Return a copy of the cache entry when local_cache exists:#36656
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How could dup_entry ever be nil?
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in case you try to access a key that doesn't exist
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Nevermind, I'm stuck in the past, turns out nil.dup actually exist on modern rubies TIL.
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I think this would have to be a deep copy. .dup alone will still let you modify nested objects if present
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No because dup_value! does a Marshal.load
- When the local cache exists (during the request lifecycle), the entry returned from the LocalStore is passed as a reference which means mutable object can accidentaly get modified. This behaviour seems unnecessarily unsafe and is prone to issues like it happened in our application. This patch dup the `Entry` returned from the cache and dup it's internal value.
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| def fetch_entry(key, options = nil) # :nodoc: | ||
| @data.fetch(key) { @data[key] = yield } | ||
| entry = @data.fetch(key) { @data[key] = yield } | ||
| dup_entry = entry.dup |
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Why you need to dup the entry itself?
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the @data hash keeps a reference to the entry, if I don't dup it, user can modify the underlying value which would affect the cache.
The local cache need to protect against mutations of both the initial received value, and the value returned. See: - rails#36656 - rails#37587 Because of this, the overhead of the local cache end up being quite significant. On a single read, the value will be deep duped twice. So unless the value is one of the type benefiting from a fast path, we'll have to do two `Marshal.load(Marshal.dump(value))` roundtrips, which for large values can be very expensive. By using a specialized `LocalEntry` type instead, we can store the `Marshal` payload rather than the original value. This way the overhead is reduced to a single `Marshal.dump` on writes and a single `Marshal.load` on reads.
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Return a copy of the cache entry when local_cache exists:
When the local cache exists (during the request lifecycle), the
entry returned from the LocalStore is passed as a reference which
means mutable object can accidentaly get modified.
This behaviour seems unnecessarily unsafe and is prone to
issues like it happened in our application.
This patch dup the
Entryreturned from the cache and dup it'sinternal value.
cc/ @rafaelfranca @casperisfine