[#2628] Add a defcustom controlling nREPL's print buffer size

This makes it possible to stream results in bigger chunks (and as a corollary -
fontify as Clojure bigger results).

Author: bbatsov

CHANGELOG.md

@@ -13,6 +13,7 @@
 * Add support for nREPL 0.8 `completions` op. It's used if `cider-nrepl` is not available.
 * Add `browser` to the list of supported ClojureScript REPL types.
 * Add an interactive command to toggle Clojure font-locking in the REPL (`cider-repl-toggle-clojure-font-lock`).
+* Add a defcustom controlling nREPL's print buffer size (`cider-print-buffer-size`). It's set to 4K by default, nREPL own default is 1k.
 
 ### Changes
 

cider-client.el

@@ -264,6 +264,17 @@ Set to nil for no limit."
   :group 'cider
   :package-version '(cider . "0.21.0"))
 
+(defcustom cider-print-buffer-size (* 4 1024)
+  "The size in bytes of each value/output chunk when using print streaming.
+Smaller values mean smaller data chunks and faster feedback, but they also mean
+smaller results that can be font-locked as Clojure in the REPL buffers, as only
+a single chunk result can be font-locked.
+
+The default value in nREPL is 1024."
+  :type 'integer
+  :group 'cider
+  :package-version '(cider . "0.25.0"))
+
 (defun cider--print-fn ()
   "Return the value to send in the nrepl.middleware.print/print slot."
   (pcase cider-print-fn
@@ -313,6 +324,8 @@ is included in the request if non-nil."
                  `(("nrepl.middleware.print/print" ,(cider--print-fn))))
                (when cider-print-quota
                  `(("nrepl.middleware.print/quota" ,cider-print-quota)))
+               (when cider-print-buffer-size
+                 `(("nrepl.middleware.print/buffer-size" ,cider-print-buffer-size)))
                (unless (nrepl-dict-empty-p print-options)
                  `(("nrepl.middleware.print/options" ,print-options))))))
 

doc/modules/ROOT/pages/repl/configuration.adoc

@@ -152,6 +152,18 @@ each chunk is font-locked by itself and the results can't really be combined
 * The font-locking of results is an expensive operation which involves copying the value
 to a temporary buffer, where we check its integrity and do the actual font-locking.
 
+By default CIDER instructs nREPL to stream data in 4K chunks, but you can easily modify this:
+
+[source,lisp]
+----
+;; let's stream data in 8K chunks
+(setq cider-print-buffer-size (8 * 1024))
+----
+
+Setting this to `nil` will result in using nREPL's default `buffer-size` of 1024 bytes.
+The smaller the print buffer size the faster you'll get feedback/updates in the REPL, so generally
+it's a good idea to stick to some relatively small size.
+
 TIP: If you'd like to learn more about the font-locking of results you can check
 out the definition of `clojure-font-lock-as` and `clojure-font-lock-as-clojure`
 in `cider-util.el`.