Clerk Analyzer doesn't strip metadata from forms when hashing

[craig-latacora]

The nextjournal.clerk.analyzer/hash-codeblock function will attempt to print out a form to generate a hash. This will result in bad things happening if the form, when evaluated, tagged it's results with some metadata. That metadata will now be applied to the form itself. That's not much of a problem, except that metadata can be used to set the :tag of a value, and then a print-method can be defined for that tag. That print-method body now will try and print the form (a sequence) and likely blow up.

Here is a quick patch that strips metadata before hashing the form. This works, and I don't think that we need to be sensitive to changes in metadata. However, perhaps the more correct mechanism would collect some part of the metadata and include that as input to the hash.

(require '[nextjournal.clerk :as clerk])

;; This will compile and run just fine, but clerk errors when evaluating this as
;; a notebook
(defprotocol Foo
  (-foo [this]))

(defmethod print-method :foo [v ^java.io.Writer w]
  (.write w (str "FOO<" (-foo v) ">")))

;; When this is analyzed by clerk, you will get an exception:
;;
;; Unhandled java.lang.IllegalArgumentException
;; No implementation of method: :-foo of protocol: #'user/Foo found for class: clojure.lang.PersistentList

(defn gimme-a-foo []
  ^{:type :foo}
  (reify Foo
    (-foo [this]
      :baz)))

;; this should use the printer above
(gimme-a-foo)


;; evaluate this, and you will patch the
;; nextjournal.clerk.analyzer/hash-codeblock to strip metadata before printing
;; forms
;;
;; (in-ns 'nextjournal.clerk.analyzer)
;; (require 'clojure.walk)
;; (defn strip-form-meta [form]
;;     (clojure.walk/walk
;;      (fn [v]
;;        (if (or (instance? clojure.lang.IObj v)
;;                (instance? clojure.lang.IMeta v))
;;          (vary-meta v (constantly nil))
;;          v)) identity form))

;;   (defn hash-codeblock [->hash {:as codeblock :keys [hash form id deps vars]}]
;;     (when (and (seq deps) (not (ifn? ->hash)))
;;       (throw (ex-info "`->hash` must be `ifn?`" {:->hash ->hash :codeblock codeblock})))
;;     (let [hashed-deps (into #{} (map ->hash) deps)]
;;       (sha1-base58 (binding [*print-length* nil]
;;                      (pr-str (set/union (conj hashed-deps (if (strip-form-meta form) hash))
;;                                         vars))))))

[mk]

Hey @craig-latacora,

Thanks for the issue. Should be able to take a closer look tomorrow or Wednesday if it's still a problem that should be fixed upstream for you. Was the closing intentional or should we reopen it?