Turn rich content on by default, fetching external content on demand

Evaluating a File or URL-shaped result no longer auto-fires a slurp
request (the behavior that got the feature disabled back in 0.25, see
#2825).  The REPL now renders the URL plus a [show content] button and
nothing is transferred until it's pressed.  Inline image results are
unaffected - they involve no IO and still render automatically.

With the danger gone, cider-repl-use-content-types defaults to t, so
images finally render in the REPL out of the box.  Best paired with
cider-nrepl 0.62+, which hardens the server side (scheme allowlist,
size caps, graceful fetch errors).

Author: bbatsov

CHANGELOG.md

@@ -61,6 +61,8 @@
 ### Changes
 
 - Bump the default `cider-repl-history-size` from 500 to 5000.
+- Enable rich content in the REPL by default (`cider-repl-use-content-types` is now t): image results render inline, and results naming external content (files, URLs) render a `[show content]` button that fetches and renders it on demand. The automatic fetching that got the feature disabled back in 0.25 ([#2825](https://github.com/clojure-emacs/cider/issues/2825)) is gone - nothing is transferred until the button is pressed - and the server side is hardened in `cider-nrepl` 0.62 (URL-scheme allowlist, size caps, graceful fetch errors).
+- Bump the default `cider-repl-history-size` from 500 to 5000 and the default `cider-print-buffer-size` from 4K to 16K (fewer, larger chunks when streaming big results).
 - Rename the REPL history browser from `cider-repl-history` to `cider-history` (command, mode and options), so its names no longer clash with the REPL's unrelated input-history settings (`cider-repl-history-file`, `cider-repl-history-size`); the old names keep working as obsolete aliases.
 - Color the nREPL messages buffer with theme-aware faces (`nrepl-message-faces`, eight faces inheriting from standard font-lock faces) instead of a hardcoded color list; `nrepl-message-colors` is now obsolete, but still takes precedence when customized.
 - Consolidate the per-buffer auto-select options into a single `cider-auto-select-buffer`, which can be `t`, `nil` or a list of the popup buffers to select (e.g. `'(error inspector)`); the old options (`cider-auto-select-error-buffer`, `cider-auto-select-test-report-buffer`, `cider-doc-auto-select-buffer`, `cider-inspector-auto-select-buffer`, `cider-cheatsheet-auto-select-buffer` and `cider-log-auto-select-frameworks-buffer`) are now obsolete, but still take precedence when customized.

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

@@ -282,21 +282,30 @@ more information on configuring printing.
 
 == Displaying images in the REPL
 
-Expressions that evaluate to images can be rendered as images in the
-REPL. You can enable this behavior like this:
+Expressions that evaluate to images are rendered as images in the REPL.
+Results naming external content - a `java.io.File`, or a `java.net.URI`/`URL`
+with a `file:`, `http:` or `https:` scheme - render as the URL followed by a
+`[show content]` button; press kbd:[RET] (or click) to fetch the content and
+render it in place.  Nothing is transferred until you press the button, so
+merely evaluating a `File` or `URI` has no side effects.
+
+This behavior is enabled by default as of CIDER 2.0.  You can disable it like
+this:
 
 [source,lisp]
 ----
-(setq cider-repl-use-content-types t)
+(setq cider-repl-use-content-types nil)
 ----
 
-NOTE: This setting is disabled by default due to some rough edges with the
-feature that were never properly addressed. See this
-https://github.com/clojure-emacs/cider/issues/2825[bug report] for details.
-
 Alternatively, you can toggle this behavior on and off using kbd:[M-x cider-repl-toggle-content-types]
 or the REPL shortcut `toggle-content-types`.
 
+NOTE: In CIDER 1.x this feature was disabled by default because the content
+was fetched automatically, which meant evaluating any URL-shaped value caused
+IO (see https://github.com/clojure-emacs/cider/issues/2825[the discussion]).
+The on-demand button, together with the hardened `cider-nrepl` middleware
+(scheme allowlist, size caps, graceful fetch errors), resolves that.
+
 == REPL type detection
 
 Normally CIDER would detect automatically the type of a REPL (Clojure or ClojureScript), based

doc/modules/ROOT/pages/troubleshooting.adoc

@@ -136,15 +136,20 @@ java.net.UnknownServiceException: protocol doesn't support input
 	at cider.nrepl.middleware.slurp$handle_slurp.invoke(slurp.clj:109)
 ----
 
-You can disable the middleware like this:
+This affected CIDER 1.x, where the content was fetched automatically as soon
+as a URL-shaped value was returned.  As of CIDER 2.0 (with `cider-nrepl`
+0.62+) content is only fetched when you press the result's `[show content]`
+button, non-fetchable URL schemes like `mailto:` are never offered for
+fetching, and a failed fetch reports a plain-text error instead of crashing
+the handler.
+
+If you're stuck on older versions, you can disable the feature like this:
 
 [source,lisp]
 ----
 (setq cider-repl-use-content-types nil)
 ----
 
-That's also the default behavior.
-
 === Debugging freezes & lock-ups
 
 Sometimes a CIDER command might hang for a while (e.g. due to a bug or a

lisp/cider-repl.el

@@ -109,12 +109,15 @@ The `cider-toggle-pretty-printing' command can be used to interactively
 change the setting's value."
   :type 'boolean)
 
