Fix clang thread-safety-pointer warnings

[theuni]

This prevents a warning in an upcoming version of clang which will turn this on for us.

Submitted upstream: google#1270

Clang's -Wthread-safety-pointer warnings will become part of -Wthread-safety in some future release. Add missing annotations for functions that pass the address of guarded members. No functional change. Fixes the following warnings:

util/cache.cc:220:17: warning: passing pointer to variable 'in_use_' requires holding mutex 'mutex_' [-Wthread-safety-pointer]
  220 |     LRU_Append(&in_use_, e);
      |                 ^
util/cache.cc:235:17: warning: passing pointer to variable 'lru_' requires holding mutex 'mutex_' [-Wthread-safety-pointer]
  235 |     LRU_Append(&lru_, e);

[davidgumberg]

Tested and review ACK 7daf4ed

in_use and lru_ are guarded by mutex_:

leveldb-subtree/util/cache.cc

Lines 185 to 193 in 113db49

	// Dummy head of LRU list.
	// lru.prev is newest entry, lru.next is oldest entry.
	// Entries have refs==1 and in_cache==true.
	LRUHandle lru_ GUARDED_BY(mutex_);
	// Dummy head of in-use list.
	// Entries are in use by clients, and have refs >= 2 and in_cache==true.
	LRUHandle in_use_ GUARDED_BY(mutex_);

and are accessed by LRUCache::Ref() and LRUCache::Unref() respectively:

leveldb-subtree/util/cache.cc

Lines 217 to 237 in 113db49

	void LRUCache::Ref(LRUHandle* e) {
	if (e->refs == 1 && e->in_cache) { // If on lru_ list, move to in_use_ list.
	LRU_Remove(e);
	LRU_Append(&in_use_, e);
	}
	e->refs++;
	}
	void LRUCache::Unref(LRUHandle* e) {
	assert(e->refs > 0);
	e->refs--;
	if (e->refs == 0) { // Deallocate.
	assert(!e->in_cache);
	(*e->deleter)(e->key(), e->value);
	free(e);
	} else if (e->in_cache && e->refs == 1) {
	// No longer in use; move to lru_ list.
	LRU_Remove(e);
	LRU_Append(&lru_, e);
	}
	}

The annotations added here are correct.

---

Attempting to build with -Wthread-safety-pointer on master fails for me with errors identical to those in the PR description

$ clang --version
clang version 21.0.0git (git@github.com:llvm/llvm-project.git 1f1c725b68b9d72b5cd3fc95c57ce4244bc85c8e)
$ CC=clang CXX=clang++ cmake -B build -DCMAKE_CXX_FLAGS=-Wthread-safety-pointer
/leveldb-subtree/util/cache.cc:220:17: error: passing pointer to variable 'in_use_' requires holding mutex 'mutex_' [-Werror,-Wthread-safety-pointer]
  220 |     LRU_Append(&in_use_, e);
      |                 ^
/leveldb-subtree/util/cache.cc:235:17: error: passing pointer to variable 'lru_' requires holding mutex 'mutex_' [-Werror,-Wthread-safety-pointer]
  235 |     LRU_Append(&lru_, e);
      | 

And this branch fixes those thread safety warnings.

[fanquake]

ACK 7daf4ed

Clang's -Wthread-safety-pointer warnings will become part of -Wthread-safety in some future release.

For anyone else wondering, Wthread-safety-pointer is first appearing in Clang 21.x.