Use one atomic operation in (*Cache).decreaseSize

The previous implementation was susceptible to a race condition (of
correctness) since c.decreaseSize is called without a lock in
(*Cache).WriteMulti.

There were already tests which asserted the correctness of the result of
decreaseSize, so no tests were added or modified.

CHANGELOG.md

@@ -23,6 +23,7 @@ The stress tool `influx_stress` will be removed in a subsequent release. We reco
 
 ### Bugfixes
 
+- [#7786](https://github.com/influxdata/influxdb/pull/7786): Fix potential race condition in correctness of tsm1_cache memBytes statistic.
 - [#7784](https://github.com/influxdata/influxdb/pull/7784): Fix broken error return on meta client's UpdateUser and DropContinuousQuery methods.
 - [#7741](https://github.com/influxdata/influxdb/pull/7741): Fix string quoting and significantly improve performance of `influx_inspect export`.
 - [#7698](https://github.com/influxdata/influxdb/pull/7698): CLI was caching db/rp for insert into statements.

tsdb/engine/tsm1/cache.go

@@ -423,8 +423,9 @@ func (c *Cache) increaseSize(delta uint64) {
 
 // decreaseSize decreases size by delta.
 func (c *Cache) decreaseSize(delta uint64) {
-	size := atomic.LoadUint64(&c.size)
-	atomic.StoreUint64(&c.size, size-delta)
+	// Since there isn't an explicit atomic operation for subtraction,
+	// use addition and overflow to effectively subtract delta from c.size.
+	atomic.AddUint64(&c.size, math.MaxUint64-delta+1)
 }
 
 // MaxSize returns the maximum number of bytes the cache may consume.