-(defcustom cider-repl-use-content-types nil
+(defcustom cider-repl-use-content-types t
   "Control whether REPL results are presented using content-type information.
+When enabled, evaluation results that are images render inline in the REPL,
+and results naming external content (files, URLs) offer a button that
+fetches and renders it on demand.
 The `cider-repl-toggle-content-types' command can be used to interactively
 change the setting's value."
   :type 'boolean
-  :package-version '(cider . "0.17.0"))
+  :package-version '(cider . "2.0.0"))
 
 (defcustom cider-repl-auto-detect-type t
   "Control whether to auto-detect the REPL type using track-state information.
@@ -1154,15 +1157,47 @@ Part of the default `cider-repl-content-type-handler-alist'."
                              (cider-repl--image image 'svg t)
                              show-prefix bol))
 
-(defun cider-repl-handle-external-body (type buffer _ &optional _show-prefix _bol)
-  "Handler for slurping external content into BUFFER.
-Handles an external-body TYPE by issuing a slurp request to fetch the content."
-  (if-let* ((args        (cadr type))
-            (access-type (nrepl-dict-get args "access-type")))
-      (cider-nrepl-send-request
-       (list "op" "cider/slurp" "url" (nrepl-dict-get args access-type))
-       (cider-repl-handler buffer)))
-  nil)
+(defun cider-repl--fetch-external-body (buffer url)
+  "Fetch URL's content from the nREPL server and render it in BUFFER."
+  (cider-nrepl-send-request (list "op" "cider/slurp" "url" url)
+                            (cider-repl-handler buffer)))
+
+(defun cider-repl--display-external-body (buffer url &optional show-prefix bol)
+  "Insert URL and a button that fetches its content into BUFFER.
+For compatibility with the rest of CIDER's REPL machinery, supports
+SHOW-PREFIX and BOL."
+  (with-current-buffer buffer
+    (save-excursion
+      (cider-save-marker cider-repl-output-start
+        (goto-char cider-repl-output-end)
+        (when (and bol (not (bolp)))
+          (insert-before-markers "\n"))
+        (when show-prefix
+          (insert-before-markers
+           (propertize cider-repl-result-prefix 'font-lock-face 'font-lock-comment-face)))
+        (insert-before-markers
+         (propertize url 'font-lock-face 'cider-repl-result-face)
+         " ")
+        (let ((start (point)))
+          (insert-before-markers "[show content]")
+          (make-text-button start (point)
+                            'follow-link t
+                            'help-echo (format "Fetch %s and render it here" url)
+                            'action (lambda (_button)
+                                      (cider-repl--fetch-external-body buffer url)))))))
+  t)
+
+(defun cider-repl-handle-external-body (type buffer _ &optional show-prefix bol)
+  "Handler for referencing external content in BUFFER.
+Handles an external-body TYPE by rendering the reference URL followed by a
+button that fetches and renders the content on demand, via a slurp
+request.  Nothing is transferred (or connected to) until the button is
+pressed, so merely evaluating a `File' or `URI' has no side effects."
+  (when-let* ((args        (cadr type))
+              (access-type (nrepl-dict-get args "access-type"))
+              (url         (nrepl-dict-get args access-type)))
+    (cider-repl--display-external-body buffer url show-prefix bol))
+  t)
 
 (defvar cider-repl-content-type-handler-alist
   `(("message/external-body" . ,#'cider-repl-handle-external-body)

test/cider-repl-tests.el

@@ -641,3 +641,28 @@ PROPERTY should be a symbol of either 'text, 'ansi-context or
     (cider-debug-sesman-friendly-session-p)
     (expect 'cider-sessions :to-have-been-called)
     (expect 'cider--sesman-friendly-session-p :to-have-been-called-times 2)))
+
+(describe "cider-repl-handle-external-body"
+  (it "renders a fetch button for the URL instead of fetching automatically"
+    (let (rendered requested)
+      (cl-letf (((symbol-function 'cider-repl--display-external-body)
+                 (lambda (_buffer url &rest _) (setq rendered url) t))
+                ((symbol-function 'cider-nrepl-send-request)
+                 (lambda (&rest _) (setq requested t))))
+        (expect (cider-repl-handle-external-body
+                 (list "message/external-body"
+                       (nrepl-dict "access-type" "URL" "URL" "file:/tmp/x.png"))
+                 (current-buffer) "")
+                :to-be-truthy)
+        (expect rendered :to-equal "file:/tmp/x.png")
+        (expect requested :to-be nil))))
+
+  (it "does nothing (but still allows the prompt) without an access type"
+    (let (rendered)
+      (cl-letf (((symbol-function 'cider-repl--display-external-body)
+                 (lambda (_buffer url &rest _) (setq rendered url) t)))
+        (expect (cider-repl-handle-external-body
+                 (list "message/external-body" (nrepl-dict))
+                 (current-buffer) "")
+                :to-be-truthy)
+        (expect rendered :to-be nil)))))