[PR] fix: guard LRUCache against non-positive capacity [casbin]

["nikolauspschuetz (via GitHub)" <gi...@apache.org>]

nikolauspschuetz opened a new pull request, #1749:
URL: https://github.com/apache/casbin/pull/1749

   ### What
   
   `NewLRUCache` and `NewSyncLRUCache` are exported public API, but calling `Put` on a cache created with a non-positive capacity panics with a nil pointer dereference.
   
   Reproduce:
   
   ```go
   c := util.NewLRUCache(0)
   c.Put("a", 1) // panic: runtime error: invalid memory address or nil pointer dereference
   ```
   
   On the first insert, the eviction branch is taken (`len(cache.m) >= cache.capacity` is `0 >= 0`), which calls `cache.remove(cache.tail.prev, false)`. On an empty list `cache.tail.prev` is the `head` sentinel, whose `prev` is `nil`, so `remove` dereferences `nil` at `util/util.go`.
   
   ### Fix
   
   Return early from `Put` when `capacity <= 0` — such a cache holds nothing, so there is nothing to store and no eviction to perform. This keeps `Get` returning `(nil, false)` for every key, consistent with a zero-capacity cache.
   
   Guarding the eviction path instead would let a zero-capacity cache incorrectly hold one entry, so the early return is the more correct minimal fix.
   
   ### Test
   
   Added `TestLRUCacheNonPositiveCapacity` in `util/util_test.go`. It panics on `master` and passes with this change:
   
   - before: `panic: runtime error: invalid memory address or nil pointer dereference` (in `Put`)
   - after: `ok  github.com/casbin/casbin/v3/util`
   
   ---
   for the record: fix found and drafted with AI assistance; reviewed and verified by me.
   


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Re: [PR] fix: guard LRUCache against non-positive capacity [casbin]

["hsluoyz (via GitHub)" <gi...@apache.org>]

hsluoyz merged PR #1749:
URL: https://github.com/apache/casbin/pull/1749


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