Bug #20307: `Hash#update` from compare_by_identity hash can have unfrozen string keys

• Status: Closed
• Assignee: -
• Target version: -
• Backport: 3.0: REQUIRED, 3.1: REQUIRED, 3.2: REQUIRED, 3.3: REQUIRED

[nobu (Nobuyoshi Nakada)]

I don't think this behavior is expected.

i = Hash.new.compare_by_identity
k = "a"
i[k] = 0
h = {}.update(i)
p h.compare_by_identity?  # => false
p h["a"]                  # => 0

k.upcase!
# `k` is still in `h`.
p h.keys.include?(k)      # => true

# but not found.
p((h.fetch(k) rescue $!)) # => #<KeyError: key not found: "A">
h["A"] = 1  
p h                       # => {"A"=>0, "A"=>1}

I expect h to still have "a"=>0 entry.

[nobu (Nobuyoshi Nakada)]

• Description updated (diff)

[nobu (Nobuyoshi Nakada)]

• Description updated (diff)

[nobu (Nobuyoshi Nakada)]

• Description updated (diff)

[nobu (Nobuyoshi Nakada)]

• Description updated (diff)

[nobu (Nobuyoshi Nakada)]

• Description updated (diff)

https://github.com/ruby/ruby/pull/10099

[Dan0042 (Daniel DeLorme)]

After the code above, we can do

h.rehash
p h      #=> {"A"=>1}

When a key is clobbered like that due to a rehash, maybe there should be a warning or an error? It seems to me a pretty clear symptom of a bug, either in the ruby code or in the interpreter itself.

[byroot (Jean Boussier)]

I don't understand the bug report nor the patch.

If you make a frozen copy of the provided key, you change the key. e.g.

cache = {}.compare_by_indentity
key = Random.bytes(5)

cache[key] = 1
p cache[key]

I'd expect cache[key] to return 1, with what I understand of your patch, it wouldn't anymore.

Mutable hash keys aren't a new thing, it's only that regular hash have this special behavior for strings to keep a frozen copy of the string key, but they don't do that for any other type.

I don't see why identity hashes would need to copy this behavior.

[byroot (Jean Boussier)]

ignore me, I misunderstood the reproduction script. h is not compared by identity.

[nobu (Nobuyoshi Nakada)]

• Status changed from Open to Closed

Applied in changeset git|f36a71e26995b69ff72bc132bbcf40ad89571414.

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[Bug #20307] Fix Hash#update to make frozen copy of string keys