server/item/inventory: fix data race between a slot change and SlotFunc or Close - #1417
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server/item/inventory: fix data race between a slot change and SlotFunc or Close#1417schphe wants to merge 4 commits into
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…nc or Close setItem returns a closure that its callers run after the inventory has been unlocked, so that the function called on a slot change does not run while the inventory is locked. That closure read inv.f when it ran rather than when it was built, while SlotFunc and Close write inv.f under the lock, so a slot being written concurrently with either was a data race on a type documented as safe for concurrent use. The function is now read while the lock is still held. A change already in progress therefore calls the function that was installed when it started, which is the same guarantee the lock gives every other field.
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What happens and why
setItem returns a closure that its callers run after the inventory has been
unlocked, so that the function called on a slot change does not run while the
inventory is locked. That closure read inv.f when it ran rather than when it
was built, while SlotFunc and Close write inv.f under the lock, so a slot
being written concurrently with either was a data race on a type documented as
safe for concurrent use.
The function is now read while the lock is still held. A change already in
progress therefore calls the function that was installed when it started,
which is the same guarantee the lock gives every other field.
Verification
Regression test reports
WARNING: DATA RACEwithout the fix, undergo test -race. Note the repository's CI does not run with the race detector, so this test only earns its place if run that way.