroll the window
     ;; down by a single line to make the emitted output visible.
@@ -734,7 +758,9 @@ of the line.  If BOL is non-nil insert at the beginning of the line."
             (insert-before-markers (cider-font-lock-as-clojure string))
           (cider-propertize-region
               '(font-lock-face cider-repl-result-face rear-nonsticky (font-lock-face))
-            (insert-before-markers string)))))))
+            (insert-before-markers string)))
+        (when cider-repl-buffer-size-limit
+          (cider-clear-old-repl-output-if-limit-exceeded))))))
 
 (defun cider-repl-newline-and-indent ()
   "Insert a newline, then indent the next line.