Serializing+deserializing lists breaks their hash
Description
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:
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(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)))1
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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)
falseApproach: 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
Environment
Tested on OS X 10.10.3, Clojure 1.6.0, Java 1.8.0_45-b14, but don't think this is a factor.
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