Serializing+deserializing lists breaks their hash

clojure.lang.PersistentList implements Serializable but a serialization roundtrip results in a hash of 0. This appears to be caused by ASeq marking its _hash property as transient; other collection base classes don't do this. I don't know if there is a rationale for the distinction, but the result is that serializing, then deserializing, a list results in its _hash property being 0 instead of either its previous, calculated value, or -1, which would trigger recalculation.

This means that any code that relies on a list's hash value can break in subtle ways.

Examples:

(import '[java.io ByteArrayInputStream ByteArrayOutputStream ObjectInputStream ObjectOutputStream])

(defn obj->bytes [obj]
  (let [bytestream (ByteArrayOutputStream.)]
    (doto (ObjectOutputStream. bytestream) (.writeObject obj))
    (.toByteArray bytestream)))

(defn bytes->obj [bs]
  (.readObject (ObjectInputStream. (ByteArrayInputStream. bs))))

(defn round-trip [x] (bytes->obj (obj->bytes x)))

user=> (hash '(1))
-1381383523
user=> (hash (round-trip '(1)))
0
user=> #{'(1) (round-trip '(1))}
#{(1) (1)}
user=> (def m {'(1) 1})
#'user/m
user=> (= m (round-trip m))
true
user=> (= (round-trip m) m)
false

Approach: Use 0 as the "uncomputed" hash value, not -1. The auto initialized value on serialization construction will then automatically trigger a re-computation.

Alternate approaches:

• Implement a readObject method that sets the _hash property to -1. This is somewhat complicated in the face of subclasses, etc.

Note: Also need to consider other classes that use -1 instead of 0 as the uncomputed hash value: APersistentMap, APersistentSet, APersistentVector, PersistentQueue - however I believe in those cases they are not transient and thus may avoid the issue. Perhaps they should be transient though.

Patch: clj-1766-2.patch
Screened by: Alex Miller